Merge https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next

[linux-2.6-microblaze.git] / net / ipv4 / tcp_input.c

// SPDX-License-Identifier: GPL-2.0
/*
 * INET         An implementation of the TCP/IP protocol suite for the LINUX
 *              operating system.  INET is implemented using the  BSD Socket
 *              interface as the means of communication with the user level.
 *
 *              Implementation of the Transmission Control Protocol(TCP).
 *
 * Authors:     Ross Biro
 *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
 *              Mark Evans, <evansmp@uhura.aston.ac.uk>
 *              Corey Minyard <wf-rch!minyard@relay.EU.net>
 *              Florian La Roche, <flla@stud.uni-sb.de>
 *              Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
 *              Linus Torvalds, <torvalds@cs.helsinki.fi>
 *              Alan Cox, <gw4pts@gw4pts.ampr.org>
 *              Matthew Dillon, <dillon@apollo.west.oic.com>
 *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 *              Jorge Cwik, <jorge@laser.satlink.net>
 */

/*
 * Changes:
 *              Pedro Roque     :       Fast Retransmit/Recovery.
 *                                      Two receive queues.
 *                                      Retransmit queue handled by TCP.
 *                                      Better retransmit timer handling.
 *                                      New congestion avoidance.
 *                                      Header prediction.
 *                                      Variable renaming.
 *
 *              Eric            :       Fast Retransmit.
 *              Randy Scott     :       MSS option defines.
 *              Eric Schenk     :       Fixes to slow start algorithm.
 *              Eric Schenk     :       Yet another double ACK bug.
 *              Eric Schenk     :       Delayed ACK bug fixes.
 *              Eric Schenk     :       Floyd style fast retrans war avoidance.
 *              David S. Miller :       Don't allow zero congestion window.
 *              Eric Schenk     :       Fix retransmitter so that it sends
 *                                      next packet on ack of previous packet.
 *              Andi Kleen      :       Moved open_request checking here
 *                                      and process RSTs for open_requests.
 *              Andi Kleen      :       Better prune_queue, and other fixes.
 *              Andrey Savochkin:       Fix RTT measurements in the presence of
 *                                      timestamps.
 *              Andrey Savochkin:       Check sequence numbers correctly when
 *                                      removing SACKs due to in sequence incoming
 *                                      data segments.
 *              Andi Kleen:             Make sure we never ack data there is not
 *                                      enough room for. Also make this condition
 *                                      a fatal error if it might still happen.
 *              Andi Kleen:             Add tcp_measure_rcv_mss to make
 *                                      connections with MSS<min(MTU,ann. MSS)
 *                                      work without delayed acks.
 *              Andi Kleen:             Process packets with PSH set in the
 *                                      fast path.
 *              J Hadi Salim:           ECN support
 *              Andrei Gurtov,
 *              Pasi Sarolahti,
 *              Panu Kuhlberg:          Experimental audit of TCP (re)transmission
 *                                      engine. Lots of bugs are found.
 *              Pasi Sarolahti:         F-RTO for dealing with spurious RTOs
 */

#define pr_fmt(fmt) "TCP: " fmt

#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/module.h>
#include <linux/sysctl.h>
#include <linux/kernel.h>
#include <linux/prefetch.h>
#include <net/dst.h>
#include <net/tcp.h>
#include <net/inet_common.h>
#include <linux/ipsec.h>
#include <asm/unaligned.h>
#include <linux/errqueue.h>
#include <trace/events/tcp.h>
#include <linux/jump_label_ratelimit.h>
#include <net/busy_poll.h>
#include <net/mptcp.h>

int sysctl_tcp_max_orphans __read_mostly = NR_FILE;

#define FLAG_DATA               0x01 /* Incoming frame contained data.          */
#define FLAG_WIN_UPDATE         0x02 /* Incoming ACK was a window update.       */
#define FLAG_DATA_ACKED         0x04 /* This ACK acknowledged new data.         */
#define FLAG_RETRANS_DATA_ACKED 0x08 /* "" "" some of which was retransmitted.  */
#define FLAG_SYN_ACKED          0x10 /* This ACK acknowledged SYN.              */
#define FLAG_DATA_SACKED        0x20 /* New SACK.                               */
#define FLAG_ECE                0x40 /* ECE in this ACK                         */
#define FLAG_LOST_RETRANS       0x80 /* This ACK marks some retransmission lost */
#define FLAG_SLOWPATH           0x100 /* Do not skip RFC checks for window update.*/
#define FLAG_ORIG_SACK_ACKED    0x200 /* Never retransmitted data are (s)acked  */
#define FLAG_SND_UNA_ADVANCED   0x400 /* Snd_una was changed (!= FLAG_DATA_ACKED) */
#define FLAG_DSACKING_ACK       0x800 /* SACK blocks contained D-SACK info */
#define FLAG_SET_XMIT_TIMER     0x1000 /* Set TLP or RTO timer */
#define FLAG_SACK_RENEGING      0x2000 /* snd_una advanced to a sacked seq */
#define FLAG_UPDATE_TS_RECENT   0x4000 /* tcp_replace_ts_recent() */
#define FLAG_NO_CHALLENGE_ACK   0x8000 /* do not call tcp_send_challenge_ack()  */
#define FLAG_ACK_MAYBE_DELAYED  0x10000 /* Likely a delayed ACK */
#define FLAG_DSACK_TLP          0x20000 /* DSACK for tail loss probe */

#define FLAG_ACKED              (FLAG_DATA_ACKED|FLAG_SYN_ACKED)
#define FLAG_NOT_DUP            (FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
#define FLAG_CA_ALERT           (FLAG_DATA_SACKED|FLAG_ECE|FLAG_DSACKING_ACK)
#define FLAG_FORWARD_PROGRESS   (FLAG_ACKED|FLAG_DATA_SACKED)

#define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH)
#define TCP_HP_BITS (~(TCP_RESERVED_BITS|TCP_FLAG_PSH))

#define REXMIT_NONE     0 /* no loss recovery to do */
#define REXMIT_LOST     1 /* retransmit packets marked lost */
#define REXMIT_NEW      2 /* FRTO-style transmit of unsent/new packets */

#if IS_ENABLED(CONFIG_TLS_DEVICE)
static DEFINE_STATIC_KEY_DEFERRED_FALSE(clean_acked_data_enabled, HZ);

void clean_acked_data_enable(struct inet_connection_sock *icsk,
                             void (*cad)(struct sock *sk, u32 ack_seq))
{
        icsk->icsk_clean_acked = cad;
        static_branch_deferred_inc(&clean_acked_data_enabled);
}
EXPORT_SYMBOL_GPL(clean_acked_data_enable);

void clean_acked_data_disable(struct inet_connection_sock *icsk)
{
        static_branch_slow_dec_deferred(&clean_acked_data_enabled);
        icsk->icsk_clean_acked = NULL;
}
EXPORT_SYMBOL_GPL(clean_acked_data_disable);

void clean_acked_data_flush(void)
{
        static_key_deferred_flush(&clean_acked_data_enabled);
}
EXPORT_SYMBOL_GPL(clean_acked_data_flush);
#endif

#ifdef CONFIG_CGROUP_BPF
static void bpf_skops_parse_hdr(struct sock *sk, struct sk_buff *skb)
{
        bool unknown_opt = tcp_sk(sk)->rx_opt.saw_unknown &&
                BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
                                       BPF_SOCK_OPS_PARSE_UNKNOWN_HDR_OPT_CB_FLAG);
        bool parse_all_opt = BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
                                                    BPF_SOCK_OPS_PARSE_ALL_HDR_OPT_CB_FLAG);
        struct bpf_sock_ops_kern sock_ops;

        if (likely(!unknown_opt && !parse_all_opt))
                return;

        /* The skb will be handled in the
         * bpf_skops_established() or
         * bpf_skops_write_hdr_opt().
         */
        switch (sk->sk_state) {
        case TCP_SYN_RECV:
        case TCP_SYN_SENT:
        case TCP_LISTEN:
                return;
        }

        sock_owned_by_me(sk);

        memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
        sock_ops.op = BPF_SOCK_OPS_PARSE_HDR_OPT_CB;
        sock_ops.is_fullsock = 1;
        sock_ops.sk = sk;
        bpf_skops_init_skb(&sock_ops, skb, tcp_hdrlen(skb));

        BPF_CGROUP_RUN_PROG_SOCK_OPS(&sock_ops);
}

static void bpf_skops_established(struct sock *sk, int bpf_op,
                                  struct sk_buff *skb)
{
        struct bpf_sock_ops_kern sock_ops;

        sock_owned_by_me(sk);

        memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
        sock_ops.op = bpf_op;
        sock_ops.is_fullsock = 1;
        sock_ops.sk = sk;
        /* sk with TCP_REPAIR_ON does not have skb in tcp_finish_connect */
        if (skb)
                bpf_skops_init_skb(&sock_ops, skb, tcp_hdrlen(skb));

        BPF_CGROUP_RUN_PROG_SOCK_OPS(&sock_ops);
}
#else
static void bpf_skops_parse_hdr(struct sock *sk, struct sk_buff *skb)
{
}

static void bpf_skops_established(struct sock *sk, int bpf_op,
                                  struct sk_buff *skb)
{
}
#endif

static void tcp_gro_dev_warn(struct sock *sk, const struct sk_buff *skb,
                             unsigned int len)
{
        static bool __once __read_mostly;

        if (!__once) {
                struct net_device *dev;

                __once = true;

                rcu_read_lock();
                dev = dev_get_by_index_rcu(sock_net(sk), skb->skb_iif);
                if (!dev || len >= dev->mtu)
                        pr_warn("%s: Driver has suspect GRO implementation, TCP performance may be compromised.\n",
                                dev ? dev->name : "Unknown driver");
                rcu_read_unlock();
        }
}

/* Adapt the MSS value used to make delayed ack decision to the
 * real world.
 */
static void tcp_measure_rcv_mss(struct sock *sk, const struct sk_buff *skb)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        const unsigned int lss = icsk->icsk_ack.last_seg_size;
        unsigned int len;

        icsk->icsk_ack.last_seg_size = 0;

        /* skb->len may jitter because of SACKs, even if peer
         * sends good full-sized frames.
         */
        len = skb_shinfo(skb)->gso_size ? : skb->len;
        if (len >= icsk->icsk_ack.rcv_mss) {
                icsk->icsk_ack.rcv_mss = min_t(unsigned int, len,
                                               tcp_sk(sk)->advmss);
                /* Account for possibly-removed options */
                if (unlikely(len > icsk->icsk_ack.rcv_mss +
                                   MAX_TCP_OPTION_SPACE))
                        tcp_gro_dev_warn(sk, skb, len);
        } else {
                /* Otherwise, we make more careful check taking into account,
                 * that SACKs block is variable.
                 *
                 * "len" is invariant segment length, including TCP header.
                 */
                len += skb->data - skb_transport_header(skb);
                if (len >= TCP_MSS_DEFAULT + sizeof(struct tcphdr) ||
                    /* If PSH is not set, packet should be
                     * full sized, provided peer TCP is not badly broken.
                     * This observation (if it is correct 8)) allows
                     * to handle super-low mtu links fairly.
                     */
                    (len >= TCP_MIN_MSS + sizeof(struct tcphdr) &&
                     !(tcp_flag_word(tcp_hdr(skb)) & TCP_REMNANT))) {
                        /* Subtract also invariant (if peer is RFC compliant),
                         * tcp header plus fixed timestamp option length.
                         * Resulting "len" is MSS free of SACK jitter.
                         */
                        len -= tcp_sk(sk)->tcp_header_len;
                        icsk->icsk_ack.last_seg_size = len;
                        if (len == lss) {
                                icsk->icsk_ack.rcv_mss = len;
                                return;
                        }
                }
                if (icsk->icsk_ack.pending & ICSK_ACK_PUSHED)
                        icsk->icsk_ack.pending |= ICSK_ACK_PUSHED2;
                icsk->icsk_ack.pending |= ICSK_ACK_PUSHED;
        }
}

static void tcp_incr_quickack(struct sock *sk, unsigned int max_quickacks)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        unsigned int quickacks = tcp_sk(sk)->rcv_wnd / (2 * icsk->icsk_ack.rcv_mss);

        if (quickacks == 0)
                quickacks = 2;
        quickacks = min(quickacks, max_quickacks);
        if (quickacks > icsk->icsk_ack.quick)
                icsk->icsk_ack.quick = quickacks;
}

void tcp_enter_quickack_mode(struct sock *sk, unsigned int max_quickacks)
{
        struct inet_connection_sock *icsk = inet_csk(sk);

        tcp_incr_quickack(sk, max_quickacks);
        inet_csk_exit_pingpong_mode(sk);
        icsk->icsk_ack.ato = TCP_ATO_MIN;
}
EXPORT_SYMBOL(tcp_enter_quickack_mode);

/* Send ACKs quickly, if "quick" count is not exhausted
 * and the session is not interactive.
 */

static bool tcp_in_quickack_mode(struct sock *sk)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        const struct dst_entry *dst = __sk_dst_get(sk);

        return (dst && dst_metric(dst, RTAX_QUICKACK)) ||
                (icsk->icsk_ack.quick && !inet_csk_in_pingpong_mode(sk));
}

static void tcp_ecn_queue_cwr(struct tcp_sock *tp)
{
        if (tp->ecn_flags & TCP_ECN_OK)
                tp->ecn_flags |= TCP_ECN_QUEUE_CWR;
}

static void tcp_ecn_accept_cwr(struct sock *sk, const struct sk_buff *skb)
{
        if (tcp_hdr(skb)->cwr) {
                tcp_sk(sk)->ecn_flags &= ~TCP_ECN_DEMAND_CWR;

                /* If the sender is telling us it has entered CWR, then its
                 * cwnd may be very low (even just 1 packet), so we should ACK
                 * immediately.
                 */
                if (TCP_SKB_CB(skb)->seq != TCP_SKB_CB(skb)->end_seq)
                        inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
        }
}

static void tcp_ecn_withdraw_cwr(struct tcp_sock *tp)
{
        tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
}

static void __tcp_ecn_check_ce(struct sock *sk, const struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);

        switch (TCP_SKB_CB(skb)->ip_dsfield & INET_ECN_MASK) {
        case INET_ECN_NOT_ECT:
                /* Funny extension: if ECT is not set on a segment,
                 * and we already seen ECT on a previous segment,
                 * it is probably a retransmit.
                 */
                if (tp->ecn_flags & TCP_ECN_SEEN)
                        tcp_enter_quickack_mode(sk, 2);
                break;
        case INET_ECN_CE:
                if (tcp_ca_needs_ecn(sk))
                        tcp_ca_event(sk, CA_EVENT_ECN_IS_CE);

                if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR)) {
                        /* Better not delay acks, sender can have a very low cwnd */
                        tcp_enter_quickack_mode(sk, 2);
                        tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
                }
                tp->ecn_flags |= TCP_ECN_SEEN;
                break;
        default:
                if (tcp_ca_needs_ecn(sk))
                        tcp_ca_event(sk, CA_EVENT_ECN_NO_CE);
                tp->ecn_flags |= TCP_ECN_SEEN;
                break;
        }
}

static void tcp_ecn_check_ce(struct sock *sk, const struct sk_buff *skb)
{
        if (tcp_sk(sk)->ecn_flags & TCP_ECN_OK)
                __tcp_ecn_check_ce(sk, skb);
}

static void tcp_ecn_rcv_synack(struct tcp_sock *tp, const struct tcphdr *th)
{
        if ((tp->ecn_flags & TCP_ECN_OK) && (!th->ece || th->cwr))
                tp->ecn_flags &= ~TCP_ECN_OK;
}

static void tcp_ecn_rcv_syn(struct tcp_sock *tp, const struct tcphdr *th)
{
        if ((tp->ecn_flags & TCP_ECN_OK) && (!th->ece || !th->cwr))
                tp->ecn_flags &= ~TCP_ECN_OK;
}

static bool tcp_ecn_rcv_ecn_echo(const struct tcp_sock *tp, const struct tcphdr *th)
{
        if (th->ece && !th->syn && (tp->ecn_flags & TCP_ECN_OK))
                return true;
        return false;
}

/* Buffer size and advertised window tuning.
 *
 * 1. Tuning sk->sk_sndbuf, when connection enters established state.
 */

static void tcp_sndbuf_expand(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
        int sndmem, per_mss;
        u32 nr_segs;

        /* Worst case is non GSO/TSO : each frame consumes one skb
         * and skb->head is kmalloced using power of two area of memory
         */
        per_mss = max_t(u32, tp->rx_opt.mss_clamp, tp->mss_cache) +
                  MAX_TCP_HEADER +
                  SKB_DATA_ALIGN(sizeof(struct skb_shared_info));

        per_mss = roundup_pow_of_two(per_mss) +
                  SKB_DATA_ALIGN(sizeof(struct sk_buff));

        nr_segs = max_t(u32, TCP_INIT_CWND, tcp_snd_cwnd(tp));
        nr_segs = max_t(u32, nr_segs, tp->reordering + 1);

        /* Fast Recovery (RFC 5681 3.2) :
         * Cubic needs 1.7 factor, rounded to 2 to include
         * extra cushion (application might react slowly to EPOLLOUT)
         */
        sndmem = ca_ops->sndbuf_expand ? ca_ops->sndbuf_expand(sk) : 2;
        sndmem *= nr_segs * per_mss;

        if (sk->sk_sndbuf < sndmem)
                WRITE_ONCE(sk->sk_sndbuf,
                           min(sndmem, sock_net(sk)->ipv4.sysctl_tcp_wmem[2]));
}

/* 2. Tuning advertised window (window_clamp, rcv_ssthresh)
 *
 * All tcp_full_space() is split to two parts: "network" buffer, allocated
 * forward and advertised in receiver window (tp->rcv_wnd) and
 * "application buffer", required to isolate scheduling/application
 * latencies from network.
 * window_clamp is maximal advertised window. It can be less than
 * tcp_full_space(), in this case tcp_full_space() - window_clamp
 * is reserved for "application" buffer. The less window_clamp is
 * the smoother our behaviour from viewpoint of network, but the lower
 * throughput and the higher sensitivity of the connection to losses. 8)
 *
 * rcv_ssthresh is more strict window_clamp used at "slow start"
 * phase to predict further behaviour of this connection.
 * It is used for two goals:
 * - to enforce header prediction at sender, even when application
 *   requires some significant "application buffer". It is check #1.
 * - to prevent pruning of receive queue because of misprediction
 *   of receiver window. Check #2.
 *
 * The scheme does not work when sender sends good segments opening
 * window and then starts to feed us spaghetti. But it should work
 * in common situations. Otherwise, we have to rely on queue collapsing.
 */

/* Slow part of check#2. */
static int __tcp_grow_window(const struct sock *sk, const struct sk_buff *skb,
                             unsigned int skbtruesize)
{
        struct tcp_sock *tp = tcp_sk(sk);
        /* Optimize this! */
        int truesize = tcp_win_from_space(sk, skbtruesize) >> 1;
        int window = tcp_win_from_space(sk, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]) >> 1;

        while (tp->rcv_ssthresh <= window) {
                if (truesize <= skb->len)
                        return 2 * inet_csk(sk)->icsk_ack.rcv_mss;

                truesize >>= 1;
                window >>= 1;
        }
        return 0;
}

/* Even if skb appears to have a bad len/truesize ratio, TCP coalescing
 * can play nice with us, as sk_buff and skb->head might be either
 * freed or shared with up to MAX_SKB_FRAGS segments.
 * Only give a boost to drivers using page frag(s) to hold the frame(s),
 * and if no payload was pulled in skb->head before reaching us.
 */
static u32 truesize_adjust(bool adjust, const struct sk_buff *skb)
{
        u32 truesize = skb->truesize;

        if (adjust && !skb_headlen(skb)) {
                truesize -= SKB_TRUESIZE(skb_end_offset(skb));
                /* paranoid check, some drivers might be buggy */
                if (unlikely((int)truesize < (int)skb->len))
                        truesize = skb->truesize;
        }
        return truesize;
}

static void tcp_grow_window(struct sock *sk, const struct sk_buff *skb,
                            bool adjust)
{
        struct tcp_sock *tp = tcp_sk(sk);
        int room;

        room = min_t(int, tp->window_clamp, tcp_space(sk)) - tp->rcv_ssthresh;

        if (room <= 0)
                return;

        /* Check #1 */
        if (!tcp_under_memory_pressure(sk)) {
                unsigned int truesize = truesize_adjust(adjust, skb);
                int incr;

                /* Check #2. Increase window, if skb with such overhead
                 * will fit to rcvbuf in future.
                 */
                if (tcp_win_from_space(sk, truesize) <= skb->len)
                        incr = 2 * tp->advmss;
                else
                        incr = __tcp_grow_window(sk, skb, truesize);

                if (incr) {
                        incr = max_t(int, incr, 2 * skb->len);
                        tp->rcv_ssthresh += min(room, incr);
                        inet_csk(sk)->icsk_ack.quick |= 1;
                }
        } else {
                /* Under pressure:
                 * Adjust rcv_ssthresh according to reserved mem
                 */
                tcp_adjust_rcv_ssthresh(sk);
        }
}

/* 3. Try to fixup all. It is made immediately after connection enters
 *    established state.
 */
static void tcp_init_buffer_space(struct sock *sk)
{
        int tcp_app_win = sock_net(sk)->ipv4.sysctl_tcp_app_win;
        struct tcp_sock *tp = tcp_sk(sk);
        int maxwin;

        if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))
                tcp_sndbuf_expand(sk);

        tcp_mstamp_refresh(tp);
        tp->rcvq_space.time = tp->tcp_mstamp;
        tp->rcvq_space.seq = tp->copied_seq;

        maxwin = tcp_full_space(sk);

        if (tp->window_clamp >= maxwin) {
                tp->window_clamp = maxwin;

                if (tcp_app_win && maxwin > 4 * tp->advmss)
                        tp->window_clamp = max(maxwin -
                                               (maxwin >> tcp_app_win),
                                               4 * tp->advmss);
        }

        /* Force reservation of one segment. */
        if (tcp_app_win &&
            tp->window_clamp > 2 * tp->advmss &&
            tp->window_clamp + tp->advmss > maxwin)
                tp->window_clamp = max(2 * tp->advmss, maxwin - tp->advmss);

        tp->rcv_ssthresh = min(tp->rcv_ssthresh, tp->window_clamp);
        tp->snd_cwnd_stamp = tcp_jiffies32;
        tp->rcvq_space.space = min3(tp->rcv_ssthresh, tp->rcv_wnd,
                                    (u32)TCP_INIT_CWND * tp->advmss);
}

/* 4. Recalculate window clamp after socket hit its memory bounds. */
static void tcp_clamp_window(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct net *net = sock_net(sk);

        icsk->icsk_ack.quick = 0;

        if (sk->sk_rcvbuf < net->ipv4.sysctl_tcp_rmem[2] &&
            !(sk->sk_userlocks & SOCK_RCVBUF_LOCK) &&
            !tcp_under_memory_pressure(sk) &&
            sk_memory_allocated(sk) < sk_prot_mem_limits(sk, 0)) {
                WRITE_ONCE(sk->sk_rcvbuf,
                           min(atomic_read(&sk->sk_rmem_alloc),
                               net->ipv4.sysctl_tcp_rmem[2]));
        }
        if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf)
                tp->rcv_ssthresh = min(tp->window_clamp, 2U * tp->advmss);
}

/* Initialize RCV_MSS value.
 * RCV_MSS is an our guess about MSS used by the peer.
 * We haven't any direct information about the MSS.
 * It's better to underestimate the RCV_MSS rather than overestimate.
 * Overestimations make us ACKing less frequently than needed.
 * Underestimations are more easy to detect and fix by tcp_measure_rcv_mss().
 */
void tcp_initialize_rcv_mss(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        unsigned int hint = min_t(unsigned int, tp->advmss, tp->mss_cache);

        hint = min(hint, tp->rcv_wnd / 2);
        hint = min(hint, TCP_MSS_DEFAULT);
        hint = max(hint, TCP_MIN_MSS);

        inet_csk(sk)->icsk_ack.rcv_mss = hint;
}
EXPORT_SYMBOL(tcp_initialize_rcv_mss);

/* Receiver "autotuning" code.
 *
 * The algorithm for RTT estimation w/o timestamps is based on
 * Dynamic Right-Sizing (DRS) by Wu Feng and Mike Fisk of LANL.
 * <https://public.lanl.gov/radiant/pubs.html#DRS>
 *
 * More detail on this code can be found at
 * <http://staff.psc.edu/jheffner/>,
 * though this reference is out of date.  A new paper
 * is pending.
 */
static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
{
        u32 new_sample = tp->rcv_rtt_est.rtt_us;
        long m = sample;

        if (new_sample != 0) {
                /* If we sample in larger samples in the non-timestamp
                 * case, we could grossly overestimate the RTT especially
                 * with chatty applications or bulk transfer apps which
                 * are stalled on filesystem I/O.
                 *
                 * Also, since we are only going for a minimum in the
                 * non-timestamp case, we do not smooth things out
                 * else with timestamps disabled convergence takes too
                 * long.
                 */
                if (!win_dep) {
                        m -= (new_sample >> 3);
                        new_sample += m;
                } else {
                        m <<= 3;
                        if (m < new_sample)
                                new_sample = m;
                }
        } else {
                /* No previous measure. */
                new_sample = m << 3;
        }

        tp->rcv_rtt_est.rtt_us = new_sample;
}

static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
{
        u32 delta_us;

        if (tp->rcv_rtt_est.time == 0)
                goto new_measure;
        if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
                return;
        delta_us = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcv_rtt_est.time);
        if (!delta_us)
                delta_us = 1;
        tcp_rcv_rtt_update(tp, delta_us, 1);

new_measure:
        tp->rcv_rtt_est.seq = tp->rcv_nxt + tp->rcv_wnd;
        tp->rcv_rtt_est.time = tp->tcp_mstamp;
}

static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
                                          const struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tp->rx_opt.rcv_tsecr == tp->rcv_rtt_last_tsecr)
                return;
        tp->rcv_rtt_last_tsecr = tp->rx_opt.rcv_tsecr;

        if (TCP_SKB_CB(skb)->end_seq -
            TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss) {
                u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;
                u32 delta_us;

                if (likely(delta < INT_MAX / (USEC_PER_SEC / TCP_TS_HZ))) {
                        if (!delta)
                                delta = 1;
                        delta_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
                        tcp_rcv_rtt_update(tp, delta_us, 0);
                }
        }
}

/*
 * This function should be called every time data is copied to user space.
 * It calculates the appropriate TCP receive buffer space.
 */
void tcp_rcv_space_adjust(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 copied;
        int time;

        trace_tcp_rcv_space_adjust(sk);

        tcp_mstamp_refresh(tp);
        time = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcvq_space.time);
        if (time < (tp->rcv_rtt_est.rtt_us >> 3) || tp->rcv_rtt_est.rtt_us == 0)
                return;

        /* Number of bytes copied to user in last RTT */
        copied = tp->copied_seq - tp->rcvq_space.seq;
        if (copied <= tp->rcvq_space.space)
                goto new_measure;

        /* A bit of theory :
         * copied = bytes received in previous RTT, our base window
         * To cope with packet losses, we need a 2x factor
         * To cope with slow start, and sender growing its cwin by 100 %
         * every RTT, we need a 4x factor, because the ACK we are sending
         * now is for the next RTT, not the current one :
         * <prev RTT . ><current RTT .. ><next RTT .... >
         */

        if (sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf &&
            !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
                int rcvmem, rcvbuf;
                u64 rcvwin, grow;

                /* minimal window to cope with packet losses, assuming
                 * steady state. Add some cushion because of small variations.
                 */
                rcvwin = ((u64)copied << 1) + 16 * tp->advmss;

                /* Accommodate for sender rate increase (eg. slow start) */
                grow = rcvwin * (copied - tp->rcvq_space.space);
                do_div(grow, tp->rcvq_space.space);
                rcvwin += (grow << 1);

                rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER);
                while (tcp_win_from_space(sk, rcvmem) < tp->advmss)
                        rcvmem += 128;

                do_div(rcvwin, tp->advmss);
                rcvbuf = min_t(u64, rcvwin * rcvmem,
                               sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);
                if (rcvbuf > sk->sk_rcvbuf) {
                        WRITE_ONCE(sk->sk_rcvbuf, rcvbuf);

                        /* Make the window clamp follow along.  */
                        tp->window_clamp = tcp_win_from_space(sk, rcvbuf);
                }
        }
        tp->rcvq_space.space = copied;

new_measure:
        tp->rcvq_space.seq = tp->copied_seq;
        tp->rcvq_space.time = tp->tcp_mstamp;
}

/* There is something which you must keep in mind when you analyze the
 * behavior of the tp->ato delayed ack timeout interval.  When a
 * connection starts up, we want to ack as quickly as possible.  The
 * problem is that "good" TCP's do slow start at the beginning of data
 * transmission.  The means that until we send the first few ACK's the
 * sender will sit on his end and only queue most of his data, because
 * he can only send snd_cwnd unacked packets at any given time.  For
 * each ACK we send, he increments snd_cwnd and transmits more of his
 * queue.  -DaveM
 */
static void tcp_event_data_recv(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);
        u32 now;

        inet_csk_schedule_ack(sk);

        tcp_measure_rcv_mss(sk, skb);

        tcp_rcv_rtt_measure(tp);

        now = tcp_jiffies32;

        if (!icsk->icsk_ack.ato) {
                /* The _first_ data packet received, initialize
                 * delayed ACK engine.
                 */
                tcp_incr_quickack(sk, TCP_MAX_QUICKACKS);
                icsk->icsk_ack.ato = TCP_ATO_MIN;
        } else {
                int m = now - icsk->icsk_ack.lrcvtime;

                if (m <= TCP_ATO_MIN / 2) {
                        /* The fastest case is the first. */
                        icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + TCP_ATO_MIN / 2;
                } else if (m < icsk->icsk_ack.ato) {
                        icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + m;
                        if (icsk->icsk_ack.ato > icsk->icsk_rto)
                                icsk->icsk_ack.ato = icsk->icsk_rto;
                } else if (m > icsk->icsk_rto) {
                        /* Too long gap. Apparently sender failed to
                         * restart window, so that we send ACKs quickly.
                         */
                        tcp_incr_quickack(sk, TCP_MAX_QUICKACKS);
                }
        }
        icsk->icsk_ack.lrcvtime = now;

        tcp_ecn_check_ce(sk, skb);

        if (skb->len >= 128)
                tcp_grow_window(sk, skb, true);
}

/* Called to compute a smoothed rtt estimate. The data fed to this
 * routine either comes from timestamps, or from segments that were
 * known _not_ to have been retransmitted [see Karn/Partridge
 * Proceedings SIGCOMM 87]. The algorithm is from the SIGCOMM 88
 * piece by Van Jacobson.
 * NOTE: the next three routines used to be one big routine.
 * To save cycles in the RFC 1323 implementation it was better to break
 * it up into three procedures. -- erics
 */
static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
{
        struct tcp_sock *tp = tcp_sk(sk);
        long m = mrtt_us; /* RTT */
        u32 srtt = tp->srtt_us;

        /*      The following amusing code comes from Jacobson's
         *      article in SIGCOMM '88.  Note that rtt and mdev
         *      are scaled versions of rtt and mean deviation.
         *      This is designed to be as fast as possible
         *      m stands for "measurement".
         *
         *      On a 1990 paper the rto value is changed to:
         *      RTO = rtt + 4 * mdev
         *
         * Funny. This algorithm seems to be very broken.
         * These formulae increase RTO, when it should be decreased, increase
         * too slowly, when it should be increased quickly, decrease too quickly
         * etc. I guess in BSD RTO takes ONE value, so that it is absolutely
         * does not matter how to _calculate_ it. Seems, it was trap
         * that VJ failed to avoid. 8)
         */
        if (srtt != 0) {
                m -= (srtt >> 3);       /* m is now error in rtt est */
                srtt += m;              /* rtt = 7/8 rtt + 1/8 new */
                if (m < 0) {
                        m = -m;         /* m is now abs(error) */
                        m -= (tp->mdev_us >> 2);   /* similar update on mdev */
                        /* This is similar to one of Eifel findings.
                         * Eifel blocks mdev updates when rtt decreases.
                         * This solution is a bit different: we use finer gain
                         * for mdev in this case (alpha*beta).
                         * Like Eifel it also prevents growth of rto,
                         * but also it limits too fast rto decreases,
                         * happening in pure Eifel.
                         */
                        if (m > 0)
                                m >>= 3;
                } else {
                        m -= (tp->mdev_us >> 2);   /* similar update on mdev */
                }
                tp->mdev_us += m;               /* mdev = 3/4 mdev + 1/4 new */
                if (tp->mdev_us > tp->mdev_max_us) {
                        tp->mdev_max_us = tp->mdev_us;
                        if (tp->mdev_max_us > tp->rttvar_us)
                                tp->rttvar_us = tp->mdev_max_us;
                }
                if (after(tp->snd_una, tp->rtt_seq)) {
                        if (tp->mdev_max_us < tp->rttvar_us)
                                tp->rttvar_us -= (tp->rttvar_us - tp->mdev_max_us) >> 2;
                        tp->rtt_seq = tp->snd_nxt;
                        tp->mdev_max_us = tcp_rto_min_us(sk);

                        tcp_bpf_rtt(sk);
                }
        } else {
                /* no previous measure. */
                srtt = m << 3;          /* take the measured time to be rtt */
                tp->mdev_us = m << 1;   /* make sure rto = 3*rtt */
                tp->rttvar_us = max(tp->mdev_us, tcp_rto_min_us(sk));
                tp->mdev_max_us = tp->rttvar_us;
                tp->rtt_seq = tp->snd_nxt;

                tcp_bpf_rtt(sk);
        }
        tp->srtt_us = max(1U, srtt);
}

static void tcp_update_pacing_rate(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        u64 rate;

        /* set sk_pacing_rate to 200 % of current rate (mss * cwnd / srtt) */
        rate = (u64)tp->mss_cache * ((USEC_PER_SEC / 100) << 3);

        /* current rate is (cwnd * mss) / srtt
         * In Slow Start [1], set sk_pacing_rate to 200 % the current rate.
         * In Congestion Avoidance phase, set it to 120 % the current rate.
         *
         * [1] : Normal Slow Start condition is (tp->snd_cwnd < tp->snd_ssthresh)
         *       If snd_cwnd >= (tp->snd_ssthresh / 2), we are approaching
         *       end of slow start and should slow down.
         */
        if (tcp_snd_cwnd(tp) < tp->snd_ssthresh / 2)
                rate *= sock_net(sk)->ipv4.sysctl_tcp_pacing_ss_ratio;
        else
                rate *= sock_net(sk)->ipv4.sysctl_tcp_pacing_ca_ratio;

        rate *= max(tcp_snd_cwnd(tp), tp->packets_out);

        if (likely(tp->srtt_us))
                do_div(rate, tp->srtt_us);

        /* WRITE_ONCE() is needed because sch_fq fetches sk_pacing_rate
         * without any lock. We want to make sure compiler wont store
         * intermediate values in this location.
         */
        WRITE_ONCE(sk->sk_pacing_rate, min_t(u64, rate,
                                             sk->sk_max_pacing_rate));
}

/* Calculate rto without backoff.  This is the second half of Van Jacobson's
 * routine referred to above.
 */
static void tcp_set_rto(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        /* Old crap is replaced with new one. 8)
         *
         * More seriously:
         * 1. If rtt variance happened to be less 50msec, it is hallucination.
         *    It cannot be less due to utterly erratic ACK generation made
         *    at least by solaris and freebsd. "Erratic ACKs" has _nothing_
         *    to do with delayed acks, because at cwnd>2 true delack timeout
         *    is invisible. Actually, Linux-2.4 also generates erratic
         *    ACKs in some circumstances.
         */
        inet_csk(sk)->icsk_rto = __tcp_set_rto(tp);

        /* 2. Fixups made earlier cannot be right.
         *    If we do not estimate RTO correctly without them,
         *    all the algo is pure shit and should be replaced
         *    with correct one. It is exactly, which we pretend to do.
         */

        /* NOTE: clamping at TCP_RTO_MIN is not required, current algo
         * guarantees that rto is higher.
         */
        tcp_bound_rto(sk);
}

__u32 tcp_init_cwnd(const struct tcp_sock *tp, const struct dst_entry *dst)
{
        __u32 cwnd = (dst ? dst_metric(dst, RTAX_INITCWND) : 0);

        if (!cwnd)
                cwnd = TCP_INIT_CWND;
        return min_t(__u32, cwnd, tp->snd_cwnd_clamp);
}

struct tcp_sacktag_state {
        /* Timestamps for earliest and latest never-retransmitted segment
         * that was SACKed. RTO needs the earliest RTT to stay conservative,
         * but congestion control should still get an accurate delay signal.
         */
        u64     first_sackt;
        u64     last_sackt;
        u32     reord;
        u32     sack_delivered;
        int     flag;
        unsigned int mss_now;
        struct rate_sample *rate;
};

/* Take a notice that peer is sending D-SACKs. Skip update of data delivery
 * and spurious retransmission information if this DSACK is unlikely caused by
 * sender's action:
 * - DSACKed sequence range is larger than maximum receiver's window.
 * - Total no. of DSACKed segments exceed the total no. of retransmitted segs.
 */
static u32 tcp_dsack_seen(struct tcp_sock *tp, u32 start_seq,
                          u32 end_seq, struct tcp_sacktag_state *state)
{
        u32 seq_len, dup_segs = 1;

        if (!before(start_seq, end_seq))
                return 0;

        seq_len = end_seq - start_seq;
        /* Dubious DSACK: DSACKed range greater than maximum advertised rwnd */
        if (seq_len > tp->max_window)
                return 0;
        if (seq_len > tp->mss_cache)
                dup_segs = DIV_ROUND_UP(seq_len, tp->mss_cache);
        else if (tp->tlp_high_seq && tp->tlp_high_seq == end_seq)
                state->flag |= FLAG_DSACK_TLP;

        tp->dsack_dups += dup_segs;
        /* Skip the DSACK if dup segs weren't retransmitted by sender */
        if (tp->dsack_dups > tp->total_retrans)
                return 0;

        tp->rx_opt.sack_ok |= TCP_DSACK_SEEN;
        /* We increase the RACK ordering window in rounds where we receive
         * DSACKs that may have been due to reordering causing RACK to trigger
         * a spurious fast recovery. Thus RACK ignores DSACKs that happen
         * without having seen reordering, or that match TLP probes (TLP
         * is timer-driven, not triggered by RACK).
         */
        if (tp->reord_seen && !(state->flag & FLAG_DSACK_TLP))
                tp->rack.dsack_seen = 1;

        state->flag |= FLAG_DSACKING_ACK;
        /* A spurious retransmission is delivered */
        state->sack_delivered += dup_segs;

        return dup_segs;
}

/* It's reordering when higher sequence was delivered (i.e. sacked) before
 * some lower never-retransmitted sequence ("low_seq"). The maximum reordering
 * distance is approximated in full-mss packet distance ("reordering").
 */
static void tcp_check_sack_reordering(struct sock *sk, const u32 low_seq,
                                      const int ts)
{
        struct tcp_sock *tp = tcp_sk(sk);
        const u32 mss = tp->mss_cache;
        u32 fack, metric;

        fack = tcp_highest_sack_seq(tp);
        if (!before(low_seq, fack))
                return;

        metric = fack - low_seq;
        if ((metric > tp->reordering * mss) && mss) {
#if FASTRETRANS_DEBUG > 1
                pr_debug("Disorder%d %d %u f%u s%u rr%d\n",
                         tp->rx_opt.sack_ok, inet_csk(sk)->icsk_ca_state,
                         tp->reordering,
                         0,
                         tp->sacked_out,
                         tp->undo_marker ? tp->undo_retrans : 0);
#endif
                tp->reordering = min_t(u32, (metric + mss - 1) / mss,
                                       sock_net(sk)->ipv4.sysctl_tcp_max_reordering);
        }

        /* This exciting event is worth to be remembered. 8) */
        tp->reord_seen++;
        NET_INC_STATS(sock_net(sk),
                      ts ? LINUX_MIB_TCPTSREORDER : LINUX_MIB_TCPSACKREORDER);
}

 /* This must be called before lost_out or retrans_out are updated
  * on a new loss, because we want to know if all skbs previously
  * known to be lost have already been retransmitted, indicating
  * that this newly lost skb is our next skb to retransmit.
  */
static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
{
        if ((!tp->retransmit_skb_hint && tp->retrans_out >= tp->lost_out) ||
            (tp->retransmit_skb_hint &&
             before(TCP_SKB_CB(skb)->seq,
                    TCP_SKB_CB(tp->retransmit_skb_hint)->seq)))
                tp->retransmit_skb_hint = skb;
}

/* Sum the number of packets on the wire we have marked as lost, and
 * notify the congestion control module that the given skb was marked lost.
 */
static void tcp_notify_skb_loss_event(struct tcp_sock *tp, const struct sk_buff *skb)
{
        tp->lost += tcp_skb_pcount(skb);
}

void tcp_mark_skb_lost(struct sock *sk, struct sk_buff *skb)
{
        __u8 sacked = TCP_SKB_CB(skb)->sacked;
        struct tcp_sock *tp = tcp_sk(sk);

        if (sacked & TCPCB_SACKED_ACKED)
                return;

        tcp_verify_retransmit_hint(tp, skb);
        if (sacked & TCPCB_LOST) {
                if (sacked & TCPCB_SACKED_RETRANS) {
                        /* Account for retransmits that are lost again */
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                        tp->retrans_out -= tcp_skb_pcount(skb);
                        NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT,
                                      tcp_skb_pcount(skb));
                        tcp_notify_skb_loss_event(tp, skb);
                }
        } else {
                tp->lost_out += tcp_skb_pcount(skb);
                TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                tcp_notify_skb_loss_event(tp, skb);
        }
}

/* Updates the delivered and delivered_ce counts */
static void tcp_count_delivered(struct tcp_sock *tp, u32 delivered,
                                bool ece_ack)
{
        tp->delivered += delivered;
        if (ece_ack)
                tp->delivered_ce += delivered;
}

/* This procedure tags the retransmission queue when SACKs arrive.
 *
 * We have three tag bits: SACKED(S), RETRANS(R) and LOST(L).
 * Packets in queue with these bits set are counted in variables
 * sacked_out, retrans_out and lost_out, correspondingly.
 *
 * Valid combinations are:
 * Tag  InFlight        Description
 * 0    1               - orig segment is in flight.
 * S    0               - nothing flies, orig reached receiver.
 * L    0               - nothing flies, orig lost by net.
 * R    2               - both orig and retransmit are in flight.
 * L|R  1               - orig is lost, retransmit is in flight.
 * S|R  1               - orig reached receiver, retrans is still in flight.
 * (L|S|R is logically valid, it could occur when L|R is sacked,
 *  but it is equivalent to plain S and code short-curcuits it to S.
 *  L|S is logically invalid, it would mean -1 packet in flight 8))
 *
 * These 6 states form finite state machine, controlled by the following events:
 * 1. New ACK (+SACK) arrives. (tcp_sacktag_write_queue())
 * 2. Retransmission. (tcp_retransmit_skb(), tcp_xmit_retransmit_queue())
 * 3. Loss detection event of two flavors:
 *      A. Scoreboard estimator decided the packet is lost.
 *         A'. Reno "three dupacks" marks head of queue lost.
 *      B. SACK arrives sacking SND.NXT at the moment, when the
 *         segment was retransmitted.
 * 4. D-SACK added new rule: D-SACK changes any tag to S.
 *
 * It is pleasant to note, that state diagram turns out to be commutative,
 * so that we are allowed not to be bothered by order of our actions,
 * when multiple events arrive simultaneously. (see the function below).
 *
 * Reordering detection.
 * --------------------
 * Reordering metric is maximal distance, which a packet can be displaced
 * in packet stream. With SACKs we can estimate it:
 *
 * 1. SACK fills old hole and the corresponding segment was not
 *    ever retransmitted -> reordering. Alas, we cannot use it
 *    when segment was retransmitted.
 * 2. The last flaw is solved with D-SACK. D-SACK arrives
 *    for retransmitted and already SACKed segment -> reordering..
 * Both of these heuristics are not used in Loss state, when we cannot
 * account for retransmits accurately.
 *
 * SACK block validation.
 * ----------------------
 *
 * SACK block range validation checks that the received SACK block fits to
 * the expected sequence limits, i.e., it is between SND.UNA and SND.NXT.
 * Note that SND.UNA is not included to the range though being valid because
 * it means that the receiver is rather inconsistent with itself reporting
 * SACK reneging when it should advance SND.UNA. Such SACK block this is
 * perfectly valid, however, in light of RFC2018 which explicitly states
 * that "SACK block MUST reflect the newest segment.  Even if the newest
 * segment is going to be discarded ...", not that it looks very clever
 * in case of head skb. Due to potentional receiver driven attacks, we
 * choose to avoid immediate execution of a walk in write queue due to
 * reneging and defer head skb's loss recovery to standard loss recovery
 * procedure that will eventually trigger (nothing forbids us doing this).
 *
 * Implements also blockage to start_seq wrap-around. Problem lies in the
 * fact that though start_seq (s) is before end_seq (i.e., not reversed),
 * there's no guarantee that it will be before snd_nxt (n). The problem
 * happens when start_seq resides between end_seq wrap (e_w) and snd_nxt
 * wrap (s_w):
 *
 *         <- outs wnd ->                          <- wrapzone ->
 *         u     e      n                         u_w   e_w  s n_w
 *         |     |      |                          |     |   |  |
 * |<------------+------+----- TCP seqno space --------------+---------->|
 * ...-- <2^31 ->|                                           |<--------...
 * ...---- >2^31 ------>|                                    |<--------...
 *
 * Current code wouldn't be vulnerable but it's better still to discard such
 * crazy SACK blocks. Doing this check for start_seq alone closes somewhat
 * similar case (end_seq after snd_nxt wrap) as earlier reversed check in
 * snd_nxt wrap -> snd_una region will then become "well defined", i.e.,
 * equal to the ideal case (infinite seqno space without wrap caused issues).
 *
 * With D-SACK the lower bound is extended to cover sequence space below
 * SND.UNA down to undo_marker, which is the last point of interest. Yet
 * again, D-SACK block must not to go across snd_una (for the same reason as
 * for the normal SACK blocks, explained above). But there all simplicity
 * ends, TCP might receive valid D-SACKs below that. As long as they reside
 * fully below undo_marker they do not affect behavior in anyway and can
 * therefore be safely ignored. In rare cases (which are more or less
 * theoretical ones), the D-SACK will nicely cross that boundary due to skb
 * fragmentation and packet reordering past skb's retransmission. To consider
 * them correctly, the acceptable range must be extended even more though
 * the exact amount is rather hard to quantify. However, tp->max_window can
 * be used as an exaggerated estimate.
 */
static bool tcp_is_sackblock_valid(struct tcp_sock *tp, bool is_dsack,
                                   u32 start_seq, u32 end_seq)
{
        /* Too far in future, or reversed (interpretation is ambiguous) */
        if (after(end_seq, tp->snd_nxt) || !before(start_seq, end_seq))
                return false;

        /* Nasty start_seq wrap-around check (see comments above) */
        if (!before(start_seq, tp->snd_nxt))
                return false;

        /* In outstanding window? ...This is valid exit for D-SACKs too.
         * start_seq == snd_una is non-sensical (see comments above)
         */
        if (after(start_seq, tp->snd_una))
                return true;

        if (!is_dsack || !tp->undo_marker)
                return false;

        /* ...Then it's D-SACK, and must reside below snd_una completely */
        if (after(end_seq, tp->snd_una))
                return false;

        if (!before(start_seq, tp->undo_marker))
                return true;

        /* Too old */
        if (!after(end_seq, tp->undo_marker))
                return false;

        /* Undo_marker boundary crossing (overestimates a lot). Known already:
         *   start_seq < undo_marker and end_seq >= undo_marker.
         */
        return !before(start_seq, end_seq - tp->max_window);
}

static bool tcp_check_dsack(struct sock *sk, const struct sk_buff *ack_skb,
                            struct tcp_sack_block_wire *sp, int num_sacks,
                            u32 prior_snd_una, struct tcp_sacktag_state *state)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 start_seq_0 = get_unaligned_be32(&sp[0].start_seq);
        u32 end_seq_0 = get_unaligned_be32(&sp[0].end_seq);
        u32 dup_segs;

        if (before(start_seq_0, TCP_SKB_CB(ack_skb)->ack_seq)) {
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKRECV);
        } else if (num_sacks > 1) {
                u32 end_seq_1 = get_unaligned_be32(&sp[1].end_seq);
                u32 start_seq_1 = get_unaligned_be32(&sp[1].start_seq);

                if (after(end_seq_0, end_seq_1) || before(start_seq_0, start_seq_1))
                        return false;
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKOFORECV);
        } else {
                return false;
        }

        dup_segs = tcp_dsack_seen(tp, start_seq_0, end_seq_0, state);
        if (!dup_segs) {        /* Skip dubious DSACK */
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKIGNOREDDUBIOUS);
                return false;
        }

        NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPDSACKRECVSEGS, dup_segs);

        /* D-SACK for already forgotten data... Do dumb counting. */
        if (tp->undo_marker && tp->undo_retrans > 0 &&
            !after(end_seq_0, prior_snd_una) &&
            after(end_seq_0, tp->undo_marker))
                tp->undo_retrans = max_t(int, 0, tp->undo_retrans - dup_segs);

        return true;
}

/* Check if skb is fully within the SACK block. In presence of GSO skbs,
 * the incoming SACK may not exactly match but we can find smaller MSS
 * aligned portion of it that matches. Therefore we might need to fragment
 * which may fail and creates some hassle (caller must handle error case
 * returns).
 *
 * FIXME: this could be merged to shift decision code
 */
static int tcp_match_skb_to_sack(struct sock *sk, struct sk_buff *skb,
                                  u32 start_seq, u32 end_seq)
{
        int err;
        bool in_sack;
        unsigned int pkt_len;
        unsigned int mss;

        in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
                  !before(end_seq, TCP_SKB_CB(skb)->end_seq);

        if (tcp_skb_pcount(skb) > 1 && !in_sack &&
            after(TCP_SKB_CB(skb)->end_seq, start_seq)) {
                mss = tcp_skb_mss(skb);
                in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);

                if (!in_sack) {
                        pkt_len = start_seq - TCP_SKB_CB(skb)->seq;
                        if (pkt_len < mss)
                                pkt_len = mss;
                } else {
                        pkt_len = end_seq - TCP_SKB_CB(skb)->seq;
                        if (pkt_len < mss)
                                return -EINVAL;
                }

                /* Round if necessary so that SACKs cover only full MSSes
                 * and/or the remaining small portion (if present)
                 */
                if (pkt_len > mss) {
                        unsigned int new_len = (pkt_len / mss) * mss;
                        if (!in_sack && new_len < pkt_len)
                                new_len += mss;
                        pkt_len = new_len;
                }

                if (pkt_len >= skb->len && !in_sack)
                        return 0;

                err = tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
                                   pkt_len, mss, GFP_ATOMIC);
                if (err < 0)
                        return err;
        }

        return in_sack;
}

/* Mark the given newly-SACKed range as such, adjusting counters and hints. */
static u8 tcp_sacktag_one(struct sock *sk,
                          struct tcp_sacktag_state *state, u8 sacked,
                          u32 start_seq, u32 end_seq,
                          int dup_sack, int pcount,
                          u64 xmit_time)
{
        struct tcp_sock *tp = tcp_sk(sk);

        /* Account D-SACK for retransmitted packet. */
        if (dup_sack && (sacked & TCPCB_RETRANS)) {
                if (tp->undo_marker && tp->undo_retrans > 0 &&
                    after(end_seq, tp->undo_marker))
                        tp->undo_retrans = max_t(int, 0, tp->undo_retrans - pcount);
                if ((sacked & TCPCB_SACKED_ACKED) &&
                    before(start_seq, state->reord))
                                state->reord = start_seq;
        }

        /* Nothing to do; acked frame is about to be dropped (was ACKed). */
        if (!after(end_seq, tp->snd_una))
                return sacked;

        if (!(sacked & TCPCB_SACKED_ACKED)) {
                tcp_rack_advance(tp, sacked, end_seq, xmit_time);

                if (sacked & TCPCB_SACKED_RETRANS) {
                        /* If the segment is not tagged as lost,
                         * we do not clear RETRANS, believing
                         * that retransmission is still in flight.
                         */
                        if (sacked & TCPCB_LOST) {
                                sacked &= ~(TCPCB_LOST|TCPCB_SACKED_RETRANS);
                                tp->lost_out -= pcount;
                                tp->retrans_out -= pcount;
                        }
                } else {
                        if (!(sacked & TCPCB_RETRANS)) {
                                /* New sack for not retransmitted frame,
                                 * which was in hole. It is reordering.
                                 */
                                if (before(start_seq,
                                           tcp_highest_sack_seq(tp)) &&
                                    before(start_seq, state->reord))
                                        state->reord = start_seq;

                                if (!after(end_seq, tp->high_seq))
                                        state->flag |= FLAG_ORIG_SACK_ACKED;
                                if (state->first_sackt == 0)
                                        state->first_sackt = xmit_time;
                                state->last_sackt = xmit_time;
                        }

                        if (sacked & TCPCB_LOST) {
                                sacked &= ~TCPCB_LOST;
                                tp->lost_out -= pcount;
                        }
                }

                sacked |= TCPCB_SACKED_ACKED;
                state->flag |= FLAG_DATA_SACKED;
                tp->sacked_out += pcount;
                /* Out-of-order packets delivered */
                state->sack_delivered += pcount;

                /* Lost marker hint past SACKed? Tweak RFC3517 cnt */
                if (tp->lost_skb_hint &&
                    before(start_seq, TCP_SKB_CB(tp->lost_skb_hint)->seq))
                        tp->lost_cnt_hint += pcount;
        }

        /* D-SACK. We can detect redundant retransmission in S|R and plain R
         * frames and clear it. undo_retrans is decreased above, L|R frames
         * are accounted above as well.
         */
        if (dup_sack && (sacked & TCPCB_SACKED_RETRANS)) {
                sacked &= ~TCPCB_SACKED_RETRANS;
                tp->retrans_out -= pcount;
        }

        return sacked;
}

/* Shift newly-SACKed bytes from this skb to the immediately previous
 * already-SACKed sk_buff. Mark the newly-SACKed bytes as such.
 */
static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *prev,
                            struct sk_buff *skb,
                            struct tcp_sacktag_state *state,
                            unsigned int pcount, int shifted, int mss,
                            bool dup_sack)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 start_seq = TCP_SKB_CB(skb)->seq;   /* start of newly-SACKed */
        u32 end_seq = start_seq + shifted;      /* end of newly-SACKed */

        BUG_ON(!pcount);

        /* Adjust counters and hints for the newly sacked sequence
         * range but discard the return value since prev is already
         * marked. We must tag the range first because the seq
         * advancement below implicitly advances
         * tcp_highest_sack_seq() when skb is highest_sack.
         */
        tcp_sacktag_one(sk, state, TCP_SKB_CB(skb)->sacked,
                        start_seq, end_seq, dup_sack, pcount,
                        tcp_skb_timestamp_us(skb));
        tcp_rate_skb_delivered(sk, skb, state->rate);

        if (skb == tp->lost_skb_hint)
                tp->lost_cnt_hint += pcount;

        TCP_SKB_CB(prev)->end_seq += shifted;
        TCP_SKB_CB(skb)->seq += shifted;

        tcp_skb_pcount_add(prev, pcount);
        WARN_ON_ONCE(tcp_skb_pcount(skb) < pcount);
        tcp_skb_pcount_add(skb, -pcount);

        /* When we're adding to gso_segs == 1, gso_size will be zero,
         * in theory this shouldn't be necessary but as long as DSACK
         * code can come after this skb later on it's better to keep
         * setting gso_size to something.
         */
        if (!TCP_SKB_CB(prev)->tcp_gso_size)
                TCP_SKB_CB(prev)->tcp_gso_size = mss;

        /* CHECKME: To clear or not to clear? Mimics normal skb currently */
        if (tcp_skb_pcount(skb) <= 1)
                TCP_SKB_CB(skb)->tcp_gso_size = 0;

        /* Difference in this won't matter, both ACKed by the same cumul. ACK */
        TCP_SKB_CB(prev)->sacked |= (TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS);

        if (skb->len > 0) {
                BUG_ON(!tcp_skb_pcount(skb));
                NET_INC_STATS(sock_net(sk), LINUX_MIB_SACKSHIFTED);
                return false;
        }

        /* Whole SKB was eaten :-) */

        if (skb == tp->retransmit_skb_hint)
                tp->retransmit_skb_hint = prev;
        if (skb == tp->lost_skb_hint) {
                tp->lost_skb_hint = prev;
                tp->lost_cnt_hint -= tcp_skb_pcount(prev);
        }

        TCP_SKB_CB(prev)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
        TCP_SKB_CB(prev)->eor = TCP_SKB_CB(skb)->eor;
        if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
                TCP_SKB_CB(prev)->end_seq++;

        if (skb == tcp_highest_sack(sk))
                tcp_advance_highest_sack(sk, skb);

        tcp_skb_collapse_tstamp(prev, skb);
        if (unlikely(TCP_SKB_CB(prev)->tx.delivered_mstamp))
                TCP_SKB_CB(prev)->tx.delivered_mstamp = 0;

        tcp_rtx_queue_unlink_and_free(skb, sk);

        NET_INC_STATS(sock_net(sk), LINUX_MIB_SACKMERGED);

        return true;
}

/* I wish gso_size would have a bit more sane initialization than
 * something-or-zero which complicates things
 */
static int tcp_skb_seglen(const struct sk_buff *skb)
{
        return tcp_skb_pcount(skb) == 1 ? skb->len : tcp_skb_mss(skb);
}

/* Shifting pages past head area doesn't work */
static int skb_can_shift(const struct sk_buff *skb)
{
        return !skb_headlen(skb) && skb_is_nonlinear(skb);
}

int tcp_skb_shift(struct sk_buff *to, struct sk_buff *from,
                  int pcount, int shiftlen)
{
        /* TCP min gso_size is 8 bytes (TCP_MIN_GSO_SIZE)
         * Since TCP_SKB_CB(skb)->tcp_gso_segs is 16 bits, we need
         * to make sure not storing more than 65535 * 8 bytes per skb,
         * even if current MSS is bigger.
         */
        if (unlikely(to->len + shiftlen >= 65535 * TCP_MIN_GSO_SIZE))
                return 0;
        if (unlikely(tcp_skb_pcount(to) + pcount > 65535))
                return 0;
        return skb_shift(to, from, shiftlen);
}

/* Try collapsing SACK blocks spanning across multiple skbs to a single
 * skb.
 */
static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
                                          struct tcp_sacktag_state *state,
                                          u32 start_seq, u32 end_seq,
                                          bool dup_sack)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *prev;
        int mss;
        int pcount = 0;
        int len;
        int in_sack;

        /* Normally R but no L won't result in plain S */
        if (!dup_sack &&
            (TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_RETRANS)) == TCPCB_SACKED_RETRANS)
                goto fallback;
        if (!skb_can_shift(skb))
                goto fallback;
        /* This frame is about to be dropped (was ACKed). */
        if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
                goto fallback;

        /* Can only happen with delayed DSACK + discard craziness */
        prev = skb_rb_prev(skb);
        if (!prev)
                goto fallback;

        if ((TCP_SKB_CB(prev)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED)
                goto fallback;

        if (!tcp_skb_can_collapse(prev, skb))
                goto fallback;

        in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
                  !before(end_seq, TCP_SKB_CB(skb)->end_seq);

        if (in_sack) {
                len = skb->len;
                pcount = tcp_skb_pcount(skb);
                mss = tcp_skb_seglen(skb);

                /* TODO: Fix DSACKs to not fragment already SACKed and we can
                 * drop this restriction as unnecessary
                 */
                if (mss != tcp_skb_seglen(prev))
                        goto fallback;
        } else {
                if (!after(TCP_SKB_CB(skb)->end_seq, start_seq))
                        goto noop;
                /* CHECKME: This is non-MSS split case only?, this will
                 * cause skipped skbs due to advancing loop btw, original
                 * has that feature too
                 */
                if (tcp_skb_pcount(skb) <= 1)
                        goto noop;

                in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);
                if (!in_sack) {
                        /* TODO: head merge to next could be attempted here
                         * if (!after(TCP_SKB_CB(skb)->end_seq, end_seq)),
                         * though it might not be worth of the additional hassle
                         *
                         * ...we can probably just fallback to what was done
                         * previously. We could try merging non-SACKed ones
                         * as well but it probably isn't going to buy off
                         * because later SACKs might again split them, and
                         * it would make skb timestamp tracking considerably
                         * harder problem.
                         */
                        goto fallback;
                }

                len = end_seq - TCP_SKB_CB(skb)->seq;
                BUG_ON(len < 0);
                BUG_ON(len > skb->len);

                /* MSS boundaries should be honoured or else pcount will
                 * severely break even though it makes things bit trickier.
                 * Optimize common case to avoid most of the divides
                 */
                mss = tcp_skb_mss(skb);

                /* TODO: Fix DSACKs to not fragment already SACKed and we can
                 * drop this restriction as unnecessary
                 */
                if (mss != tcp_skb_seglen(prev))
                        goto fallback;

                if (len == mss) {
                        pcount = 1;
                } else if (len < mss) {
                        goto noop;
                } else {
                        pcount = len / mss;
                        len = pcount * mss;
                }
        }

        /* tcp_sacktag_one() won't SACK-tag ranges below snd_una */
        if (!after(TCP_SKB_CB(skb)->seq + len, tp->snd_una))
                goto fallback;

        if (!tcp_skb_shift(prev, skb, pcount, len))
                goto fallback;
        if (!tcp_shifted_skb(sk, prev, skb, state, pcount, len, mss, dup_sack))
                goto out;

        /* Hole filled allows collapsing with the next as well, this is very
         * useful when hole on every nth skb pattern happens
         */
        skb = skb_rb_next(prev);
        if (!skb)
                goto out;

        if (!skb_can_shift(skb) ||
            ((TCP_SKB_CB(skb)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED) ||
            (mss != tcp_skb_seglen(skb)))
                goto out;

        if (!tcp_skb_can_collapse(prev, skb))
                goto out;
        len = skb->len;
        pcount = tcp_skb_pcount(skb);
        if (tcp_skb_shift(prev, skb, pcount, len))
                tcp_shifted_skb(sk, prev, skb, state, pcount,
                                len, mss, 0);

out:
        return prev;

noop:
        return skb;

fallback:
        NET_INC_STATS(sock_net(sk), LINUX_MIB_SACKSHIFTFALLBACK);
        return NULL;
}

static struct sk_buff *tcp_sacktag_walk(struct sk_buff *skb, struct sock *sk,
                                        struct tcp_sack_block *next_dup,
                                        struct tcp_sacktag_state *state,
                                        u32 start_seq, u32 end_seq,
                                        bool dup_sack_in)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *tmp;

        skb_rbtree_walk_from(skb) {
                int in_sack = 0;
                bool dup_sack = dup_sack_in;

                /* queue is in-order => we can short-circuit the walk early */
                if (!before(TCP_SKB_CB(skb)->seq, end_seq))
                        break;

                if (next_dup  &&
                    before(TCP_SKB_CB(skb)->seq, next_dup->end_seq)) {
                        in_sack = tcp_match_skb_to_sack(sk, skb,
                                                        next_dup->start_seq,
                                                        next_dup->end_seq);
                        if (in_sack > 0)
                                dup_sack = true;
                }

                /* skb reference here is a bit tricky to get right, since
                 * shifting can eat and free both this skb and the next,
                 * so not even _safe variant of the loop is enough.
                 */
                if (in_sack <= 0) {
                        tmp = tcp_shift_skb_data(sk, skb, state,
                                                 start_seq, end_seq, dup_sack);
                        if (tmp) {
                                if (tmp != skb) {
                                        skb = tmp;
                                        continue;
                                }

                                in_sack = 0;
                        } else {
                                in_sack = tcp_match_skb_to_sack(sk, skb,
                                                                start_seq,
                                                                end_seq);
                        }
                }

                if (unlikely(in_sack < 0))
                        break;

                if (in_sack) {
                        TCP_SKB_CB(skb)->sacked =
                                tcp_sacktag_one(sk,
                                                state,
                                                TCP_SKB_CB(skb)->sacked,
                                                TCP_SKB_CB(skb)->seq,
                                                TCP_SKB_CB(skb)->end_seq,
                                                dup_sack,
                                                tcp_skb_pcount(skb),
                                                tcp_skb_timestamp_us(skb));
                        tcp_rate_skb_delivered(sk, skb, state->rate);
                        if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
                                list_del_init(&skb->tcp_tsorted_anchor);

                        if (!before(TCP_SKB_CB(skb)->seq,
                                    tcp_highest_sack_seq(tp)))
                                tcp_advance_highest_sack(sk, skb);
                }
        }
        return skb;
}

static struct sk_buff *tcp_sacktag_bsearch(struct sock *sk, u32 seq)
{
        struct rb_node *parent, **p = &sk->tcp_rtx_queue.rb_node;
        struct sk_buff *skb;

        while (*p) {
                parent = *p;
                skb = rb_to_skb(parent);
                if (before(seq, TCP_SKB_CB(skb)->seq)) {
                        p = &parent->rb_left;
                        continue;
                }
                if (!before(seq, TCP_SKB_CB(skb)->end_seq)) {
                        p = &parent->rb_right;
                        continue;
                }
                return skb;
        }
        return NULL;
}

static struct sk_buff *tcp_sacktag_skip(struct sk_buff *skb, struct sock *sk,
                                        u32 skip_to_seq)
{
        if (skb && after(TCP_SKB_CB(skb)->seq, skip_to_seq))
                return skb;

        return tcp_sacktag_bsearch(sk, skip_to_seq);
}

static struct sk_buff *tcp_maybe_skipping_dsack(struct sk_buff *skb,
                                                struct sock *sk,
                                                struct tcp_sack_block *next_dup,
                                                struct tcp_sacktag_state *state,
                                                u32 skip_to_seq)
{
        if (!next_dup)
                return skb;

        if (before(next_dup->start_seq, skip_to_seq)) {
                skb = tcp_sacktag_skip(skb, sk, next_dup->start_seq);
                skb = tcp_sacktag_walk(skb, sk, NULL, state,
                                       next_dup->start_seq, next_dup->end_seq,
                                       1);
        }

        return skb;
}

static int tcp_sack_cache_ok(const struct tcp_sock *tp, const struct tcp_sack_block *cache)
{
        return cache < tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
}

static int
tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
                        u32 prior_snd_una, struct tcp_sacktag_state *state)
{
        struct tcp_sock *tp = tcp_sk(sk);
        const unsigned char *ptr = (skb_transport_header(ack_skb) +
                                    TCP_SKB_CB(ack_skb)->sacked);
        struct tcp_sack_block_wire *sp_wire = (struct tcp_sack_block_wire *)(ptr+2);
        struct tcp_sack_block sp[TCP_NUM_SACKS];
        struct tcp_sack_block *cache;
        struct sk_buff *skb;
        int num_sacks = min(TCP_NUM_SACKS, (ptr[1] - TCPOLEN_SACK_BASE) >> 3);
        int used_sacks;
        bool found_dup_sack = false;
        int i, j;
        int first_sack_index;

        state->flag = 0;
        state->reord = tp->snd_nxt;

        if (!tp->sacked_out)
                tcp_highest_sack_reset(sk);

        found_dup_sack = tcp_check_dsack(sk, ack_skb, sp_wire,
                                         num_sacks, prior_snd_una, state);

        /* Eliminate too old ACKs, but take into
         * account more or less fresh ones, they can
         * contain valid SACK info.
         */
        if (before(TCP_SKB_CB(ack_skb)->ack_seq, prior_snd_una - tp->max_window))
                return 0;

        if (!tp->packets_out)
                goto out;

        used_sacks = 0;
        first_sack_index = 0;
        for (i = 0; i < num_sacks; i++) {
                bool dup_sack = !i && found_dup_sack;

                sp[used_sacks].start_seq = get_unaligned_be32(&sp_wire[i].start_seq);
                sp[used_sacks].end_seq = get_unaligned_be32(&sp_wire[i].end_seq);

                if (!tcp_is_sackblock_valid(tp, dup_sack,
                                            sp[used_sacks].start_seq,
                                            sp[used_sacks].end_seq)) {
                        int mib_idx;

                        if (dup_sack) {
                                if (!tp->undo_marker)
                                        mib_idx = LINUX_MIB_TCPDSACKIGNOREDNOUNDO;
                                else
                                        mib_idx = LINUX_MIB_TCPDSACKIGNOREDOLD;
                        } else {
                                /* Don't count olds caused by ACK reordering */
                                if ((TCP_SKB_CB(ack_skb)->ack_seq != tp->snd_una) &&
                                    !after(sp[used_sacks].end_seq, tp->snd_una))
                                        continue;
                                mib_idx = LINUX_MIB_TCPSACKDISCARD;
                        }

                        NET_INC_STATS(sock_net(sk), mib_idx);
                        if (i == 0)
                                first_sack_index = -1;
                        continue;
                }

                /* Ignore very old stuff early */
                if (!after(sp[used_sacks].end_seq, prior_snd_una)) {
                        if (i == 0)
                                first_sack_index = -1;
                        continue;
                }

                used_sacks++;
        }

        /* order SACK blocks to allow in order walk of the retrans queue */
        for (i = used_sacks - 1; i > 0; i--) {
                for (j = 0; j < i; j++) {
                        if (after(sp[j].start_seq, sp[j + 1].start_seq)) {
                                swap(sp[j], sp[j + 1]);

                                /* Track where the first SACK block goes to */
                                if (j == first_sack_index)
                                        first_sack_index = j + 1;
                        }
                }
        }

        state->mss_now = tcp_current_mss(sk);
        skb = NULL;
        i = 0;

        if (!tp->sacked_out) {
                /* It's already past, so skip checking against it */
                cache = tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
        } else {
                cache = tp->recv_sack_cache;
                /* Skip empty blocks in at head of the cache */
                while (tcp_sack_cache_ok(tp, cache) && !cache->start_seq &&
                       !cache->end_seq)
                        cache++;
        }

        while (i < used_sacks) {
                u32 start_seq = sp[i].start_seq;
                u32 end_seq = sp[i].end_seq;
                bool dup_sack = (found_dup_sack && (i == first_sack_index));
                struct tcp_sack_block *next_dup = NULL;

                if (found_dup_sack && ((i + 1) == first_sack_index))
                        next_dup = &sp[i + 1];

                /* Skip too early cached blocks */
                while (tcp_sack_cache_ok(tp, cache) &&
                       !before(start_seq, cache->end_seq))
                        cache++;

                /* Can skip some work by looking recv_sack_cache? */
                if (tcp_sack_cache_ok(tp, cache) && !dup_sack &&
                    after(end_seq, cache->start_seq)) {

                        /* Head todo? */
                        if (before(start_seq, cache->start_seq)) {
                                skb = tcp_sacktag_skip(skb, sk, start_seq);
                                skb = tcp_sacktag_walk(skb, sk, next_dup,
                                                       state,
                                                       start_seq,
                                                       cache->start_seq,
                                                       dup_sack);
                        }

                        /* Rest of the block already fully processed? */
                        if (!after(end_seq, cache->end_seq))
                                goto advance_sp;

                        skb = tcp_maybe_skipping_dsack(skb, sk, next_dup,
                                                       state,
                                                       cache->end_seq);

                        /* ...tail remains todo... */
                        if (tcp_highest_sack_seq(tp) == cache->end_seq) {
                                /* ...but better entrypoint exists! */
                                skb = tcp_highest_sack(sk);
                                if (!skb)
                                        break;
                                cache++;
                                goto walk;
                        }

                        skb = tcp_sacktag_skip(skb, sk, cache->end_seq);
                        /* Check overlap against next cached too (past this one already) */
                        cache++;
                        continue;
                }

                if (!before(start_seq, tcp_highest_sack_seq(tp))) {
                        skb = tcp_highest_sack(sk);
                        if (!skb)
                                break;
                }
                skb = tcp_sacktag_skip(skb, sk, start_seq);

walk:
                skb = tcp_sacktag_walk(skb, sk, next_dup, state,
                                       start_seq, end_seq, dup_sack);

advance_sp:
                i++;
        }

        /* Clear the head of the cache sack blocks so we can skip it next time */
        for (i = 0; i < ARRAY_SIZE(tp->recv_sack_cache) - used_sacks; i++) {
                tp->recv_sack_cache[i].start_seq = 0;
                tp->recv_sack_cache[i].end_seq = 0;
        }
        for (j = 0; j < used_sacks; j++)
                tp->recv_sack_cache[i++] = sp[j];

        if (inet_csk(sk)->icsk_ca_state != TCP_CA_Loss || tp->undo_marker)
                tcp_check_sack_reordering(sk, state->reord, 0);

        tcp_verify_left_out(tp);
out:

#if FASTRETRANS_DEBUG > 0
        WARN_ON((int)tp->sacked_out < 0);
        WARN_ON((int)tp->lost_out < 0);
        WARN_ON((int)tp->retrans_out < 0);
        WARN_ON((int)tcp_packets_in_flight(tp) < 0);
#endif
        return state->flag;
}

/* Limits sacked_out so that sum with lost_out isn't ever larger than
 * packets_out. Returns false if sacked_out adjustement wasn't necessary.
 */
static bool tcp_limit_reno_sacked(struct tcp_sock *tp)
{
        u32 holes;

        holes = max(tp->lost_out, 1U);
        holes = min(holes, tp->packets_out);

        if ((tp->sacked_out + holes) > tp->packets_out) {
                tp->sacked_out = tp->packets_out - holes;
                return true;
        }
        return false;
}

/* If we receive more dupacks than we expected counting segments
 * in assumption of absent reordering, interpret this as reordering.
 * The only another reason could be bug in receiver TCP.
 */
static void tcp_check_reno_reordering(struct sock *sk, const int addend)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (!tcp_limit_reno_sacked(tp))
                return;

        tp->reordering = min_t(u32, tp->packets_out + addend,
                               sock_net(sk)->ipv4.sysctl_tcp_max_reordering);
        tp->reord_seen++;
        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRENOREORDER);
}

/* Emulate SACKs for SACKless connection: account for a new dupack. */

static void tcp_add_reno_sack(struct sock *sk, int num_dupack, bool ece_ack)
{
        if (num_dupack) {
                struct tcp_sock *tp = tcp_sk(sk);
                u32 prior_sacked = tp->sacked_out;
                s32 delivered;

                tp->sacked_out += num_dupack;
                tcp_check_reno_reordering(sk, 0);
                delivered = tp->sacked_out - prior_sacked;
                if (delivered > 0)
                        tcp_count_delivered(tp, delivered, ece_ack);
                tcp_verify_left_out(tp);
        }
}

/* Account for ACK, ACKing some data in Reno Recovery phase. */

static void tcp_remove_reno_sacks(struct sock *sk, int acked, bool ece_ack)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (acked > 0) {
                /* One ACK acked hole. The rest eat duplicate ACKs. */
                tcp_count_delivered(tp, max_t(int, acked - tp->sacked_out, 1),
                                    ece_ack);
                if (acked - 1 >= tp->sacked_out)
                        tp->sacked_out = 0;
                else
                        tp->sacked_out -= acked - 1;
        }
        tcp_check_reno_reordering(sk, acked);
        tcp_verify_left_out(tp);
}

static inline void tcp_reset_reno_sack(struct tcp_sock *tp)
{
        tp->sacked_out = 0;
}

void tcp_clear_retrans(struct tcp_sock *tp)
{
        tp->retrans_out = 0;
        tp->lost_out = 0;
        tp->undo_marker = 0;
        tp->undo_retrans = -1;
        tp->sacked_out = 0;
}

static inline void tcp_init_undo(struct tcp_sock *tp)
{
        tp->undo_marker = tp->snd_una;
        /* Retransmission still in flight may cause DSACKs later. */
        tp->undo_retrans = tp->retrans_out ? : -1;
}

static bool tcp_is_rack(const struct sock *sk)
{
        return sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_LOSS_DETECTION;
}

/* If we detect SACK reneging, forget all SACK information
 * and reset tags completely, otherwise preserve SACKs. If receiver
 * dropped its ofo queue, we will know this due to reneging detection.
 */
static void tcp_timeout_mark_lost(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb, *head;
        bool is_reneg;                  /* is receiver reneging on SACKs? */

        head = tcp_rtx_queue_head(sk);
        is_reneg = head && (TCP_SKB_CB(head)->sacked & TCPCB_SACKED_ACKED);
        if (is_reneg) {
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSACKRENEGING);
                tp->sacked_out = 0;
                /* Mark SACK reneging until we recover from this loss event. */
                tp->is_sack_reneg = 1;
        } else if (tcp_is_reno(tp)) {
                tcp_reset_reno_sack(tp);
        }

        skb = head;
        skb_rbtree_walk_from(skb) {
                if (is_reneg)
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
                else if (tcp_is_rack(sk) && skb != head &&
                         tcp_rack_skb_timeout(tp, skb, 0) > 0)
                        continue; /* Don't mark recently sent ones lost yet */
                tcp_mark_skb_lost(sk, skb);
        }
        tcp_verify_left_out(tp);
        tcp_clear_all_retrans_hints(tp);
}

/* Enter Loss state. */
void tcp_enter_loss(struct sock *sk)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        struct net *net = sock_net(sk);
        bool new_recovery = icsk->icsk_ca_state < TCP_CA_Recovery;

        tcp_timeout_mark_lost(sk);

        /* Reduce ssthresh if it has not yet been made inside this window. */
        if (icsk->icsk_ca_state <= TCP_CA_Disorder ||
            !after(tp->high_seq, tp->snd_una) ||
            (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
                tp->prior_ssthresh = tcp_current_ssthresh(sk);
                tp->prior_cwnd = tcp_snd_cwnd(tp);
                tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
                tcp_ca_event(sk, CA_EVENT_LOSS);
                tcp_init_undo(tp);
        }
        tcp_snd_cwnd_set(tp, tcp_packets_in_flight(tp) + 1);
        tp->snd_cwnd_cnt   = 0;
        tp->snd_cwnd_stamp = tcp_jiffies32;

        /* Timeout in disordered state after receiving substantial DUPACKs
         * suggests that the degree of reordering is over-estimated.
         */
        if (icsk->icsk_ca_state <= TCP_CA_Disorder &&
            tp->sacked_out >= net->ipv4.sysctl_tcp_reordering)
                tp->reordering = min_t(unsigned int, tp->reordering,
                                       net->ipv4.sysctl_tcp_reordering);
        tcp_set_ca_state(sk, TCP_CA_Loss);
        tp->high_seq = tp->snd_nxt;
        tcp_ecn_queue_cwr(tp);

        /* F-RTO RFC5682 sec 3.1 step 1: retransmit SND.UNA if no previous
         * loss recovery is underway except recurring timeout(s) on
         * the same SND.UNA (sec 3.2). Disable F-RTO on path MTU probing
         */
        tp->frto = net->ipv4.sysctl_tcp_frto &&
                   (new_recovery || icsk->icsk_retransmits) &&
                   !inet_csk(sk)->icsk_mtup.probe_size;
}

/* If ACK arrived pointing to a remembered SACK, it means that our
 * remembered SACKs do not reflect real state of receiver i.e.
 * receiver _host_ is heavily congested (or buggy).
 *
 * To avoid big spurious retransmission bursts due to transient SACK
 * scoreboard oddities that look like reneging, we give the receiver a
 * little time (max(RTT/2, 10ms)) to send us some more ACKs that will
 * restore sanity to the SACK scoreboard. If the apparent reneging
 * persists until this RTO then we'll clear the SACK scoreboard.
 */
static bool tcp_check_sack_reneging(struct sock *sk, int flag)
{
        if (flag & FLAG_SACK_RENEGING) {
                struct tcp_sock *tp = tcp_sk(sk);
                unsigned long delay = max(usecs_to_jiffies(tp->srtt_us >> 4),
                                          msecs_to_jiffies(10));

                inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                          delay, TCP_RTO_MAX);
                return true;
        }
        return false;
}

/* Heurestics to calculate number of duplicate ACKs. There's no dupACKs
 * counter when SACK is enabled (without SACK, sacked_out is used for
 * that purpose).
 *
 * With reordering, holes may still be in flight, so RFC3517 recovery
 * uses pure sacked_out (total number of SACKed segments) even though
 * it violates the RFC that uses duplicate ACKs, often these are equal
 * but when e.g. out-of-window ACKs or packet duplication occurs,
 * they differ. Since neither occurs due to loss, TCP should really
 * ignore them.
 */
static inline int tcp_dupack_heuristics(const struct tcp_sock *tp)
{
        return tp->sacked_out + 1;
}

/* Linux NewReno/SACK/ECN state machine.
 * --------------------------------------
 *
 * "Open"       Normal state, no dubious events, fast path.
 * "Disorder"   In all the respects it is "Open",
 *              but requires a bit more attention. It is entered when
 *              we see some SACKs or dupacks. It is split of "Open"
 *              mainly to move some processing from fast path to slow one.
 * "CWR"        CWND was reduced due to some Congestion Notification event.
 *              It can be ECN, ICMP source quench, local device congestion.
 * "Recovery"   CWND was reduced, we are fast-retransmitting.
 * "Loss"       CWND was reduced due to RTO timeout or SACK reneging.
 *
 * tcp_fastretrans_alert() is entered:
 * - each incoming ACK, if state is not "Open"
 * - when arrived ACK is unusual, namely:
 *      * SACK
 *      * Duplicate ACK.
 *      * ECN ECE.
 *
 * Counting packets in flight is pretty simple.
 *
 *      in_flight = packets_out - left_out + retrans_out
 *
 *      packets_out is SND.NXT-SND.UNA counted in packets.
 *
 *      retrans_out is number of retransmitted segments.
 *
 *      left_out is number of segments left network, but not ACKed yet.
 *
 *              left_out = sacked_out + lost_out
 *
 *     sacked_out: Packets, which arrived to receiver out of order
 *                 and hence not ACKed. With SACKs this number is simply
 *                 amount of SACKed data. Even without SACKs
 *                 it is easy to give pretty reliable estimate of this number,
 *                 counting duplicate ACKs.
 *
 *       lost_out: Packets lost by network. TCP has no explicit
 *                 "loss notification" feedback from network (for now).
 *                 It means that this number can be only _guessed_.
 *                 Actually, it is the heuristics to predict lossage that
 *                 distinguishes different algorithms.
 *
 *      F.e. after RTO, when all the queue is considered as lost,
 *      lost_out = packets_out and in_flight = retrans_out.
 *
 *              Essentially, we have now a few algorithms detecting
 *              lost packets.
 *
 *              If the receiver supports SACK:
 *
 *              RFC6675/3517: It is the conventional algorithm. A packet is
 *              considered lost if the number of higher sequence packets
 *              SACKed is greater than or equal the DUPACK thoreshold
 *              (reordering). This is implemented in tcp_mark_head_lost and
 *              tcp_update_scoreboard.
 *
 *              RACK (draft-ietf-tcpm-rack-01): it is a newer algorithm
 *              (2017-) that checks timing instead of counting DUPACKs.
 *              Essentially a packet is considered lost if it's not S/ACKed
 *              after RTT + reordering_window, where both metrics are
 *              dynamically measured and adjusted. This is implemented in
 *              tcp_rack_mark_lost.
 *
 *              If the receiver does not support SACK:
 *
 *              NewReno (RFC6582): in Recovery we assume that one segment
 *              is lost (classic Reno). While we are in Recovery and
 *              a partial ACK arrives, we assume that one more packet
 *              is lost (NewReno). This heuristics are the same in NewReno
 *              and SACK.
 *
 * Really tricky (and requiring careful tuning) part of algorithm
 * is hidden in functions tcp_time_to_recover() and tcp_xmit_retransmit_queue().
 * The first determines the moment _when_ we should reduce CWND and,
 * hence, slow down forward transmission. In fact, it determines the moment
 * when we decide that hole is caused by loss, rather than by a reorder.
 *
 * tcp_xmit_retransmit_queue() decides, _what_ we should retransmit to fill
 * holes, caused by lost packets.
 *
 * And the most logically complicated part of algorithm is undo
 * heuristics. We detect false retransmits due to both too early
 * fast retransmit (reordering) and underestimated RTO, analyzing
 * timestamps and D-SACKs. When we detect that some segments were
 * retransmitted by mistake and CWND reduction was wrong, we undo
 * window reduction and abort recovery phase. This logic is hidden
 * inside several functions named tcp_try_undo_<something>.
 */

/* This function decides, when we should leave Disordered state
 * and enter Recovery phase, reducing congestion window.
 *
 * Main question: may we further continue forward transmission
 * with the same cwnd?
 */
static bool tcp_time_to_recover(struct sock *sk, int flag)
{
        struct tcp_sock *tp = tcp_sk(sk);

        /* Trick#1: The loss is proven. */
        if (tp->lost_out)
                return true;

        /* Not-A-Trick#2 : Classic rule... */
        if (!tcp_is_rack(sk) && tcp_dupack_heuristics(tp) > tp->reordering)
                return true;

        return false;
}

/* Detect loss in event "A" above by marking head of queue up as lost.
 * For RFC3517 SACK, a segment is considered lost if it
 * has at least tp->reordering SACKed seqments above it; "packets" refers to
 * the maximum SACKed segments to pass before reaching this limit.
 */
static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
        int cnt;
        /* Use SACK to deduce losses of new sequences sent during recovery */
        const u32 loss_high = tp->snd_nxt;

        WARN_ON(packets > tp->packets_out);
        skb = tp->lost_skb_hint;
        if (skb) {
                /* Head already handled? */
                if (mark_head && after(TCP_SKB_CB(skb)->seq, tp->snd_una))
                        return;
                cnt = tp->lost_cnt_hint;
        } else {
                skb = tcp_rtx_queue_head(sk);
                cnt = 0;
        }

        skb_rbtree_walk_from(skb) {
                /* TODO: do this better */
                /* this is not the most efficient way to do this... */
                tp->lost_skb_hint = skb;
                tp->lost_cnt_hint = cnt;

                if (after(TCP_SKB_CB(skb)->end_seq, loss_high))
                        break;

                if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
                        cnt += tcp_skb_pcount(skb);

                if (cnt > packets)
                        break;

                if (!(TCP_SKB_CB(skb)->sacked & TCPCB_LOST))
                        tcp_mark_skb_lost(sk, skb);

                if (mark_head)
                        break;
        }
        tcp_verify_left_out(tp);
}

/* Account newly detected lost packet(s) */

static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tcp_is_sack(tp)) {
                int sacked_upto = tp->sacked_out - tp->reordering;
                if (sacked_upto >= 0)
                        tcp_mark_head_lost(sk, sacked_upto, 0);
                else if (fast_rexmit)
                        tcp_mark_head_lost(sk, 1, 1);
        }
}

static bool tcp_tsopt_ecr_before(const struct tcp_sock *tp, u32 when)
{
        return tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
               before(tp->rx_opt.rcv_tsecr, when);
}

/* skb is spurious retransmitted if the returned timestamp echo
 * reply is prior to the skb transmission time
 */
static bool tcp_skb_spurious_retrans(const struct tcp_sock *tp,
                                     const struct sk_buff *skb)
{
        return (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS) &&
               tcp_tsopt_ecr_before(tp, tcp_skb_timestamp(skb));
}

/* Nothing was retransmitted or returned timestamp is less
 * than timestamp of the first retransmission.
 */
static inline bool tcp_packet_delayed(const struct tcp_sock *tp)
{
        return tp->retrans_stamp &&
               tcp_tsopt_ecr_before(tp, tp->retrans_stamp);
}

/* Undo procedures. */

/* We can clear retrans_stamp when there are no retransmissions in the
 * window. It would seem that it is trivially available for us in
 * tp->retrans_out, however, that kind of assumptions doesn't consider
 * what will happen if errors occur when sending retransmission for the
 * second time. ...It could the that such segment has only
 * TCPCB_EVER_RETRANS set at the present time. It seems that checking
 * the head skb is enough except for some reneging corner cases that
 * are not worth the effort.
 *
 * Main reason for all this complexity is the fact that connection dying
 * time now depends on the validity of the retrans_stamp, in particular,
 * that successive retransmissions of a segment must not advance
 * retrans_stamp under any conditions.
 */
static bool tcp_any_retrans_done(const struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;

        if (tp->retrans_out)
                return true;

        skb = tcp_rtx_queue_head(sk);
        if (unlikely(skb && TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS))
                return true;

        return false;
}

static void DBGUNDO(struct sock *sk, const char *msg)
{
#if FASTRETRANS_DEBUG > 1
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_sock *inet = inet_sk(sk);

        if (sk->sk_family == AF_INET) {
                pr_debug("Undo %s %pI4/%u c%u l%u ss%u/%u p%u\n",
                         msg,
                         &inet->inet_daddr, ntohs(inet->inet_dport),
                         tcp_snd_cwnd(tp), tcp_left_out(tp),
                         tp->snd_ssthresh, tp->prior_ssthresh,
                         tp->packets_out);
        }
#if IS_ENABLED(CONFIG_IPV6)
        else if (sk->sk_family == AF_INET6) {
                pr_debug("Undo %s %pI6/%u c%u l%u ss%u/%u p%u\n",
                         msg,
                         &sk->sk_v6_daddr, ntohs(inet->inet_dport),
                         tcp_snd_cwnd(tp), tcp_left_out(tp),
                         tp->snd_ssthresh, tp->prior_ssthresh,
                         tp->packets_out);
        }
#endif
#endif
}

static void tcp_undo_cwnd_reduction(struct sock *sk, bool unmark_loss)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (unmark_loss) {
                struct sk_buff *skb;

                skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
                }
                tp->lost_out = 0;
                tcp_clear_all_retrans_hints(tp);
        }

        if (tp->prior_ssthresh) {
                const struct inet_connection_sock *icsk = inet_csk(sk);

                tcp_snd_cwnd_set(tp, icsk->icsk_ca_ops->undo_cwnd(sk));

                if (tp->prior_ssthresh > tp->snd_ssthresh) {
                        tp->snd_ssthresh = tp->prior_ssthresh;
                        tcp_ecn_withdraw_cwr(tp);
                }
        }
        tp->snd_cwnd_stamp = tcp_jiffies32;
        tp->undo_marker = 0;
        tp->rack.advanced = 1; /* Force RACK to re-exam losses */
}

static inline bool tcp_may_undo(const struct tcp_sock *tp)
{
        return tp->undo_marker && (!tp->undo_retrans || tcp_packet_delayed(tp));
}

/* People celebrate: "We love our President!" */
static bool tcp_try_undo_recovery(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tcp_may_undo(tp)) {
                int mib_idx;

                /* Happy end! We did not retransmit anything
                 * or our original transmission succeeded.
                 */
                DBGUNDO(sk, inet_csk(sk)->icsk_ca_state == TCP_CA_Loss ? "loss" : "retrans");
                tcp_undo_cwnd_reduction(sk, false);
                if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss)
                        mib_idx = LINUX_MIB_TCPLOSSUNDO;
                else
                        mib_idx = LINUX_MIB_TCPFULLUNDO;

                NET_INC_STATS(sock_net(sk), mib_idx);
        } else if (tp->rack.reo_wnd_persist) {
                tp->rack.reo_wnd_persist--;
        }
        if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) {
                /* Hold old state until something *above* high_seq
                 * is ACKed. For Reno it is MUST to prevent false
                 * fast retransmits (RFC2582). SACK TCP is safe. */
                if (!tcp_any_retrans_done(sk))
                        tp->retrans_stamp = 0;
                return true;
        }
        tcp_set_ca_state(sk, TCP_CA_Open);
        tp->is_sack_reneg = 0;
        return false;
}

/* Try to undo cwnd reduction, because D-SACKs acked all retransmitted data */
static bool tcp_try_undo_dsack(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tp->undo_marker && !tp->undo_retrans) {
                tp->rack.reo_wnd_persist = min(TCP_RACK_RECOVERY_THRESH,
                                               tp->rack.reo_wnd_persist + 1);
                DBGUNDO(sk, "D-SACK");
                tcp_undo_cwnd_reduction(sk, false);
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKUNDO);
                return true;
        }
        return false;
}

/* Undo during loss recovery after partial ACK or using F-RTO. */
static bool tcp_try_undo_loss(struct sock *sk, bool frto_undo)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (frto_undo || tcp_may_undo(tp)) {
                tcp_undo_cwnd_reduction(sk, true);

                DBGUNDO(sk, "partial loss");
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSSUNDO);
                if (frto_undo)
                        NET_INC_STATS(sock_net(sk),
                                        LINUX_MIB_TCPSPURIOUSRTOS);
                inet_csk(sk)->icsk_retransmits = 0;
                if (frto_undo || tcp_is_sack(tp)) {
                        tcp_set_ca_state(sk, TCP_CA_Open);
                        tp->is_sack_reneg = 0;
                }
                return true;
        }
        return false;
}

/* The cwnd reduction in CWR and Recovery uses the PRR algorithm in RFC 6937.
 * It computes the number of packets to send (sndcnt) based on packets newly
 * delivered:
 *   1) If the packets in flight is larger than ssthresh, PRR spreads the
 *      cwnd reductions across a full RTT.
 *   2) Otherwise PRR uses packet conservation to send as much as delivered.
 *      But when SND_UNA is acked without further losses,
 *      slow starts cwnd up to ssthresh to speed up the recovery.
 */
static void tcp_init_cwnd_reduction(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        tp->high_seq = tp->snd_nxt;
        tp->tlp_high_seq = 0;
        tp->snd_cwnd_cnt = 0;
        tp->prior_cwnd = tcp_snd_cwnd(tp);
        tp->prr_delivered = 0;
        tp->prr_out = 0;
        tp->snd_ssthresh = inet_csk(sk)->icsk_ca_ops->ssthresh(sk);
        tcp_ecn_queue_cwr(tp);
}

void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int newly_lost, int flag)
{
        struct tcp_sock *tp = tcp_sk(sk);
        int sndcnt = 0;
        int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);

        if (newly_acked_sacked <= 0 || WARN_ON_ONCE(!tp->prior_cwnd))
                return;

        tp->prr_delivered += newly_acked_sacked;
        if (delta < 0) {
                u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +
                               tp->prior_cwnd - 1;
                sndcnt = div_u64(dividend, tp->prior_cwnd) - tp->prr_out;
        } else {
                sndcnt = max_t(int, tp->prr_delivered - tp->prr_out,
                               newly_acked_sacked);
                if (flag & FLAG_SND_UNA_ADVANCED && !newly_lost)
                        sndcnt++;
                sndcnt = min(delta, sndcnt);
        }
        /* Force a fast retransmit upon entering fast recovery */
        sndcnt = max(sndcnt, (tp->prr_out ? 0 : 1));
        tcp_snd_cwnd_set(tp, tcp_packets_in_flight(tp) + sndcnt);
}

static inline void tcp_end_cwnd_reduction(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (inet_csk(sk)->icsk_ca_ops->cong_control)
                return;

        /* Reset cwnd to ssthresh in CWR or Recovery (unless it's undone) */
        if (tp->snd_ssthresh < TCP_INFINITE_SSTHRESH &&
            (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR || tp->undo_marker)) {
                tcp_snd_cwnd_set(tp, tp->snd_ssthresh);
                tp->snd_cwnd_stamp = tcp_jiffies32;
        }
        tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
}

/* Enter CWR state. Disable cwnd undo since congestion is proven with ECN */
void tcp_enter_cwr(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        tp->prior_ssthresh = 0;
        if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
                tp->undo_marker = 0;
                tcp_init_cwnd_reduction(sk);
                tcp_set_ca_state(sk, TCP_CA_CWR);
        }
}
EXPORT_SYMBOL(tcp_enter_cwr);

static void tcp_try_keep_open(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        int state = TCP_CA_Open;

        if (tcp_left_out(tp) || tcp_any_retrans_done(sk))
                state = TCP_CA_Disorder;

        if (inet_csk(sk)->icsk_ca_state != state) {
                tcp_set_ca_state(sk, state);
                tp->high_seq = tp->snd_nxt;
        }
}

static void tcp_try_to_open(struct sock *sk, int flag)
{
        struct tcp_sock *tp = tcp_sk(sk);

        tcp_verify_left_out(tp);

        if (!tcp_any_retrans_done(sk))
                tp->retrans_stamp = 0;

        if (flag & FLAG_ECE)
                tcp_enter_cwr(sk);

        if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
                tcp_try_keep_open(sk);
        }
}

static void tcp_mtup_probe_failed(struct sock *sk)
{
        struct inet_connection_sock *icsk = inet_csk(sk);

        icsk->icsk_mtup.search_high = icsk->icsk_mtup.probe_size - 1;
        icsk->icsk_mtup.probe_size = 0;
        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMTUPFAIL);
}

static void tcp_mtup_probe_success(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);
        u64 val;

        tp->prior_ssthresh = tcp_current_ssthresh(sk);

        val = (u64)tcp_snd_cwnd(tp) * tcp_mss_to_mtu(sk, tp->mss_cache);
        do_div(val, icsk->icsk_mtup.probe_size);
        DEBUG_NET_WARN_ON_ONCE((u32)val != val);
        tcp_snd_cwnd_set(tp, max_t(u32, 1U, val));

        tp->snd_cwnd_cnt = 0;
        tp->snd_cwnd_stamp = tcp_jiffies32;
        tp->snd_ssthresh = tcp_current_ssthresh(sk);

        icsk->icsk_mtup.search_low = icsk->icsk_mtup.probe_size;
        icsk->icsk_mtup.probe_size = 0;
        tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMTUPSUCCESS);
}

/* Do a simple retransmit without using the backoff mechanisms in
 * tcp_timer. This is used for path mtu discovery.
 * The socket is already locked here.
 */
void tcp_simple_retransmit(struct sock *sk)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
        int mss;

        /* A fastopen SYN request is stored as two separate packets within
         * the retransmit queue, this is done by tcp_send_syn_data().
         * As a result simply checking the MSS of the frames in the queue
         * will not work for the SYN packet.
         *
         * Us being here is an indication of a path MTU issue so we can
         * assume that the fastopen SYN was lost and just mark all the
         * frames in the retransmit queue as lost. We will use an MSS of
         * -1 to mark all frames as lost, otherwise compute the current MSS.
         */
        if (tp->syn_data && sk->sk_state == TCP_SYN_SENT)
                mss = -1;
        else
                mss = tcp_current_mss(sk);

        skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
                if (tcp_skb_seglen(skb) > mss)
                        tcp_mark_skb_lost(sk, skb);
        }

        tcp_clear_retrans_hints_partial(tp);

        if (!tp->lost_out)
                return;

        if (tcp_is_reno(tp))
                tcp_limit_reno_sacked(tp);

        tcp_verify_left_out(tp);

        /* Don't muck with the congestion window here.
         * Reason is that we do not increase amount of _data_
         * in network, but units changed and effective
         * cwnd/ssthresh really reduced now.
         */
        if (icsk->icsk_ca_state != TCP_CA_Loss) {
                tp->high_seq = tp->snd_nxt;
                tp->snd_ssthresh = tcp_current_ssthresh(sk);
                tp->prior_ssthresh = 0;
                tp->undo_marker = 0;
                tcp_set_ca_state(sk, TCP_CA_Loss);
        }
        tcp_xmit_retransmit_queue(sk);
}
EXPORT_SYMBOL(tcp_simple_retransmit);

void tcp_enter_recovery(struct sock *sk, bool ece_ack)
{
        struct tcp_sock *tp = tcp_sk(sk);
        int mib_idx;

        if (tcp_is_reno(tp))
                mib_idx = LINUX_MIB_TCPRENORECOVERY;
        else
                mib_idx = LINUX_MIB_TCPSACKRECOVERY;

        NET_INC_STATS(sock_net(sk), mib_idx);

        tp->prior_ssthresh = 0;
        tcp_init_undo(tp);

        if (!tcp_in_cwnd_reduction(sk)) {
                if (!ece_ack)
                        tp->prior_ssthresh = tcp_current_ssthresh(sk);
                tcp_init_cwnd_reduction(sk);
        }
        tcp_set_ca_state(sk, TCP_CA_Recovery);
}

/* Process an ACK in CA_Loss state. Move to CA_Open if lost data are
 * recovered or spurious. Otherwise retransmits more on partial ACKs.
 */
static void tcp_process_loss(struct sock *sk, int flag, int num_dupack,
                             int *rexmit)
{
        struct tcp_sock *tp = tcp_sk(sk);
        bool recovered = !before(tp->snd_una, tp->high_seq);

        if ((flag & FLAG_SND_UNA_ADVANCED || rcu_access_pointer(tp->fastopen_rsk)) &&
            tcp_try_undo_loss(sk, false))
                return;

        if (tp->frto) { /* F-RTO RFC5682 sec 3.1 (sack enhanced version). */
                /* Step 3.b. A timeout is spurious if not all data are
                 * lost, i.e., never-retransmitted data are (s)acked.
                 */
                if ((flag & FLAG_ORIG_SACK_ACKED) &&
                    tcp_try_undo_loss(sk, true))
                        return;

                if (after(tp->snd_nxt, tp->high_seq)) {
                        if (flag & FLAG_DATA_SACKED || num_dupack)
                                tp->frto = 0; /* Step 3.a. loss was real */
                } else if (flag & FLAG_SND_UNA_ADVANCED && !recovered) {
                        tp->high_seq = tp->snd_nxt;
                        /* Step 2.b. Try send new data (but deferred until cwnd
                         * is updated in tcp_ack()). Otherwise fall back to
                         * the conventional recovery.
                         */
                        if (!tcp_write_queue_empty(sk) &&
                            after(tcp_wnd_end(tp), tp->snd_nxt)) {
                                *rexmit = REXMIT_NEW;
                                return;
                        }
                        tp->frto = 0;
                }
        }

        if (recovered) {
                /* F-RTO RFC5682 sec 3.1 step 2.a and 1st part of step 3.a */
                tcp_try_undo_recovery(sk);
                return;
        }
        if (tcp_is_reno(tp)) {
                /* A Reno DUPACK means new data in F-RTO step 2.b above are
                 * delivered. Lower inflight to clock out (re)tranmissions.
                 */
                if (after(tp->snd_nxt, tp->high_seq) && num_dupack)
                        tcp_add_reno_sack(sk, num_dupack, flag & FLAG_ECE);
                else if (flag & FLAG_SND_UNA_ADVANCED)
                        tcp_reset_reno_sack(tp);
        }
        *rexmit = REXMIT_LOST;
}

static bool tcp_force_fast_retransmit(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        return after(tcp_highest_sack_seq(tp),
                     tp->snd_una + tp->reordering * tp->mss_cache);
}

/* Undo during fast recovery after partial ACK. */
static bool tcp_try_undo_partial(struct sock *sk, u32 prior_snd_una,
                                 bool *do_lost)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tp->undo_marker && tcp_packet_delayed(tp)) {
                /* Plain luck! Hole if filled with delayed
                 * packet, rather than with a retransmit. Check reordering.
                 */
                tcp_check_sack_reordering(sk, prior_snd_una, 1);

                /* We are getting evidence that the reordering degree is higher
                 * than we realized. If there are no retransmits out then we
                 * can undo. Otherwise we clock out new packets but do not
                 * mark more packets lost or retransmit more.
                 */
                if (tp->retrans_out)
                        return true;

                if (!tcp_any_retrans_done(sk))
                        tp->retrans_stamp = 0;

                DBGUNDO(sk, "partial recovery");
                tcp_undo_cwnd_reduction(sk, true);
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPARTIALUNDO);
                tcp_try_keep_open(sk);
        } else {
                /* Partial ACK arrived. Force fast retransmit. */
                *do_lost = tcp_force_fast_retransmit(sk);
        }
        return false;
}

static void tcp_identify_packet_loss(struct sock *sk, int *ack_flag)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tcp_rtx_queue_empty(sk))
                return;

        if (unlikely(tcp_is_reno(tp))) {
                tcp_newreno_mark_lost(sk, *ack_flag & FLAG_SND_UNA_ADVANCED);
        } else if (tcp_is_rack(sk)) {
                u32 prior_retrans = tp->retrans_out;

                if (tcp_rack_mark_lost(sk))
                        *ack_flag &= ~FLAG_SET_XMIT_TIMER;
                if (prior_retrans > tp->retrans_out)
                        *ack_flag |= FLAG_LOST_RETRANS;
        }
}

/* Process an event, which can update packets-in-flight not trivially.
 * Main goal of this function is to calculate new estimate for left_out,
 * taking into account both packets sitting in receiver's buffer and
 * packets lost by network.
 *
 * Besides that it updates the congestion state when packet loss or ECN
 * is detected. But it does not reduce the cwnd, it is done by the
 * congestion control later.
 *
 * It does _not_ decide what to send, it is made in function
 * tcp_xmit_retransmit_queue().
 */
static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una,
                                  int num_dupack, int *ack_flag, int *rexmit)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        int fast_rexmit = 0, flag = *ack_flag;
        bool ece_ack = flag & FLAG_ECE;
        bool do_lost = num_dupack || ((flag & FLAG_DATA_SACKED) &&
                                      tcp_force_fast_retransmit(sk));

        if (!tp->packets_out && tp->sacked_out)
                tp->sacked_out = 0;

        /* Now state machine starts.
         * A. ECE, hence prohibit cwnd undoing, the reduction is required. */
        if (ece_ack)
                tp->prior_ssthresh = 0;

        /* B. In all the states check for reneging SACKs. */
        if (tcp_check_sack_reneging(sk, flag))
                return;

        /* C. Check consistency of the current state. */
        tcp_verify_left_out(tp);

        /* D. Check state exit conditions. State can be terminated
         *    when high_seq is ACKed. */
        if (icsk->icsk_ca_state == TCP_CA_Open) {
                WARN_ON(tp->retrans_out != 0 && !tp->syn_data);
                tp->retrans_stamp = 0;
        } else if (!before(tp->snd_una, tp->high_seq)) {
                switch (icsk->icsk_ca_state) {
                case TCP_CA_CWR:
                        /* CWR is to be held something *above* high_seq
                         * is ACKed for CWR bit to reach receiver. */
                        if (tp->snd_una != tp->high_seq) {
                                tcp_end_cwnd_reduction(sk);
                                tcp_set_ca_state(sk, TCP_CA_Open);
                        }
                        break;

                case TCP_CA_Recovery:
                        if (tcp_is_reno(tp))
                                tcp_reset_reno_sack(tp);
                        if (tcp_try_undo_recovery(sk))
                                return;
                        tcp_end_cwnd_reduction(sk);
                        break;
                }
        }

        /* E. Process state. */
        switch (icsk->icsk_ca_state) {
        case TCP_CA_Recovery:
                if (!(flag & FLAG_SND_UNA_ADVANCED)) {
                        if (tcp_is_reno(tp))
                                tcp_add_reno_sack(sk, num_dupack, ece_ack);
                } else if (tcp_try_undo_partial(sk, prior_snd_una, &do_lost))
                        return;

                if (tcp_try_undo_dsack(sk))
                        tcp_try_keep_open(sk);

                tcp_identify_packet_loss(sk, ack_flag);
                if (icsk->icsk_ca_state != TCP_CA_Recovery) {
                        if (!tcp_time_to_recover(sk, flag))
                                return;
                        /* Undo reverts the recovery state. If loss is evident,
                         * starts a new recovery (e.g. reordering then loss);
                         */
                        tcp_enter_recovery(sk, ece_ack);
                }
                break;
        case TCP_CA_Loss:
                tcp_process_loss(sk, flag, num_dupack, rexmit);
                tcp_identify_packet_loss(sk, ack_flag);
                if (!(icsk->icsk_ca_state == TCP_CA_Open ||
                      (*ack_flag & FLAG_LOST_RETRANS)))
                        return;
                /* Change state if cwnd is undone or retransmits are lost */
                fallthrough;
        default:
                if (tcp_is_reno(tp)) {
                        if (flag & FLAG_SND_UNA_ADVANCED)
                                tcp_reset_reno_sack(tp);
                        tcp_add_reno_sack(sk, num_dupack, ece_ack);
                }

                if (icsk->icsk_ca_state <= TCP_CA_Disorder)
                        tcp_try_undo_dsack(sk);

                tcp_identify_packet_loss(sk, ack_flag);
                if (!tcp_time_to_recover(sk, flag)) {
                        tcp_try_to_open(sk, flag);
                        return;
                }

                /* MTU probe failure: don't reduce cwnd */
                if (icsk->icsk_ca_state < TCP_CA_CWR &&
                    icsk->icsk_mtup.probe_size &&
                    tp->snd_una == tp->mtu_probe.probe_seq_start) {
                        tcp_mtup_probe_failed(sk);
                        /* Restores the reduction we did in tcp_mtup_probe() */
                        tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
                        tcp_simple_retransmit(sk);
                        return;
                }

                /* Otherwise enter Recovery state */
                tcp_enter_recovery(sk, ece_ack);
                fast_rexmit = 1;
        }

        if (!tcp_is_rack(sk) && do_lost)
                tcp_update_scoreboard(sk, fast_rexmit);
        *rexmit = REXMIT_LOST;
}

static void tcp_update_rtt_min(struct sock *sk, u32 rtt_us, const int flag)
{
        u32 wlen = sock_net(sk)->ipv4.sysctl_tcp_min_rtt_wlen * HZ;
        struct tcp_sock *tp = tcp_sk(sk);

        if ((flag & FLAG_ACK_MAYBE_DELAYED) && rtt_us > tcp_min_rtt(tp)) {
                /* If the remote keeps returning delayed ACKs, eventually
                 * the min filter would pick it up and overestimate the
                 * prop. delay when it expires. Skip suspected delayed ACKs.
                 */
                return;
        }
        minmax_running_min(&tp->rtt_min, wlen, tcp_jiffies32,
                           rtt_us ? : jiffies_to_usecs(1));
}

static bool tcp_ack_update_rtt(struct sock *sk, const int flag,
                               long seq_rtt_us, long sack_rtt_us,
                               long ca_rtt_us, struct rate_sample *rs)
{
        const struct tcp_sock *tp = tcp_sk(sk);

        /* Prefer RTT measured from ACK's timing to TS-ECR. This is because
         * broken middle-boxes or peers may corrupt TS-ECR fields. But
         * Karn's algorithm forbids taking RTT if some retransmitted data
         * is acked (RFC6298).
         */
        if (seq_rtt_us < 0)
                seq_rtt_us = sack_rtt_us;

        /* RTTM Rule: A TSecr value received in a segment is used to
         * update the averaged RTT measurement only if the segment
         * acknowledges some new data, i.e., only if it advances the
         * left edge of the send window.
         * See draft-ietf-tcplw-high-performance-00, section 3.3.
         */
        if (seq_rtt_us < 0 && tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
            flag & FLAG_ACKED) {
                u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;

                if (likely(delta < INT_MAX / (USEC_PER_SEC / TCP_TS_HZ))) {
                        if (!delta)
                                delta = 1;
                        seq_rtt_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
                        ca_rtt_us = seq_rtt_us;
                }
        }
        rs->rtt_us = ca_rtt_us; /* RTT of last (S)ACKed packet (or -1) */
        if (seq_rtt_us < 0)
                return false;

        /* ca_rtt_us >= 0 is counting on the invariant that ca_rtt_us is
         * always taken together with ACK, SACK, or TS-opts. Any negative
         * values will be skipped with the seq_rtt_us < 0 check above.
         */
        tcp_update_rtt_min(sk, ca_rtt_us, flag);
        tcp_rtt_estimator(sk, seq_rtt_us);
        tcp_set_rto(sk);

        /* RFC6298: only reset backoff on valid RTT measurement. */
        inet_csk(sk)->icsk_backoff = 0;
        return true;
}

/* Compute time elapsed between (last) SYNACK and the ACK completing 3WHS. */
void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req)
{
        struct rate_sample rs;
        long rtt_us = -1L;

        if (req && !req->num_retrans && tcp_rsk(req)->snt_synack)
                rtt_us = tcp_stamp_us_delta(tcp_clock_us(), tcp_rsk(req)->snt_synack);

        tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, rtt_us, -1L, rtt_us, &rs);
}


static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);

        icsk->icsk_ca_ops->cong_avoid(sk, ack, acked);
        tcp_sk(sk)->snd_cwnd_stamp = tcp_jiffies32;
}

/* Restart timer after forward progress on connection.
 * RFC2988 recommends to restart timer to now+rto.
 */
void tcp_rearm_rto(struct sock *sk)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);

        /* If the retrans timer is currently being used by Fast Open
         * for SYN-ACK retrans purpose, stay put.
         */
        if (rcu_access_pointer(tp->fastopen_rsk))
                return;

        if (!tp->packets_out) {
                inet_csk_clear_xmit_timer(sk, ICSK_TIME_RETRANS);
        } else {
                u32 rto = inet_csk(sk)->icsk_rto;
                /* Offset the time elapsed after installing regular RTO */
                if (icsk->icsk_pending == ICSK_TIME_REO_TIMEOUT ||
                    icsk->icsk_pending == ICSK_TIME_LOSS_PROBE) {
                        s64 delta_us = tcp_rto_delta_us(sk);
                        /* delta_us may not be positive if the socket is locked
                         * when the retrans timer fires and is rescheduled.
                         */
                        rto = usecs_to_jiffies(max_t(int, delta_us, 1));
                }
                tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
                                     TCP_RTO_MAX);
        }
}

/* Try to schedule a loss probe; if that doesn't work, then schedule an RTO. */
static void tcp_set_xmit_timer(struct sock *sk)
{
        if (!tcp_schedule_loss_probe(sk, true))
                tcp_rearm_rto(sk);
}

/* If we get here, the whole TSO packet has not been acked. */
static u32 tcp_tso_acked(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 packets_acked;

        BUG_ON(!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una));

        packets_acked = tcp_skb_pcount(skb);
        if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
                return 0;
        packets_acked -= tcp_skb_pcount(skb);

        if (packets_acked) {
                BUG_ON(tcp_skb_pcount(skb) == 0);
                BUG_ON(!before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq));
        }

        return packets_acked;
}

static void tcp_ack_tstamp(struct sock *sk, struct sk_buff *skb,
                           const struct sk_buff *ack_skb, u32 prior_snd_una)
{
        const struct skb_shared_info *shinfo;

        /* Avoid cache line misses to get skb_shinfo() and shinfo->tx_flags */
        if (likely(!TCP_SKB_CB(skb)->txstamp_ack))
                return;

        shinfo = skb_shinfo(skb);
        if (!before(shinfo->tskey, prior_snd_una) &&
            before(shinfo->tskey, tcp_sk(sk)->snd_una)) {
                tcp_skb_tsorted_save(skb) {
                        __skb_tstamp_tx(skb, ack_skb, NULL, sk, SCM_TSTAMP_ACK);
                } tcp_skb_tsorted_restore(skb);
        }
}

/* Remove acknowledged frames from the retransmission queue. If our packet
 * is before the ack sequence we can discard it as it's confirmed to have
 * arrived at the other end.
 */
static int tcp_clean_rtx_queue(struct sock *sk, const struct sk_buff *ack_skb,
                               u32 prior_fack, u32 prior_snd_una,
                               struct tcp_sacktag_state *sack, bool ece_ack)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        u64 first_ackt, last_ackt;
        struct tcp_sock *tp = tcp_sk(sk);
        u32 prior_sacked = tp->sacked_out;
        u32 reord = tp->snd_nxt; /* lowest acked un-retx un-sacked seq */
        struct sk_buff *skb, *next;
        bool fully_acked = true;
        long sack_rtt_us = -1L;
        long seq_rtt_us = -1L;
        long ca_rtt_us = -1L;
        u32 pkts_acked = 0;
        bool rtt_update;
        int flag = 0;

        first_ackt = 0;

        for (skb = skb_rb_first(&sk->tcp_rtx_queue); skb; skb = next) {
                struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
                const u32 start_seq = scb->seq;
                u8 sacked = scb->sacked;
                u32 acked_pcount;

                /* Determine how many packets and what bytes were acked, tso and else */
                if (after(scb->end_seq, tp->snd_una)) {
                        if (tcp_skb_pcount(skb) == 1 ||
                            !after(tp->snd_una, scb->seq))
                                break;

                        acked_pcount = tcp_tso_acked(sk, skb);
                        if (!acked_pcount)
                                break;
                        fully_acked = false;
                } else {
                        acked_pcount = tcp_skb_pcount(skb);
                }

                if (unlikely(sacked & TCPCB_RETRANS)) {
                        if (sacked & TCPCB_SACKED_RETRANS)
                                tp->retrans_out -= acked_pcount;
                        flag |= FLAG_RETRANS_DATA_ACKED;
                } else if (!(sacked & TCPCB_SACKED_ACKED)) {
                        last_ackt = tcp_skb_timestamp_us(skb);
                        WARN_ON_ONCE(last_ackt == 0);
                        if (!first_ackt)
                                first_ackt = last_ackt;

                        if (before(start_seq, reord))
                                reord = start_seq;
                        if (!after(scb->end_seq, tp->high_seq))
                                flag |= FLAG_ORIG_SACK_ACKED;
                }

                if (sacked & TCPCB_SACKED_ACKED) {
                        tp->sacked_out -= acked_pcount;
                } else if (tcp_is_sack(tp)) {
                        tcp_count_delivered(tp, acked_pcount, ece_ack);
                        if (!tcp_skb_spurious_retrans(tp, skb))
                                tcp_rack_advance(tp, sacked, scb->end_seq,
                                                 tcp_skb_timestamp_us(skb));
                }
                if (sacked & TCPCB_LOST)
                        tp->lost_out -= acked_pcount;

                tp->packets_out -= acked_pcount;
                pkts_acked += acked_pcount;
                tcp_rate_skb_delivered(sk, skb, sack->rate);

                /* Initial outgoing SYN's get put onto the write_queue
                 * just like anything else we transmit.  It is not
                 * true data, and if we misinform our callers that
                 * this ACK acks real data, we will erroneously exit
                 * connection startup slow start one packet too
                 * quickly.  This is severely frowned upon behavior.
                 */
                if (likely(!(scb->tcp_flags & TCPHDR_SYN))) {
                        flag |= FLAG_DATA_ACKED;
                } else {
                        flag |= FLAG_SYN_ACKED;
                        tp->retrans_stamp = 0;
                }

                if (!fully_acked)
                        break;

                tcp_ack_tstamp(sk, skb, ack_skb, prior_snd_una);

                next = skb_rb_next(skb);
                if (unlikely(skb == tp->retransmit_skb_hint))
                        tp->retransmit_skb_hint = NULL;
                if (unlikely(skb == tp->lost_skb_hint))
                        tp->lost_skb_hint = NULL;
                tcp_highest_sack_replace(sk, skb, next);
                tcp_rtx_queue_unlink_and_free(skb, sk);
        }

        if (!skb)
                tcp_chrono_stop(sk, TCP_CHRONO_BUSY);

        if (likely(between(tp->snd_up, prior_snd_una, tp->snd_una)))
                tp->snd_up = tp->snd_una;

        if (skb) {
                tcp_ack_tstamp(sk, skb, ack_skb, prior_snd_una);
                if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
                        flag |= FLAG_SACK_RENEGING;
        }

        if (likely(first_ackt) && !(flag & FLAG_RETRANS_DATA_ACKED)) {
                seq_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, first_ackt);
                ca_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, last_ackt);

                if (pkts_acked == 1 && fully_acked && !prior_sacked &&
                    (tp->snd_una - prior_snd_una) < tp->mss_cache &&
                    sack->rate->prior_delivered + 1 == tp->delivered &&
                    !(flag & (FLAG_CA_ALERT | FLAG_SYN_ACKED))) {
                        /* Conservatively mark a delayed ACK. It's typically
                         * from a lone runt packet over the round trip to
                         * a receiver w/o out-of-order or CE events.
                         */
                        flag |= FLAG_ACK_MAYBE_DELAYED;
                }
        }
        if (sack->first_sackt) {
                sack_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, sack->first_sackt);
                ca_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, sack->last_sackt);
        }
        rtt_update = tcp_ack_update_rtt(sk, flag, seq_rtt_us, sack_rtt_us,
                                        ca_rtt_us, sack->rate);

        if (flag & FLAG_ACKED) {
                flag |= FLAG_SET_XMIT_TIMER;  /* set TLP or RTO timer */
                if (unlikely(icsk->icsk_mtup.probe_size &&
                             !after(tp->mtu_probe.probe_seq_end, tp->snd_una))) {
                        tcp_mtup_probe_success(sk);
                }

                if (tcp_is_reno(tp)) {
                        tcp_remove_reno_sacks(sk, pkts_acked, ece_ack);

                        /* If any of the cumulatively ACKed segments was
                         * retransmitted, non-SACK case cannot confirm that
                         * progress was due to original transmission due to
                         * lack of TCPCB_SACKED_ACKED bits even if some of
                         * the packets may have been never retransmitted.
                         */
                        if (flag & FLAG_RETRANS_DATA_ACKED)
                                flag &= ~FLAG_ORIG_SACK_ACKED;
                } else {
                        int delta;

                        /* Non-retransmitted hole got filled? That's reordering */
                        if (before(reord, prior_fack))
                                tcp_check_sack_reordering(sk, reord, 0);

                        delta = prior_sacked - tp->sacked_out;
                        tp->lost_cnt_hint -= min(tp->lost_cnt_hint, delta);
                }
        } else if (skb && rtt_update && sack_rtt_us >= 0 &&
                   sack_rtt_us > tcp_stamp_us_delta(tp->tcp_mstamp,
                                                    tcp_skb_timestamp_us(skb))) {
                /* Do not re-arm RTO if the sack RTT is measured from data sent
                 * after when the head was last (re)transmitted. Otherwise the
                 * timeout may continue to extend in loss recovery.
                 */
                flag |= FLAG_SET_XMIT_TIMER;  /* set TLP or RTO timer */
        }

        if (icsk->icsk_ca_ops->pkts_acked) {
                struct ack_sample sample = { .pkts_acked = pkts_acked,
                                             .rtt_us = sack->rate->rtt_us };

                sample.in_flight = tp->mss_cache *
                        (tp->delivered - sack->rate->prior_delivered);
                icsk->icsk_ca_ops->pkts_acked(sk, &sample);
        }

#if FASTRETRANS_DEBUG > 0
        WARN_ON((int)tp->sacked_out < 0);
        WARN_ON((int)tp->lost_out < 0);
        WARN_ON((int)tp->retrans_out < 0);
        if (!tp->packets_out && tcp_is_sack(tp)) {
                icsk = inet_csk(sk);
                if (tp->lost_out) {
                        pr_debug("Leak l=%u %d\n",
                                 tp->lost_out, icsk->icsk_ca_state);
                        tp->lost_out = 0;
                }
                if (tp->sacked_out) {
                        pr_debug("Leak s=%u %d\n",
                                 tp->sacked_out, icsk->icsk_ca_state);
                        tp->sacked_out = 0;
                }
                if (tp->retrans_out) {
                        pr_debug("Leak r=%u %d\n",
                                 tp->retrans_out, icsk->icsk_ca_state);
                        tp->retrans_out = 0;
                }
        }
#endif
        return flag;
}

static void tcp_ack_probe(struct sock *sk)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct sk_buff *head = tcp_send_head(sk);
        const struct tcp_sock *tp = tcp_sk(sk);

        /* Was it a usable window open? */
        if (!head)
                return;
        if (!after(TCP_SKB_CB(head)->end_seq, tcp_wnd_end(tp))) {
                icsk->icsk_backoff = 0;
                icsk->icsk_probes_tstamp = 0;
                inet_csk_clear_xmit_timer(sk, ICSK_TIME_PROBE0);
                /* Socket must be waked up by subsequent tcp_data_snd_check().
                 * This function is not for random using!
                 */
        } else {
                unsigned long when = tcp_probe0_when(sk, TCP_RTO_MAX);

                when = tcp_clamp_probe0_to_user_timeout(sk, when);
                tcp_reset_xmit_timer(sk, ICSK_TIME_PROBE0, when, TCP_RTO_MAX);
        }
}

static inline bool tcp_ack_is_dubious(const struct sock *sk, const int flag)
{
        return !(flag & FLAG_NOT_DUP) || (flag & FLAG_CA_ALERT) ||
                inet_csk(sk)->icsk_ca_state != TCP_CA_Open;
}

/* Decide wheather to run the increase function of congestion control. */
static inline bool tcp_may_raise_cwnd(const struct sock *sk, const int flag)
{
        /* If reordering is high then always grow cwnd whenever data is
         * delivered regardless of its ordering. Otherwise stay conservative
         * and only grow cwnd on in-order delivery (RFC5681). A stretched ACK w/
         * new SACK or ECE mark may first advance cwnd here and later reduce
         * cwnd in tcp_fastretrans_alert() based on more states.
         */
        if (tcp_sk(sk)->reordering > sock_net(sk)->ipv4.sysctl_tcp_reordering)
                return flag & FLAG_FORWARD_PROGRESS;

        return flag & FLAG_DATA_ACKED;
}

/* The "ultimate" congestion control function that aims to replace the rigid
 * cwnd increase and decrease control (tcp_cong_avoid,tcp_*cwnd_reduction).
 * It's called toward the end of processing an ACK with precise rate
 * information. All transmission or retransmission are delayed afterwards.
 */
static void tcp_cong_control(struct sock *sk, u32 ack, u32 acked_sacked,
                             int flag, const struct rate_sample *rs)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);

        if (icsk->icsk_ca_ops->cong_control) {
                icsk->icsk_ca_ops->cong_control(sk, rs);
                return;
        }

        if (tcp_in_cwnd_reduction(sk)) {
                /* Reduce cwnd if state mandates */
                tcp_cwnd_reduction(sk, acked_sacked, rs->losses, flag);
        } else if (tcp_may_raise_cwnd(sk, flag)) {
                /* Advance cwnd if state allows */
                tcp_cong_avoid(sk, ack, acked_sacked);
        }
        tcp_update_pacing_rate(sk);
}

/* Check that window update is acceptable.
 * The function assumes that snd_una<=ack<=snd_next.
 */
static inline bool tcp_may_update_window(const struct tcp_sock *tp,
                                        const u32 ack, const u32 ack_seq,
                                        const u32 nwin)
{
        return  after(ack, tp->snd_una) ||
                after(ack_seq, tp->snd_wl1) ||
                (ack_seq == tp->snd_wl1 && nwin > tp->snd_wnd);
}

/* If we update tp->snd_una, also update tp->bytes_acked */
static void tcp_snd_una_update(struct tcp_sock *tp, u32 ack)
{
        u32 delta = ack - tp->snd_una;

        sock_owned_by_me((struct sock *)tp);
        tp->bytes_acked += delta;
        tp->snd_una = ack;
}

/* If we update tp->rcv_nxt, also update tp->bytes_received */
static void tcp_rcv_nxt_update(struct tcp_sock *tp, u32 seq)
{
        u32 delta = seq - tp->rcv_nxt;

        sock_owned_by_me((struct sock *)tp);
        tp->bytes_received += delta;
        WRITE_ONCE(tp->rcv_nxt, seq);
}

/* Update our send window.
 *
 * Window update algorithm, described in RFC793/RFC1122 (used in linux-2.2
 * and in FreeBSD. NetBSD's one is even worse.) is wrong.
 */
static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32 ack,
                                 u32 ack_seq)
{
        struct tcp_sock *tp = tcp_sk(sk);
        int flag = 0;
        u32 nwin = ntohs(tcp_hdr(skb)->window);

        if (likely(!tcp_hdr(skb)->syn))
                nwin <<= tp->rx_opt.snd_wscale;

        if (tcp_may_update_window(tp, ack, ack_seq, nwin)) {
                flag |= FLAG_WIN_UPDATE;
                tcp_update_wl(tp, ack_seq);

                if (tp->snd_wnd != nwin) {
                        tp->snd_wnd = nwin;

                        /* Note, it is the only place, where
                         * fast path is recovered for sending TCP.
                         */
                        tp->pred_flags = 0;
                        tcp_fast_path_check(sk);

                        if (!tcp_write_queue_empty(sk))
                                tcp_slow_start_after_idle_check(sk);

                        if (nwin > tp->max_window) {
                                tp->max_window = nwin;
                                tcp_sync_mss(sk, inet_csk(sk)->icsk_pmtu_cookie);
                        }
                }
        }

        tcp_snd_una_update(tp, ack);

        return flag;
}

static bool __tcp_oow_rate_limited(struct net *net, int mib_idx,
                                   u32 *last_oow_ack_time)
{
        if (*last_oow_ack_time) {
                s32 elapsed = (s32)(tcp_jiffies32 - *last_oow_ack_time);

                if (0 <= elapsed && elapsed < net->ipv4.sysctl_tcp_invalid_ratelimit) {
                        NET_INC_STATS(net, mib_idx);
                        return true;    /* rate-limited: don't send yet! */
                }
        }

        *last_oow_ack_time = tcp_jiffies32;

        return false;   /* not rate-limited: go ahead, send dupack now! */
}

/* Return true if we're currently rate-limiting out-of-window ACKs and
 * thus shouldn't send a dupack right now. We rate-limit dupacks in
 * response to out-of-window SYNs or ACKs to mitigate ACK loops or DoS
 * attacks that send repeated SYNs or ACKs for the same connection. To
 * do this, we do not send a duplicate SYNACK or ACK if the remote
 * endpoint is sending out-of-window SYNs or pure ACKs at a high rate.
 */
bool tcp_oow_rate_limited(struct net *net, const struct sk_buff *skb,
                          int mib_idx, u32 *last_oow_ack_time)
{
        /* Data packets without SYNs are not likely part of an ACK loop. */
        if ((TCP_SKB_CB(skb)->seq != TCP_SKB_CB(skb)->end_seq) &&
            !tcp_hdr(skb)->syn)
                return false;

        return __tcp_oow_rate_limited(net, mib_idx, last_oow_ack_time);
}

/* RFC 5961 7 [ACK Throttling] */
static void tcp_send_challenge_ack(struct sock *sk)
{
        /* unprotected vars, we dont care of overwrites */
        static u32 challenge_timestamp;
        static unsigned int challenge_count;
        struct tcp_sock *tp = tcp_sk(sk);
        struct net *net = sock_net(sk);
        u32 count, now;

        /* First check our per-socket dupack rate limit. */
        if (__tcp_oow_rate_limited(net,
                                   LINUX_MIB_TCPACKSKIPPEDCHALLENGE,
                                   &tp->last_oow_ack_time))
                return;

        /* Then check host-wide RFC 5961 rate limit. */
        now = jiffies / HZ;
        if (now != challenge_timestamp) {
                u32 ack_limit = net->ipv4.sysctl_tcp_challenge_ack_limit;
                u32 half = (ack_limit + 1) >> 1;

                challenge_timestamp = now;
                WRITE_ONCE(challenge_count, half + prandom_u32_max(ack_limit));
        }
        count = READ_ONCE(challenge_count);
        if (count > 0) {
                WRITE_ONCE(challenge_count, count - 1);
                NET_INC_STATS(net, LINUX_MIB_TCPCHALLENGEACK);
                tcp_send_ack(sk);
        }
}

static void tcp_store_ts_recent(struct tcp_sock *tp)
{
        tp->rx_opt.ts_recent = tp->rx_opt.rcv_tsval;
        tp->rx_opt.ts_recent_stamp = ktime_get_seconds();
}

static void tcp_replace_ts_recent(struct tcp_sock *tp, u32 seq)
{
        if (tp->rx_opt.saw_tstamp && !after(seq, tp->rcv_wup)) {
                /* PAWS bug workaround wrt. ACK frames, the PAWS discard
                 * extra check below makes sure this can only happen
                 * for pure ACK frames.  -DaveM
                 *
                 * Not only, also it occurs for expired timestamps.
                 */

                if (tcp_paws_check(&tp->rx_opt, 0))
                        tcp_store_ts_recent(tp);
        }
}

/* This routine deals with acks during a TLP episode and ends an episode by
 * resetting tlp_high_seq. Ref: TLP algorithm in draft-ietf-tcpm-rack
 */
static void tcp_process_tlp_ack(struct sock *sk, u32 ack, int flag)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (before(ack, tp->tlp_high_seq))
                return;

        if (!tp->tlp_retrans) {
                /* TLP of new data has been acknowledged */
                tp->tlp_high_seq = 0;
        } else if (flag & FLAG_DSACK_TLP) {
                /* This DSACK means original and TLP probe arrived; no loss */
                tp->tlp_high_seq = 0;
        } else if (after(ack, tp->tlp_high_seq)) {
                /* ACK advances: there was a loss, so reduce cwnd. Reset
                 * tlp_high_seq in tcp_init_cwnd_reduction()
                 */
                tcp_init_cwnd_reduction(sk);
                tcp_set_ca_state(sk, TCP_CA_CWR);
                tcp_end_cwnd_reduction(sk);
                tcp_try_keep_open(sk);
                NET_INC_STATS(sock_net(sk),
                                LINUX_MIB_TCPLOSSPROBERECOVERY);
        } else if (!(flag & (FLAG_SND_UNA_ADVANCED |
                             FLAG_NOT_DUP | FLAG_DATA_SACKED))) {
                /* Pure dupack: original and TLP probe arrived; no loss */
                tp->tlp_high_seq = 0;
        }
}

static inline void tcp_in_ack_event(struct sock *sk, u32 flags)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);

        if (icsk->icsk_ca_ops->in_ack_event)
                icsk->icsk_ca_ops->in_ack_event(sk, flags);
}

/* Congestion control has updated the cwnd already. So if we're in
 * loss recovery then now we do any new sends (for FRTO) or
 * retransmits (for CA_Loss or CA_recovery) that make sense.
 */
static void tcp_xmit_recovery(struct sock *sk, int rexmit)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (rexmit == REXMIT_NONE || sk->sk_state == TCP_SYN_SENT)
                return;

        if (unlikely(rexmit == REXMIT_NEW)) {
                __tcp_push_pending_frames(sk, tcp_current_mss(sk),
                                          TCP_NAGLE_OFF);
                if (after(tp->snd_nxt, tp->high_seq))
                        return;
                tp->frto = 0;
        }
        tcp_xmit_retransmit_queue(sk);
}

/* Returns the number of packets newly acked or sacked by the current ACK */
static u32 tcp_newly_delivered(struct sock *sk, u32 prior_delivered, int flag)
{
        const struct net *net = sock_net(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        u32 delivered;

        delivered = tp->delivered - prior_delivered;
        NET_ADD_STATS(net, LINUX_MIB_TCPDELIVERED, delivered);
        if (flag & FLAG_ECE)
                NET_ADD_STATS(net, LINUX_MIB_TCPDELIVEREDCE, delivered);

        return delivered;
}

/* This routine deals with incoming acks, but not outgoing ones. */
static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        struct tcp_sacktag_state sack_state;
        struct rate_sample rs = { .prior_delivered = 0 };
        u32 prior_snd_una = tp->snd_una;
        bool is_sack_reneg = tp->is_sack_reneg;
        u32 ack_seq = TCP_SKB_CB(skb)->seq;
        u32 ack = TCP_SKB_CB(skb)->ack_seq;
        int num_dupack = 0;
        int prior_packets = tp->packets_out;
        u32 delivered = tp->delivered;
        u32 lost = tp->lost;
        int rexmit = REXMIT_NONE; /* Flag to (re)transmit to recover losses */
        u32 prior_fack;

        sack_state.first_sackt = 0;
        sack_state.rate = &rs;
        sack_state.sack_delivered = 0;

        /* We very likely will need to access rtx queue. */
        prefetch(sk->tcp_rtx_queue.rb_node);

        /* If the ack is older than previous acks
         * then we can probably ignore it.
         */
        if (before(ack, prior_snd_una)) {
                /* RFC 5961 5.2 [Blind Data Injection Attack].[Mitigation] */
                if (before(ack, prior_snd_una - tp->max_window)) {
                        if (!(flag & FLAG_NO_CHALLENGE_ACK))
                                tcp_send_challenge_ack(sk);
                        return -SKB_DROP_REASON_TCP_TOO_OLD_ACK;
                }
                goto old_ack;
        }

        /* If the ack includes data we haven't sent yet, discard
         * this segment (RFC793 Section 3.9).
         */
        if (after(ack, tp->snd_nxt))
                return -SKB_DROP_REASON_TCP_ACK_UNSENT_DATA;

        if (after(ack, prior_snd_una)) {
                flag |= FLAG_SND_UNA_ADVANCED;
                icsk->icsk_retransmits = 0;

#if IS_ENABLED(CONFIG_TLS_DEVICE)
                if (static_branch_unlikely(&clean_acked_data_enabled.key))
                        if (icsk->icsk_clean_acked)
                                icsk->icsk_clean_acked(sk, ack);
#endif
        }

        prior_fack = tcp_is_sack(tp) ? tcp_highest_sack_seq(tp) : tp->snd_una;
        rs.prior_in_flight = tcp_packets_in_flight(tp);

        /* ts_recent update must be made after we are sure that the packet
         * is in window.
         */
        if (flag & FLAG_UPDATE_TS_RECENT)
                tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);

        if ((flag & (FLAG_SLOWPATH | FLAG_SND_UNA_ADVANCED)) ==
            FLAG_SND_UNA_ADVANCED) {
                /* Window is constant, pure forward advance.
                 * No more checks are required.
                 * Note, we use the fact that SND.UNA>=SND.WL2.
                 */
                tcp_update_wl(tp, ack_seq);
                tcp_snd_una_update(tp, ack);
                flag |= FLAG_WIN_UPDATE;

                tcp_in_ack_event(sk, CA_ACK_WIN_UPDATE);

                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPACKS);
        } else {
                u32 ack_ev_flags = CA_ACK_SLOWPATH;

                if (ack_seq != TCP_SKB_CB(skb)->end_seq)
                        flag |= FLAG_DATA;
                else
                        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPUREACKS);

                flag |= tcp_ack_update_window(sk, skb, ack, ack_seq);

                if (TCP_SKB_CB(skb)->sacked)
                        flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
                                                        &sack_state);

                if (tcp_ecn_rcv_ecn_echo(tp, tcp_hdr(skb))) {
                        flag |= FLAG_ECE;
                        ack_ev_flags |= CA_ACK_ECE;
                }

                if (sack_state.sack_delivered)
                        tcp_count_delivered(tp, sack_state.sack_delivered,
                                            flag & FLAG_ECE);

                if (flag & FLAG_WIN_UPDATE)
                        ack_ev_flags |= CA_ACK_WIN_UPDATE;

                tcp_in_ack_event(sk, ack_ev_flags);
        }

        /* This is a deviation from RFC3168 since it states that:
         * "When the TCP data sender is ready to set the CWR bit after reducing
         * the congestion window, it SHOULD set the CWR bit only on the first
         * new data packet that it transmits."
         * We accept CWR on pure ACKs to be more robust
         * with widely-deployed TCP implementations that do this.
         */
        tcp_ecn_accept_cwr(sk, skb);

        /* We passed data and got it acked, remove any soft error
         * log. Something worked...
         */
        sk->sk_err_soft = 0;
        icsk->icsk_probes_out = 0;
        tp->rcv_tstamp = tcp_jiffies32;
        if (!prior_packets)
                goto no_queue;

        /* See if we can take anything off of the retransmit queue. */
        flag |= tcp_clean_rtx_queue(sk, skb, prior_fack, prior_snd_una,
                                    &sack_state, flag & FLAG_ECE);

        tcp_rack_update_reo_wnd(sk, &rs);

        if (tp->tlp_high_seq)
                tcp_process_tlp_ack(sk, ack, flag);

        if (tcp_ack_is_dubious(sk, flag)) {
                if (!(flag & (FLAG_SND_UNA_ADVANCED |
                              FLAG_NOT_DUP | FLAG_DSACKING_ACK))) {
                        num_dupack = 1;
                        /* Consider if pure acks were aggregated in tcp_add_backlog() */
                        if (!(flag & FLAG_DATA))
                                num_dupack = max_t(u16, 1, skb_shinfo(skb)->gso_segs);
                }
                tcp_fastretrans_alert(sk, prior_snd_una, num_dupack, &flag,
                                      &rexmit);
        }

        /* If needed, reset TLP/RTO timer when RACK doesn't set. */
        if (flag & FLAG_SET_XMIT_TIMER)
                tcp_set_xmit_timer(sk);

        if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP))
                sk_dst_confirm(sk);

        delivered = tcp_newly_delivered(sk, delivered, flag);
        lost = tp->lost - lost;                 /* freshly marked lost */
        rs.is_ack_delayed = !!(flag & FLAG_ACK_MAYBE_DELAYED);
        tcp_rate_gen(sk, delivered, lost, is_sack_reneg, sack_state.rate);
        tcp_cong_control(sk, ack, delivered, flag, sack_state.rate);
        tcp_xmit_recovery(sk, rexmit);
        return 1;

no_queue:
        /* If data was DSACKed, see if we can undo a cwnd reduction. */
        if (flag & FLAG_DSACKING_ACK) {
                tcp_fastretrans_alert(sk, prior_snd_una, num_dupack, &flag,
                                      &rexmit);
                tcp_newly_delivered(sk, delivered, flag);
        }
        /* If this ack opens up a zero window, clear backoff.  It was
         * being used to time the probes, and is probably far higher than
         * it needs to be for normal retransmission.
         */
        tcp_ack_probe(sk);

        if (tp->tlp_high_seq)
                tcp_process_tlp_ack(sk, ack, flag);
        return 1;

old_ack:
        /* If data was SACKed, tag it and see if we should send more data.
         * If data was DSACKed, see if we can undo a cwnd reduction.
         */
        if (TCP_SKB_CB(skb)->sacked) {
                flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
                                                &sack_state);
                tcp_fastretrans_alert(sk, prior_snd_una, num_dupack, &flag,
                                      &rexmit);
                tcp_newly_delivered(sk, delivered, flag);
                tcp_xmit_recovery(sk, rexmit);
        }

        return 0;
}

static void tcp_parse_fastopen_option(int len, const unsigned char *cookie,
                                      bool syn, struct tcp_fastopen_cookie *foc,
                                      bool exp_opt)
{
        /* Valid only in SYN or SYN-ACK with an even length.  */
        if (!foc || !syn || len < 0 || (len & 1))
                return;

        if (len >= TCP_FASTOPEN_COOKIE_MIN &&
            len <= TCP_FASTOPEN_COOKIE_MAX)
                memcpy(foc->val, cookie, len);
        else if (len != 0)
                len = -1;
        foc->len = len;
        foc->exp = exp_opt;
}

static bool smc_parse_options(const struct tcphdr *th,
                              struct tcp_options_received *opt_rx,
                              const unsigned char *ptr,
                              int opsize)
{
#if IS_ENABLED(CONFIG_SMC)
        if (static_branch_unlikely(&tcp_have_smc)) {
                if (th->syn && !(opsize & 1) &&
                    opsize >= TCPOLEN_EXP_SMC_BASE &&
                    get_unaligned_be32(ptr) == TCPOPT_SMC_MAGIC) {
                        opt_rx->smc_ok = 1;
                        return true;
                }
        }
#endif
        return false;
}

/* Try to parse the MSS option from the TCP header. Return 0 on failure, clamped
 * value on success.
 */
u16 tcp_parse_mss_option(const struct tcphdr *th, u16 user_mss)
{
        const unsigned char *ptr = (const unsigned char *)(th + 1);
        int length = (th->doff * 4) - sizeof(struct tcphdr);
        u16 mss = 0;

        while (length > 0) {
                int opcode = *ptr++;
                int opsize;

                switch (opcode) {
                case TCPOPT_EOL:
                        return mss;
                case TCPOPT_NOP:        /* Ref: RFC 793 section 3.1 */
                        length--;
                        continue;
                default:
                        if (length < 2)
                                return mss;
                        opsize = *ptr++;
                        if (opsize < 2) /* "silly options" */
                                return mss;
                        if (opsize > length)
                                return mss;     /* fail on partial options */
                        if (opcode == TCPOPT_MSS && opsize == TCPOLEN_MSS) {
                                u16 in_mss = get_unaligned_be16(ptr);

                                if (in_mss) {
                                        if (user_mss && user_mss < in_mss)
                                                in_mss = user_mss;
                                        mss = in_mss;
                                }
                        }
                        ptr += opsize - 2;
                        length -= opsize;
                }
        }
        return mss;
}
EXPORT_SYMBOL_GPL(tcp_parse_mss_option);

/* Look for tcp options. Normally only called on SYN and SYNACK packets.
 * But, this can also be called on packets in the established flow when
 * the fast version below fails.
 */
void tcp_parse_options(const struct net *net,
                       const struct sk_buff *skb,
                       struct tcp_options_received *opt_rx, int estab,
                       struct tcp_fastopen_cookie *foc)
{
        const unsigned char *ptr;
        const struct tcphdr *th = tcp_hdr(skb);
        int length = (th->doff * 4) - sizeof(struct tcphdr);

        ptr = (const unsigned char *)(th + 1);
        opt_rx->saw_tstamp = 0;
        opt_rx->saw_unknown = 0;

        while (length > 0) {
                int opcode = *ptr++;
                int opsize;

                switch (opcode) {
                case TCPOPT_EOL:
                        return;
                case TCPOPT_NOP:        /* Ref: RFC 793 section 3.1 */
                        length--;
                        continue;
                default:
                        if (length < 2)
                                return;
                        opsize = *ptr++;
                        if (opsize < 2) /* "silly options" */
                                return;
                        if (opsize > length)
                                return; /* don't parse partial options */
                        switch (opcode) {
                        case TCPOPT_MSS:
                                if (opsize == TCPOLEN_MSS && th->syn && !estab) {
                                        u16 in_mss = get_unaligned_be16(ptr);
                                        if (in_mss) {
                                                if (opt_rx->user_mss &&
                                                    opt_rx->user_mss < in_mss)
                                                        in_mss = opt_rx->user_mss;
                                                opt_rx->mss_clamp = in_mss;
                                        }
                                }
                                break;
                        case TCPOPT_WINDOW:
                                if (opsize == TCPOLEN_WINDOW && th->syn &&
                                    !estab && net->ipv4.sysctl_tcp_window_scaling) {
                                        __u8 snd_wscale = *(__u8 *)ptr;
                                        opt_rx->wscale_ok = 1;
                                        if (snd_wscale > TCP_MAX_WSCALE) {
                                                net_info_ratelimited("%s: Illegal window scaling value %d > %u received\n",
                                                                     __func__,
                                                                     snd_wscale,
                                                                     TCP_MAX_WSCALE);
                                                snd_wscale = TCP_MAX_WSCALE;
                                        }
                                        opt_rx->snd_wscale = snd_wscale;
                                }
                                break;
                        case TCPOPT_TIMESTAMP:
                                if ((opsize == TCPOLEN_TIMESTAMP) &&
                                    ((estab && opt_rx->tstamp_ok) ||
                                     (!estab && net->ipv4.sysctl_tcp_timestamps))) {
                                        opt_rx->saw_tstamp = 1;
                                        opt_rx->rcv_tsval = get_unaligned_be32(ptr);
                                        opt_rx->rcv_tsecr = get_unaligned_be32(ptr + 4);
                                }
                                break;
                        case TCPOPT_SACK_PERM:
                                if (opsize == TCPOLEN_SACK_PERM && th->syn &&
                                    !estab && net->ipv4.sysctl_tcp_sack) {
                                        opt_rx->sack_ok = TCP_SACK_SEEN;
                                        tcp_sack_reset(opt_rx);
                                }
                                break;

                        case TCPOPT_SACK:
                                if ((opsize >= (TCPOLEN_SACK_BASE + TCPOLEN_SACK_PERBLOCK)) &&
                                   !((opsize - TCPOLEN_SACK_BASE) % TCPOLEN_SACK_PERBLOCK) &&
                                   opt_rx->sack_ok) {
                                        TCP_SKB_CB(skb)->sacked = (ptr - 2) - (unsigned char *)th;
                                }
                                break;
#ifdef CONFIG_TCP_MD5SIG
                        case TCPOPT_MD5SIG:
                                /*
                                 * The MD5 Hash has already been
                                 * checked (see tcp_v{4,6}_do_rcv()).
                                 */
                                break;
#endif
                        case TCPOPT_FASTOPEN:
                                tcp_parse_fastopen_option(
                                        opsize - TCPOLEN_FASTOPEN_BASE,
                                        ptr, th->syn, foc, false);
                                break;

                        case TCPOPT_EXP:
                                /* Fast Open option shares code 254 using a
                                 * 16 bits magic number.
                                 */
                                if (opsize >= TCPOLEN_EXP_FASTOPEN_BASE &&
                                    get_unaligned_be16(ptr) ==
                                    TCPOPT_FASTOPEN_MAGIC) {
                                        tcp_parse_fastopen_option(opsize -
                                                TCPOLEN_EXP_FASTOPEN_BASE,
                                                ptr + 2, th->syn, foc, true);
                                        break;
                                }

                                if (smc_parse_options(th, opt_rx, ptr, opsize))
                                        break;

                                opt_rx->saw_unknown = 1;
                                break;

                        default:
                                opt_rx->saw_unknown = 1;
                        }
                        ptr += opsize-2;
                        length -= opsize;
                }
        }
}
EXPORT_SYMBOL(tcp_parse_options);

static bool tcp_parse_aligned_timestamp(struct tcp_sock *tp, const struct tcphdr *th)
{
        const __be32 *ptr = (const __be32 *)(th + 1);

        if (*ptr == htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16)
                          | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP)) {
                tp->rx_opt.saw_tstamp = 1;
                ++ptr;
                tp->rx_opt.rcv_tsval = ntohl(*ptr);
                ++ptr;
                if (*ptr)
                        tp->rx_opt.rcv_tsecr = ntohl(*ptr) - tp->tsoffset;
                else
                        tp->rx_opt.rcv_tsecr = 0;
                return true;
        }
        return false;
}

/* Fast parse options. This hopes to only see timestamps.
 * If it is wrong it falls back on tcp_parse_options().
 */
static bool tcp_fast_parse_options(const struct net *net,
                                   const struct sk_buff *skb,
                                   const struct tcphdr *th, struct tcp_sock *tp)
{
        /* In the spirit of fast parsing, compare doff directly to constant
         * values.  Because equality is used, short doff can be ignored here.
         */
        if (th->doff == (sizeof(*th) / 4)) {
                tp->rx_opt.saw_tstamp = 0;
                return false;
        } else if (tp->rx_opt.tstamp_ok &&
                   th->doff == ((sizeof(*th) + TCPOLEN_TSTAMP_ALIGNED) / 4)) {
                if (tcp_parse_aligned_timestamp(tp, th))
                        return true;
        }

        tcp_parse_options(net, skb, &tp->rx_opt, 1, NULL);
        if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
                tp->rx_opt.rcv_tsecr -= tp->tsoffset;

        return true;
}

#ifdef CONFIG_TCP_MD5SIG
/*
 * Parse MD5 Signature option
 */
const u8 *tcp_parse_md5sig_option(const struct tcphdr *th)
{
        int length = (th->doff << 2) - sizeof(*th);
        const u8 *ptr = (const u8 *)(th + 1);

        /* If not enough data remaining, we can short cut */
        while (length >= TCPOLEN_MD5SIG) {
                int opcode = *ptr++;
                int opsize;

                switch (opcode) {
                case TCPOPT_EOL:
                        return NULL;
                case TCPOPT_NOP:
                        length--;
                        continue;
                default:
                        opsize = *ptr++;
                        if (opsize < 2 || opsize > length)
                                return NULL;
                        if (opcode == TCPOPT_MD5SIG)
                                return opsize == TCPOLEN_MD5SIG ? ptr : NULL;
                }
                ptr += opsize - 2;
                length -= opsize;
        }
        return NULL;
}
EXPORT_SYMBOL(tcp_parse_md5sig_option);
#endif

/* Sorry, PAWS as specified is broken wrt. pure-ACKs -DaveM
 *
 * It is not fatal. If this ACK does _not_ change critical state (seqs, window)
 * it can pass through stack. So, the following predicate verifies that
 * this segment is not used for anything but congestion avoidance or
 * fast retransmit. Moreover, we even are able to eliminate most of such
 * second order effects, if we apply some small "replay" window (~RTO)
 * to timestamp space.
 *
 * All these measures still do not guarantee that we reject wrapped ACKs
 * on networks with high bandwidth, when sequence space is recycled fastly,
 * but it guarantees that such events will be very rare and do not affect
 * connection seriously. This doesn't look nice, but alas, PAWS is really
 * buggy extension.
 *
 * [ Later note. Even worse! It is buggy for segments _with_ data. RFC
 * states that events when retransmit arrives after original data are rare.
 * It is a blatant lie. VJ forgot about fast retransmit! 8)8) It is
 * the biggest problem on large power networks even with minor reordering.
 * OK, let's give it small replay window. If peer clock is even 1hz, it is safe
 * up to bandwidth of 18Gigabit/sec. 8) ]
 */

static int tcp_disordered_ack(const struct sock *sk, const struct sk_buff *skb)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        const struct tcphdr *th = tcp_hdr(skb);
        u32 seq = TCP_SKB_CB(skb)->seq;
        u32 ack = TCP_SKB_CB(skb)->ack_seq;

        return (/* 1. Pure ACK with correct sequence number. */
                (th->ack && seq == TCP_SKB_CB(skb)->end_seq && seq == tp->rcv_nxt) &&

                /* 2. ... and duplicate ACK. */
                ack == tp->snd_una &&

                /* 3. ... and does not update window. */
                !tcp_may_update_window(tp, ack, seq, ntohs(th->window) << tp->rx_opt.snd_wscale) &&

                /* 4. ... and sits in replay window. */
                (s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) <= (inet_csk(sk)->icsk_rto * 1024) / HZ);
}

static inline bool tcp_paws_discard(const struct sock *sk,
                                   const struct sk_buff *skb)
{
        const struct tcp_sock *tp = tcp_sk(sk);

        return !tcp_paws_check(&tp->rx_opt, TCP_PAWS_WINDOW) &&
               !tcp_disordered_ack(sk, skb);
}

/* Check segment sequence number for validity.
 *
 * Segment controls are considered valid, if the segment
 * fits to the window after truncation to the window. Acceptability
 * of data (and SYN, FIN, of course) is checked separately.
 * See tcp_data_queue(), for example.
 *
 * Also, controls (RST is main one) are accepted using RCV.WUP instead
 * of RCV.NXT. Peer still did not advance his SND.UNA when we
 * delayed ACK, so that hisSND.UNA<=ourRCV.WUP.
 * (borrowed from freebsd)
 */

static inline bool tcp_sequence(const struct tcp_sock *tp, u32 seq, u32 end_seq)
{
        return  !before(end_seq, tp->rcv_wup) &&
                !after(seq, tp->rcv_nxt + tcp_receive_window(tp));
}

/* When we get a reset we do this. */
void tcp_reset(struct sock *sk, struct sk_buff *skb)
{
        trace_tcp_receive_reset(sk);

        /* mptcp can't tell us to ignore reset pkts,
         * so just ignore the return value of mptcp_incoming_options().
         */
        if (sk_is_mptcp(sk))
                mptcp_incoming_options(sk, skb);

        /* We want the right error as BSD sees it (and indeed as we do). */
        switch (sk->sk_state) {
        case TCP_SYN_SENT:
                sk->sk_err = ECONNREFUSED;
                break;
        case TCP_CLOSE_WAIT:
                sk->sk_err = EPIPE;
                break;
        case TCP_CLOSE:
                return;
        default:
                sk->sk_err = ECONNRESET;
        }
        /* This barrier is coupled with smp_rmb() in tcp_poll() */
        smp_wmb();

        tcp_write_queue_purge(sk);
        tcp_done(sk);

        if (!sock_flag(sk, SOCK_DEAD))
                sk_error_report(sk);
}

/*
 *      Process the FIN bit. This now behaves as it is supposed to work
 *      and the FIN takes effect when it is validly part of sequence
 *      space. Not before when we get holes.
 *
 *      If we are ESTABLISHED, a received fin moves us to CLOSE-WAIT
 *      (and thence onto LAST-ACK and finally, CLOSE, we never enter
 *      TIME-WAIT)
 *
 *      If we are in FINWAIT-1, a received FIN indicates simultaneous
 *      close and we go into CLOSING (and later onto TIME-WAIT)
 *
 *      If we are in FINWAIT-2, a received FIN moves us to TIME-WAIT.
 */
void tcp_fin(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        inet_csk_schedule_ack(sk);

        sk->sk_shutdown |= RCV_SHUTDOWN;
        sock_set_flag(sk, SOCK_DONE);

        switch (sk->sk_state) {
        case TCP_SYN_RECV:
        case TCP_ESTABLISHED:
                /* Move to CLOSE_WAIT */
                tcp_set_state(sk, TCP_CLOSE_WAIT);
                inet_csk_enter_pingpong_mode(sk);
                break;

        case TCP_CLOSE_WAIT:
        case TCP_CLOSING:
                /* Received a retransmission of the FIN, do
                 * nothing.
                 */
                break;
        case TCP_LAST_ACK:
                /* RFC793: Remain in the LAST-ACK state. */
                break;

        case TCP_FIN_WAIT1:
                /* This case occurs when a simultaneous close
                 * happens, we must ack the received FIN and
                 * enter the CLOSING state.
                 */
                tcp_send_ack(sk);
                tcp_set_state(sk, TCP_CLOSING);
                break;
        case TCP_FIN_WAIT2:
                /* Received a FIN -- send ACK and enter TIME_WAIT. */
                tcp_send_ack(sk);
                tcp_time_wait(sk, TCP_TIME_WAIT, 0);
                break;
        default:
                /* Only TCP_LISTEN and TCP_CLOSE are left, in these
                 * cases we should never reach this piece of code.
                 */
                pr_err("%s: Impossible, sk->sk_state=%d\n",
                       __func__, sk->sk_state);
                break;
        }

        /* It _is_ possible, that we have something out-of-order _after_ FIN.
         * Probably, we should reset in this case. For now drop them.
         */
        skb_rbtree_purge(&tp->out_of_order_queue);
        if (tcp_is_sack(tp))
                tcp_sack_reset(&tp->rx_opt);

        if (!sock_flag(sk, SOCK_DEAD)) {
                sk->sk_state_change(sk);

                /* Do not send POLL_HUP for half duplex close. */
                if (sk->sk_shutdown == SHUTDOWN_MASK ||
                    sk->sk_state == TCP_CLOSE)
                        sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_HUP);
                else
                        sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
        }
}

static inline bool tcp_sack_extend(struct tcp_sack_block *sp, u32 seq,
                                  u32 end_seq)
{
        if (!after(seq, sp->end_seq) && !after(sp->start_seq, end_seq)) {
                if (before(seq, sp->start_seq))
                        sp->start_seq = seq;
                if (after(end_seq, sp->end_seq))
                        sp->end_seq = end_seq;
                return true;
        }
        return false;
}

static void tcp_dsack_set(struct sock *sk, u32 seq, u32 end_seq)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tcp_is_sack(tp) && sock_net(sk)->ipv4.sysctl_tcp_dsack) {
                int mib_idx;

                if (before(seq, tp->rcv_nxt))
                        mib_idx = LINUX_MIB_TCPDSACKOLDSENT;
                else
                        mib_idx = LINUX_MIB_TCPDSACKOFOSENT;

                NET_INC_STATS(sock_net(sk), mib_idx);

                tp->rx_opt.dsack = 1;
                tp->duplicate_sack[0].start_seq = seq;
                tp->duplicate_sack[0].end_seq = end_seq;
        }
}

static void tcp_dsack_extend(struct sock *sk, u32 seq, u32 end_seq)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (!tp->rx_opt.dsack)
                tcp_dsack_set(sk, seq, end_seq);
        else
                tcp_sack_extend(tp->duplicate_sack, seq, end_seq);
}

static void tcp_rcv_spurious_retrans(struct sock *sk, const struct sk_buff *skb)
{
        /* When the ACK path fails or drops most ACKs, the sender would
         * timeout and spuriously retransmit the same segment repeatedly.
         * The receiver remembers and reflects via DSACKs. Leverage the
         * DSACK state and change the txhash to re-route speculatively.
         */
        if (TCP_SKB_CB(skb)->seq == tcp_sk(sk)->duplicate_sack[0].start_seq &&
            sk_rethink_txhash(sk))
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDUPLICATEDATAREHASH);
}

static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
            before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
                NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
                tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);

                if (tcp_is_sack(tp) && sock_net(sk)->ipv4.sysctl_tcp_dsack) {
                        u32 end_seq = TCP_SKB_CB(skb)->end_seq;

                        tcp_rcv_spurious_retrans(sk, skb);
                        if (after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))
                                end_seq = tp->rcv_nxt;
                        tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, end_seq);
                }
        }

        tcp_send_ack(sk);
}

/* These routines update the SACK block as out-of-order packets arrive or
 * in-order packets close up the sequence space.
 */
static void tcp_sack_maybe_coalesce(struct tcp_sock *tp)
{
        int this_sack;
        struct tcp_sack_block *sp = &tp->selective_acks[0];
        struct tcp_sack_block *swalk = sp + 1;

        /* See if the recent change to the first SACK eats into
         * or hits the sequence space of other SACK blocks, if so coalesce.
         */
        for (this_sack = 1; this_sack < tp->rx_opt.num_sacks;) {
                if (tcp_sack_extend(sp, swalk->start_seq, swalk->end_seq)) {
                        int i;

                        /* Zap SWALK, by moving every further SACK up by one slot.
                         * Decrease num_sacks.
                         */
                        tp->rx_opt.num_sacks--;
                        for (i = this_sack; i < tp->rx_opt.num_sacks; i++)
                                sp[i] = sp[i + 1];
                        continue;
                }
                this_sack++;
                swalk++;
        }
}

static void tcp_sack_compress_send_ack(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (!tp->compressed_ack)
                return;

        if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
                __sock_put(sk);

        /* Since we have to send one ack finally,
         * substract one from tp->compressed_ack to keep
         * LINUX_MIB_TCPACKCOMPRESSED accurate.
         */
        NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
                      tp->compressed_ack - 1);

        tp->compressed_ack = 0;
        tcp_send_ack(sk);
}

/* Reasonable amount of sack blocks included in TCP SACK option
 * The max is 4, but this becomes 3 if TCP timestamps are there.
 * Given that SACK packets might be lost, be conservative and use 2.
 */
#define TCP_SACK_BLOCKS_EXPECTED 2

static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct tcp_sack_block *sp = &tp->selective_acks[0];
        int cur_sacks = tp->rx_opt.num_sacks;
        int this_sack;

        if (!cur_sacks)
                goto new_sack;

        for (this_sack = 0; this_sack < cur_sacks; this_sack++, sp++) {
                if (tcp_sack_extend(sp, seq, end_seq)) {
                        if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
                                tcp_sack_compress_send_ack(sk);
                        /* Rotate this_sack to the first one. */
                        for (; this_sack > 0; this_sack--, sp--)
                                swap(*sp, *(sp - 1));
                        if (cur_sacks > 1)
                                tcp_sack_maybe_coalesce(tp);
                        return;
                }
        }

        if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
                tcp_sack_compress_send_ack(sk);

        /* Could not find an adjacent existing SACK, build a new one,
         * put it at the front, and shift everyone else down.  We
         * always know there is at least one SACK present already here.
         *
         * If the sack array is full, forget about the last one.
         */
        if (this_sack >= TCP_NUM_SACKS) {
                this_sack--;
                tp->rx_opt.num_sacks--;
                sp--;
        }
        for (; this_sack > 0; this_sack--, sp--)
                *sp = *(sp - 1);

new_sack:
        /* Build the new head SACK, and we're done. */
        sp->start_seq = seq;
        sp->end_seq = end_seq;
        tp->rx_opt.num_sacks++;
}

/* RCV.NXT advances, some SACKs should be eaten. */

static void tcp_sack_remove(struct tcp_sock *tp)
{
        struct tcp_sack_block *sp = &tp->selective_acks[0];
        int num_sacks = tp->rx_opt.num_sacks;
        int this_sack;

        /* Empty ofo queue, hence, all the SACKs are eaten. Clear. */
        if (RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
                tp->rx_opt.num_sacks = 0;
                return;
        }

        for (this_sack = 0; this_sack < num_sacks;) {
                /* Check if the start of the sack is covered by RCV.NXT. */
                if (!before(tp->rcv_nxt, sp->start_seq)) {
                        int i;

                        /* RCV.NXT must cover all the block! */
                        WARN_ON(before(tp->rcv_nxt, sp->end_seq));

                        /* Zap this SACK, by moving forward any other SACKS. */
                        for (i = this_sack+1; i < num_sacks; i++)
                                tp->selective_acks[i-1] = tp->selective_acks[i];
                        num_sacks--;
                        continue;
                }
                this_sack++;
                sp++;
        }
        tp->rx_opt.num_sacks = num_sacks;
}

/**
 * tcp_try_coalesce - try to merge skb to prior one
 * @sk: socket
 * @to: prior buffer
 * @from: buffer to add in queue
 * @fragstolen: pointer to boolean
 *
 * Before queueing skb @from after @to, try to merge them
 * to reduce overall memory use and queue lengths, if cost is small.
 * Packets in ofo or receive queues can stay a long time.
 * Better try to coalesce them right now to avoid future collapses.
 * Returns true if caller should free @from instead of queueing it
 */
static bool tcp_try_coalesce(struct sock *sk,
                             struct sk_buff *to,
                             struct sk_buff *from,
                             bool *fragstolen)
{
        int delta;

        *fragstolen = false;

        /* Its possible this segment overlaps with prior segment in queue */
        if (TCP_SKB_CB(from)->seq != TCP_SKB_CB(to)->end_seq)
                return false;

        if (!mptcp_skb_can_collapse(to, from))
                return false;

#ifdef CONFIG_TLS_DEVICE
        if (from->decrypted != to->decrypted)
                return false;
#endif

        if (!skb_try_coalesce(to, from, fragstolen, &delta))
                return false;

        atomic_add(delta, &sk->sk_rmem_alloc);
        sk_mem_charge(sk, delta);
        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
        TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
        TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
        TCP_SKB_CB(to)->tcp_flags |= TCP_SKB_CB(from)->tcp_flags;

        if (TCP_SKB_CB(from)->has_rxtstamp) {
                TCP_SKB_CB(to)->has_rxtstamp = true;
                to->tstamp = from->tstamp;
                skb_hwtstamps(to)->hwtstamp = skb_hwtstamps(from)->hwtstamp;
        }

        return true;
}

static bool tcp_ooo_try_coalesce(struct sock *sk,
                             struct sk_buff *to,
                             struct sk_buff *from,
                             bool *fragstolen)
{
        bool res = tcp_try_coalesce(sk, to, from, fragstolen);

        /* In case tcp_drop_reason() is called later, update to->gso_segs */
        if (res) {
                u32 gso_segs = max_t(u16, 1, skb_shinfo(to)->gso_segs) +
                               max_t(u16, 1, skb_shinfo(from)->gso_segs);

                skb_shinfo(to)->gso_segs = min_t(u32, gso_segs, 0xFFFF);
        }
        return res;
}

static void tcp_drop_reason(struct sock *sk, struct sk_buff *skb,
                            enum skb_drop_reason reason)
{
        sk_drops_add(sk, skb);
        kfree_skb_reason(skb, reason);
}

/* This one checks to see if we can put data from the
 * out_of_order queue into the receive_queue.
 */
static void tcp_ofo_queue(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        __u32 dsack_high = tp->rcv_nxt;
        bool fin, fragstolen, eaten;
        struct sk_buff *skb, *tail;
        struct rb_node *p;

        p = rb_first(&tp->out_of_order_queue);
        while (p) {
                skb = rb_to_skb(p);
                if (after(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
                        break;

                if (before(TCP_SKB_CB(skb)->seq, dsack_high)) {
                        __u32 dsack = dsack_high;
                        if (before(TCP_SKB_CB(skb)->end_seq, dsack_high))
                                dsack_high = TCP_SKB_CB(skb)->end_seq;
                        tcp_dsack_extend(sk, TCP_SKB_CB(skb)->seq, dsack);
                }
                p = rb_next(p);
                rb_erase(&skb->rbnode, &tp->out_of_order_queue);

                if (unlikely(!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))) {
                        tcp_drop_reason(sk, skb, SKB_DROP_REASON_TCP_OFO_DROP);
                        continue;
                }

                tail = skb_peek_tail(&sk->sk_receive_queue);
                eaten = tail && tcp_try_coalesce(sk, tail, skb, &fragstolen);
                tcp_rcv_nxt_update(tp, TCP_SKB_CB(skb)->end_seq);
                fin = TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN;
                if (!eaten)
                        __skb_queue_tail(&sk->sk_receive_queue, skb);
                else
                        kfree_skb_partial(skb, fragstolen);

                if (unlikely(fin)) {
                        tcp_fin(sk);
                        /* tcp_fin() purges tp->out_of_order_queue,
                         * so we must end this loop right now.
                         */
                        break;
                }
        }
}

static bool tcp_prune_ofo_queue(struct sock *sk);
static int tcp_prune_queue(struct sock *sk);

static int tcp_try_rmem_schedule(struct sock *sk, struct sk_buff *skb,
                                 unsigned int size)
{
        if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf ||
            !sk_rmem_schedule(sk, skb, size)) {

                if (tcp_prune_queue(sk) < 0)
                        return -1;

                while (!sk_rmem_schedule(sk, skb, size)) {
                        if (!tcp_prune_ofo_queue(sk))
                                return -1;
                }
        }
        return 0;
}

static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct rb_node **p, *parent;
        struct sk_buff *skb1;
        u32 seq, end_seq;
        bool fragstolen;

        tcp_ecn_check_ce(sk, skb);

        if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) {
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFODROP);
                sk->sk_data_ready(sk);
                tcp_drop_reason(sk, skb, SKB_DROP_REASON_PROTO_MEM);
                return;
        }

        /* Disable header prediction. */
        tp->pred_flags = 0;
        inet_csk_schedule_ack(sk);

        tp->rcv_ooopack += max_t(u16, 1, skb_shinfo(skb)->gso_segs);
        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFOQUEUE);
        seq = TCP_SKB_CB(skb)->seq;
        end_seq = TCP_SKB_CB(skb)->end_seq;

        p = &tp->out_of_order_queue.rb_node;
        if (RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
                /* Initial out of order segment, build 1 SACK. */
                if (tcp_is_sack(tp)) {
                        tp->rx_opt.num_sacks = 1;
                        tp->selective_acks[0].start_seq = seq;
                        tp->selective_acks[0].end_seq = end_seq;
                }
                rb_link_node(&skb->rbnode, NULL, p);
                rb_insert_color(&skb->rbnode, &tp->out_of_order_queue);
                tp->ooo_last_skb = skb;
                goto end;
        }

        /* In the typical case, we are adding an skb to the end of the list.
         * Use of ooo_last_skb avoids the O(Log(N)) rbtree lookup.
         */
        if (tcp_ooo_try_coalesce(sk, tp->ooo_last_skb,
                                 skb, &fragstolen)) {
coalesce_done:
                /* For non sack flows, do not grow window to force DUPACK
                 * and trigger fast retransmit.
                 */
                if (tcp_is_sack(tp))
                        tcp_grow_window(sk, skb, true);
                kfree_skb_partial(skb, fragstolen);
                skb = NULL;
                goto add_sack;
        }
        /* Can avoid an rbtree lookup if we are adding skb after ooo_last_skb */
        if (!before(seq, TCP_SKB_CB(tp->ooo_last_skb)->end_seq)) {
                parent = &tp->ooo_last_skb->rbnode;
                p = &parent->rb_right;
                goto insert;
        }

        /* Find place to insert this segment. Handle overlaps on the way. */
        parent = NULL;
        while (*p) {
                parent = *p;
                skb1 = rb_to_skb(parent);
                if (before(seq, TCP_SKB_CB(skb1)->seq)) {
                        p = &parent->rb_left;
                        continue;
                }
                if (before(seq, TCP_SKB_CB(skb1)->end_seq)) {
                        if (!after(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
                                /* All the bits are present. Drop. */
                                NET_INC_STATS(sock_net(sk),
                                              LINUX_MIB_TCPOFOMERGE);
                                tcp_drop_reason(sk, skb,
                                                SKB_DROP_REASON_TCP_OFOMERGE);
                                skb = NULL;
                                tcp_dsack_set(sk, seq, end_seq);
                                goto add_sack;
                        }
                        if (after(seq, TCP_SKB_CB(skb1)->seq)) {
                                /* Partial overlap. */
                                tcp_dsack_set(sk, seq, TCP_SKB_CB(skb1)->end_seq);
                        } else {
                                /* skb's seq == skb1's seq and skb covers skb1.
                                 * Replace skb1 with skb.
                                 */
                                rb_replace_node(&skb1->rbnode, &skb->rbnode,
                                                &tp->out_of_order_queue);
                                tcp_dsack_extend(sk,
                                                 TCP_SKB_CB(skb1)->seq,
                                                 TCP_SKB_CB(skb1)->end_seq);
                                NET_INC_STATS(sock_net(sk),
                                              LINUX_MIB_TCPOFOMERGE);
                                tcp_drop_reason(sk, skb1,
                                                SKB_DROP_REASON_TCP_OFOMERGE);
                                goto merge_right;
                        }
                } else if (tcp_ooo_try_coalesce(sk, skb1,
                                                skb, &fragstolen)) {
                        goto coalesce_done;
                }
                p = &parent->rb_right;
        }
insert:
        /* Insert segment into RB tree. */
        rb_link_node(&skb->rbnode, parent, p);
        rb_insert_color(&skb->rbnode, &tp->out_of_order_queue);

merge_right:
        /* Remove other segments covered by skb. */
        while ((skb1 = skb_rb_next(skb)) != NULL) {
                if (!after(end_seq, TCP_SKB_CB(skb1)->seq))
                        break;
                if (before(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
                        tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
                                         end_seq);
                        break;
                }
                rb_erase(&skb1->rbnode, &tp->out_of_order_queue);
                tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
                                 TCP_SKB_CB(skb1)->end_seq);
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFOMERGE);
                tcp_drop_reason(sk, skb1, SKB_DROP_REASON_TCP_OFOMERGE);
        }
        /* If there is no skb after us, we are the last_skb ! */
        if (!skb1)
                tp->ooo_last_skb = skb;

add_sack:
        if (tcp_is_sack(tp))
                tcp_sack_new_ofo_skb(sk, seq, end_seq);
end:
        if (skb) {
                /* For non sack flows, do not grow window to force DUPACK
                 * and trigger fast retransmit.
                 */
                if (tcp_is_sack(tp))
                        tcp_grow_window(sk, skb, false);
                skb_condense(skb);
                skb_set_owner_r(skb, sk);
        }
}

static int __must_check tcp_queue_rcv(struct sock *sk, struct sk_buff *skb,
                                      bool *fragstolen)
{
        int eaten;
        struct sk_buff *tail = skb_peek_tail(&sk->sk_receive_queue);

        eaten = (tail &&
                 tcp_try_coalesce(sk, tail,
                                  skb, fragstolen)) ? 1 : 0;
        tcp_rcv_nxt_update(tcp_sk(sk), TCP_SKB_CB(skb)->end_seq);
        if (!eaten) {
                __skb_queue_tail(&sk->sk_receive_queue, skb);
                skb_set_owner_r(skb, sk);
        }
        return eaten;
}

int tcp_send_rcvq(struct sock *sk, struct msghdr *msg, size_t size)
{
        struct sk_buff *skb;
        int err = -ENOMEM;
        int data_len = 0;
        bool fragstolen;

        if (size == 0)
                return 0;

        if (size > PAGE_SIZE) {
                int npages = min_t(size_t, size >> PAGE_SHIFT, MAX_SKB_FRAGS);

                data_len = npages << PAGE_SHIFT;
                size = data_len + (size & ~PAGE_MASK);
        }
        skb = alloc_skb_with_frags(size - data_len, data_len,
                                   PAGE_ALLOC_COSTLY_ORDER,
                                   &err, sk->sk_allocation);
        if (!skb)
                goto err;

        skb_put(skb, size - data_len);
        skb->data_len = data_len;
        skb->len = size;

        if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
                goto err_free;
        }

        err = skb_copy_datagram_from_iter(skb, 0, &msg->msg_iter, size);
        if (err)
                goto err_free;

        TCP_SKB_CB(skb)->seq = tcp_sk(sk)->rcv_nxt;
        TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + size;
        TCP_SKB_CB(skb)->ack_seq = tcp_sk(sk)->snd_una - 1;

        if (tcp_queue_rcv(sk, skb, &fragstolen)) {
                WARN_ON_ONCE(fragstolen); /* should not happen */
                __kfree_skb(skb);
        }
        return size;

err_free:
        kfree_skb(skb);
err:
        return err;

}

void tcp_data_ready(struct sock *sk)
{
        if (tcp_epollin_ready(sk, sk->sk_rcvlowat) || sock_flag(sk, SOCK_DONE))
                sk->sk_data_ready(sk);
}

static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        enum skb_drop_reason reason;
        bool fragstolen;
        int eaten;

        /* If a subflow has been reset, the packet should not continue
         * to be processed, drop the packet.
         */
        if (sk_is_mptcp(sk) && !mptcp_incoming_options(sk, skb)) {
                __kfree_skb(skb);
                return;
        }

        if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
                __kfree_skb(skb);
                return;
        }
        skb_dst_drop(skb);
        __skb_pull(skb, tcp_hdr(skb)->doff * 4);

        reason = SKB_DROP_REASON_NOT_SPECIFIED;
        tp->rx_opt.dsack = 0;

        /*  Queue data for delivery to the user.
         *  Packets in sequence go to the receive queue.
         *  Out of sequence packets to the out_of_order_queue.
         */
        if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
                if (tcp_receive_window(tp) == 0) {
                        reason = SKB_DROP_REASON_TCP_ZEROWINDOW;
                        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPZEROWINDOWDROP);
                        goto out_of_window;
                }

                /* Ok. In sequence. In window. */
queue_and_out:
                if (skb_queue_len(&sk->sk_receive_queue) == 0)
                        sk_forced_mem_schedule(sk, skb->truesize);
                else if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
                        reason = SKB_DROP_REASON_PROTO_MEM;
                        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
                        sk->sk_data_ready(sk);
                        goto drop;
                }

                eaten = tcp_queue_rcv(sk, skb, &fragstolen);
                if (skb->len)
                        tcp_event_data_recv(sk, skb);
                if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
                        tcp_fin(sk);

                if (!RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
                        tcp_ofo_queue(sk);

                        /* RFC5681. 4.2. SHOULD send immediate ACK, when
                         * gap in queue is filled.
                         */
                        if (RB_EMPTY_ROOT(&tp->out_of_order_queue))
                                inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
                }

                if (tp->rx_opt.num_sacks)
                        tcp_sack_remove(tp);

                tcp_fast_path_check(sk);

                if (eaten > 0)
                        kfree_skb_partial(skb, fragstolen);
                if (!sock_flag(sk, SOCK_DEAD))
                        tcp_data_ready(sk);
                return;
        }

        if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
                tcp_rcv_spurious_retrans(sk, skb);
                /* A retransmit, 2nd most common case.  Force an immediate ack. */
                reason = SKB_DROP_REASON_TCP_OLD_DATA;
                NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
                tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);

out_of_window:
                tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
                inet_csk_schedule_ack(sk);
drop:
                tcp_drop_reason(sk, skb, reason);
                return;
        }

        /* Out of window. F.e. zero window probe. */
        if (!before(TCP_SKB_CB(skb)->seq,
                    tp->rcv_nxt + tcp_receive_window(tp))) {
                reason = SKB_DROP_REASON_TCP_OVERWINDOW;
                goto out_of_window;
        }

        if (before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
                /* Partial packet, seq < rcv_next < end_seq */
                tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, tp->rcv_nxt);

                /* If window is closed, drop tail of packet. But after
                 * remembering D-SACK for its head made in previous line.
                 */
                if (!tcp_receive_window(tp)) {
                        reason = SKB_DROP_REASON_TCP_ZEROWINDOW;
                        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPZEROWINDOWDROP);
                        goto out_of_window;
                }
                goto queue_and_out;
        }

        tcp_data_queue_ofo(sk, skb);
}

static struct sk_buff *tcp_skb_next(struct sk_buff *skb, struct sk_buff_head *list)
{
        if (list)
                return !skb_queue_is_last(list, skb) ? skb->next : NULL;

        return skb_rb_next(skb);
}

static struct sk_buff *tcp_collapse_one(struct sock *sk, struct sk_buff *skb,
                                        struct sk_buff_head *list,
                                        struct rb_root *root)
{
        struct sk_buff *next = tcp_skb_next(skb, list);

        if (list)
                __skb_unlink(skb, list);
        else
                rb_erase(&skb->rbnode, root);

        __kfree_skb(skb);
        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED);

        return next;
}

/* Insert skb into rb tree, ordered by TCP_SKB_CB(skb)->seq */
void tcp_rbtree_insert(struct rb_root *root, struct sk_buff *skb)
{
        struct rb_node **p = &root->rb_node;
        struct rb_node *parent = NULL;
        struct sk_buff *skb1;

        while (*p) {
                parent = *p;
                skb1 = rb_to_skb(parent);
                if (before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb1)->seq))
                        p = &parent->rb_left;
                else
                        p = &parent->rb_right;
        }
        rb_link_node(&skb->rbnode, parent, p);
        rb_insert_color(&skb->rbnode, root);
}

/* Collapse contiguous sequence of skbs head..tail with
 * sequence numbers start..end.
 *
 * If tail is NULL, this means until the end of the queue.
 *
 * Segments with FIN/SYN are not collapsed (only because this
 * simplifies code)
 */
static void
tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
             struct sk_buff *head, struct sk_buff *tail, u32 start, u32 end)
{
        struct sk_buff *skb = head, *n;
        struct sk_buff_head tmp;
        bool end_of_skbs;

        /* First, check that queue is collapsible and find
         * the point where collapsing can be useful.
         */
restart:
        for (end_of_skbs = true; skb != NULL && skb != tail; skb = n) {
                n = tcp_skb_next(skb, list);

                /* No new bits? It is possible on ofo queue. */
                if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
                        skb = tcp_collapse_one(sk, skb, list, root);
                        if (!skb)
                                break;
                        goto restart;
                }

                /* The first skb to collapse is:
                 * - not SYN/FIN and
                 * - bloated or contains data before "start" or
                 *   overlaps to the next one and mptcp allow collapsing.
                 */
                if (!(TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)) &&
                    (tcp_win_from_space(sk, skb->truesize) > skb->len ||
                     before(TCP_SKB_CB(skb)->seq, start))) {
                        end_of_skbs = false;
                        break;
                }

                if (n && n != tail && mptcp_skb_can_collapse(skb, n) &&
                    TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(n)->seq) {
                        end_of_skbs = false;
                        break;
                }

                /* Decided to skip this, advance start seq. */
                start = TCP_SKB_CB(skb)->end_seq;
        }
        if (end_of_skbs ||
            (TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)))
                return;

        __skb_queue_head_init(&tmp);

        while (before(start, end)) {
                int copy = min_t(int, SKB_MAX_ORDER(0, 0), end - start);
                struct sk_buff *nskb;

                nskb = alloc_skb(copy, GFP_ATOMIC);
                if (!nskb)
                        break;

                memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
#ifdef CONFIG_TLS_DEVICE
                nskb->decrypted = skb->decrypted;
#endif
                TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
                if (list)
                        __skb_queue_before(list, skb, nskb);
                else
                        __skb_queue_tail(&tmp, nskb); /* defer rbtree insertion */
                skb_set_owner_r(nskb, sk);
                mptcp_skb_ext_move(nskb, skb);

                /* Copy data, releasing collapsed skbs. */
                while (copy > 0) {
                        int offset = start - TCP_SKB_CB(skb)->seq;
                        int size = TCP_SKB_CB(skb)->end_seq - start;

                        BUG_ON(offset < 0);
                        if (size > 0) {
                                size = min(copy, size);
                                if (skb_copy_bits(skb, offset, skb_put(nskb, size), size))
                                        BUG();
                                TCP_SKB_CB(nskb)->end_seq += size;
                                copy -= size;
                                start += size;
                        }
                        if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
                                skb = tcp_collapse_one(sk, skb, list, root);
                                if (!skb ||
                                    skb == tail ||
                                    !mptcp_skb_can_collapse(nskb, skb) ||
                                    (TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)))
                                        goto end;
#ifdef CONFIG_TLS_DEVICE
                                if (skb->decrypted != nskb->decrypted)
                                        goto end;
#endif
                        }
                }
        }
end:
        skb_queue_walk_safe(&tmp, skb, n)
                tcp_rbtree_insert(root, skb);
}

/* Collapse ofo queue. Algorithm: select contiguous sequence of skbs
 * and tcp_collapse() them until all the queue is collapsed.
 */
static void tcp_collapse_ofo_queue(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 range_truesize, sum_tiny = 0;
        struct sk_buff *skb, *head;
        u32 start, end;

        skb = skb_rb_first(&tp->out_of_order_queue);
new_range:
        if (!skb) {
                tp->ooo_last_skb = skb_rb_last(&tp->out_of_order_queue);
                return;
        }
        start = TCP_SKB_CB(skb)->seq;
        end = TCP_SKB_CB(skb)->end_seq;
        range_truesize = skb->truesize;

        for (head = skb;;) {
                skb = skb_rb_next(skb);

                /* Range is terminated when we see a gap or when
                 * we are at the queue end.
                 */
                if (!skb ||
                    after(TCP_SKB_CB(skb)->seq, end) ||
                    before(TCP_SKB_CB(skb)->end_seq, start)) {
                        /* Do not attempt collapsing tiny skbs */
                        if (range_truesize != head->truesize ||
                            end - start >= SKB_WITH_OVERHEAD(PAGE_SIZE)) {
                                tcp_collapse(sk, NULL, &tp->out_of_order_queue,
                                             head, skb, start, end);
                        } else {
                                sum_tiny += range_truesize;
                                if (sum_tiny > sk->sk_rcvbuf >> 3)
                                        return;
                        }
                        goto new_range;
                }

                range_truesize += skb->truesize;
                if (unlikely(before(TCP_SKB_CB(skb)->seq, start)))
                        start = TCP_SKB_CB(skb)->seq;
                if (after(TCP_SKB_CB(skb)->end_seq, end))
                        end = TCP_SKB_CB(skb)->end_seq;
        }
}

/*
 * Clean the out-of-order queue to make room.
 * We drop high sequences packets to :
 * 1) Let a chance for holes to be filled.
 * 2) not add too big latencies if thousands of packets sit there.
 *    (But if application shrinks SO_RCVBUF, we could still end up
 *     freeing whole queue here)
 * 3) Drop at least 12.5 % of sk_rcvbuf to avoid malicious attacks.
 *
 * Return true if queue has shrunk.
 */
static bool tcp_prune_ofo_queue(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct rb_node *node, *prev;
        int goal;

        if (RB_EMPTY_ROOT(&tp->out_of_order_queue))
                return false;

        NET_INC_STATS(sock_net(sk), LINUX_MIB_OFOPRUNED);
        goal = sk->sk_rcvbuf >> 3;
        node = &tp->ooo_last_skb->rbnode;
        do {
                prev = rb_prev(node);
                rb_erase(node, &tp->out_of_order_queue);
                goal -= rb_to_skb(node)->truesize;
                tcp_drop_reason(sk, rb_to_skb(node),
                                SKB_DROP_REASON_TCP_OFO_QUEUE_PRUNE);
                if (!prev || goal <= 0) {
                        if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf &&
                            !tcp_under_memory_pressure(sk))
                                break;
                        goal = sk->sk_rcvbuf >> 3;
                }
                node = prev;
        } while (node);
        tp->ooo_last_skb = rb_to_skb(prev);

        /* Reset SACK state.  A conforming SACK implementation will
         * do the same at a timeout based retransmit.  When a connection
         * is in a sad state like this, we care only about integrity
         * of the connection not performance.
         */
        if (tp->rx_opt.sack_ok)
                tcp_sack_reset(&tp->rx_opt);
        return true;
}

/* Reduce allocated memory if we can, trying to get
 * the socket within its memory limits again.
 *
 * Return less than zero if we should start dropping frames
 * until the socket owning process reads some of the data
 * to stabilize the situation.
 */
static int tcp_prune_queue(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        NET_INC_STATS(sock_net(sk), LINUX_MIB_PRUNECALLED);

        if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf)
                tcp_clamp_window(sk);
        else if (tcp_under_memory_pressure(sk))
                tcp_adjust_rcv_ssthresh(sk);

        if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
                return 0;

        tcp_collapse_ofo_queue(sk);
        if (!skb_queue_empty(&sk->sk_receive_queue))
                tcp_collapse(sk, &sk->sk_receive_queue, NULL,
                             skb_peek(&sk->sk_receive_queue),
                             NULL,
                             tp->copied_seq, tp->rcv_nxt);

        if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
                return 0;

        /* Collapsing did not help, destructive actions follow.
         * This must not ever occur. */

        tcp_prune_ofo_queue(sk);

        if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
                return 0;

        /* If we are really being abused, tell the caller to silently
         * drop receive data on the floor.  It will get retransmitted
         * and hopefully then we'll have sufficient space.
         */
        NET_INC_STATS(sock_net(sk), LINUX_MIB_RCVPRUNED);

        /* Massive buffer overcommit. */
        tp->pred_flags = 0;
        return -1;
}

static bool tcp_should_expand_sndbuf(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);

        /* If the user specified a specific send buffer setting, do
         * not modify it.
         */
        if (sk->sk_userlocks & SOCK_SNDBUF_LOCK)
                return false;

        /* If we are under global TCP memory pressure, do not expand.  */
        if (tcp_under_memory_pressure(sk)) {
                int unused_mem = sk_unused_reserved_mem(sk);

                /* Adjust sndbuf according to reserved mem. But make sure
                 * it never goes below SOCK_MIN_SNDBUF.
                 * See sk_stream_moderate_sndbuf() for more details.
                 */
                if (unused_mem > SOCK_MIN_SNDBUF)
                        WRITE_ONCE(sk->sk_sndbuf, unused_mem);

                return false;
        }

        /* If we are under soft global TCP memory pressure, do not expand.  */
        if (sk_memory_allocated(sk) >= sk_prot_mem_limits(sk, 0))
                return false;

        /* If we filled the congestion window, do not expand.  */
        if (tcp_packets_in_flight(tp) >= tcp_snd_cwnd(tp))
                return false;

        return true;
}

static void tcp_new_space(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tcp_should_expand_sndbuf(sk)) {
                tcp_sndbuf_expand(sk);
                tp->snd_cwnd_stamp = tcp_jiffies32;
        }

        INDIRECT_CALL_1(sk->sk_write_space, sk_stream_write_space, sk);
}

/* Caller made space either from:
 * 1) Freeing skbs in rtx queues (after tp->snd_una has advanced)
 * 2) Sent skbs from output queue (and thus advancing tp->snd_nxt)
 *
 * We might be able to generate EPOLLOUT to the application if:
 * 1) Space consumed in output/rtx queues is below sk->sk_sndbuf/2
 * 2) notsent amount (tp->write_seq - tp->snd_nxt) became
 *    small enough that tcp_stream_memory_free() decides it
 *    is time to generate EPOLLOUT.
 */
void tcp_check_space(struct sock *sk)
{
        /* pairs with tcp_poll() */
        smp_mb();
        if (sk->sk_socket &&
            test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
                tcp_new_space(sk);
                if (!test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
                        tcp_chrono_stop(sk, TCP_CHRONO_SNDBUF_LIMITED);
        }
}

static inline void tcp_data_snd_check(struct sock *sk)
{
        tcp_push_pending_frames(sk);
        tcp_check_space(sk);
}

/*
 * Check if sending an ack is needed.
 */
static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
{
        struct tcp_sock *tp = tcp_sk(sk);
        unsigned long rtt, delay;

            /* More than one full frame received... */
        if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss &&
             /* ... and right edge of window advances far enough.
              * (tcp_recvmsg() will send ACK otherwise).
              * If application uses SO_RCVLOWAT, we want send ack now if
              * we have not received enough bytes to satisfy the condition.
              */
            (tp->rcv_nxt - tp->copied_seq < sk->sk_rcvlowat ||
             __tcp_select_window(sk) >= tp->rcv_wnd)) ||
            /* We ACK each frame or... */
            tcp_in_quickack_mode(sk) ||
            /* Protocol state mandates a one-time immediate ACK */
            inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOW) {
send_now:
                tcp_send_ack(sk);
                return;
        }

        if (!ofo_possible || RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
                tcp_send_delayed_ack(sk);
                return;
        }

        if (!tcp_is_sack(tp) ||
            tp->compressed_ack >= sock_net(sk)->ipv4.sysctl_tcp_comp_sack_nr)
                goto send_now;

        if (tp->compressed_ack_rcv_nxt != tp->rcv_nxt) {
                tp->compressed_ack_rcv_nxt = tp->rcv_nxt;
                tp->dup_ack_counter = 0;
        }
        if (tp->dup_ack_counter < TCP_FASTRETRANS_THRESH) {
                tp->dup_ack_counter++;
                goto send_now;
        }
        tp->compressed_ack++;
        if (hrtimer_is_queued(&tp->compressed_ack_timer))
                return;

        /* compress ack timer : 5 % of rtt, but no more than tcp_comp_sack_delay_ns */

        rtt = tp->rcv_rtt_est.rtt_us;
        if (tp->srtt_us && tp->srtt_us < rtt)
                rtt = tp->srtt_us;

        delay = min_t(unsigned long, sock_net(sk)->ipv4.sysctl_tcp_comp_sack_delay_ns,
                      rtt * (NSEC_PER_USEC >> 3)/20);
        sock_hold(sk);
        hrtimer_start_range_ns(&tp->compressed_ack_timer, ns_to_ktime(delay),
                               sock_net(sk)->ipv4.sysctl_tcp_comp_sack_slack_ns,
                               HRTIMER_MODE_REL_PINNED_SOFT);
}

static inline void tcp_ack_snd_check(struct sock *sk)
{
        if (!inet_csk_ack_scheduled(sk)) {
                /* We sent a data segment already. */
                return;
        }
        __tcp_ack_snd_check(sk, 1);
}

/*
 *      This routine is only called when we have urgent data
 *      signaled. Its the 'slow' part of tcp_urg. It could be
 *      moved inline now as tcp_urg is only called from one
 *      place. We handle URGent data wrong. We have to - as
 *      BSD still doesn't use the correction from RFC961.
 *      For 1003.1g we should support a new option TCP_STDURG to permit
 *      either form (or just set the sysctl tcp_stdurg).
 */

static void tcp_check_urg(struct sock *sk, const struct tcphdr *th)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 ptr = ntohs(th->urg_ptr);

        if (ptr && !sock_net(sk)->ipv4.sysctl_tcp_stdurg)
                ptr--;
        ptr += ntohl(th->seq);

        /* Ignore urgent data that we've already seen and read. */
        if (after(tp->copied_seq, ptr))
                return;

        /* Do not replay urg ptr.
         *
         * NOTE: interesting situation not covered by specs.
         * Misbehaving sender may send urg ptr, pointing to segment,
         * which we already have in ofo queue. We are not able to fetch
         * such data and will stay in TCP_URG_NOTYET until will be eaten
         * by recvmsg(). Seems, we are not obliged to handle such wicked
         * situations. But it is worth to think about possibility of some
         * DoSes using some hypothetical application level deadlock.
         */
        if (before(ptr, tp->rcv_nxt))
                return;

        /* Do we already have a newer (or duplicate) urgent pointer? */
        if (tp->urg_data && !after(ptr, tp->urg_seq))
                return;

        /* Tell the world about our new urgent pointer. */
        sk_send_sigurg(sk);

        /* We may be adding urgent data when the last byte read was
         * urgent. To do this requires some care. We cannot just ignore
         * tp->copied_seq since we would read the last urgent byte again
         * as data, nor can we alter copied_seq until this data arrives
         * or we break the semantics of SIOCATMARK (and thus sockatmark())
         *
         * NOTE. Double Dutch. Rendering to plain English: author of comment
         * above did something sort of  send("A", MSG_OOB); send("B", MSG_OOB);
         * and expect that both A and B disappear from stream. This is _wrong_.
         * Though this happens in BSD with high probability, this is occasional.
         * Any application relying on this is buggy. Note also, that fix "works"
         * only in this artificial test. Insert some normal data between A and B and we will
         * decline of BSD again. Verdict: it is better to remove to trap
         * buggy users.
         */
        if (tp->urg_seq == tp->copied_seq && tp->urg_data &&
            !sock_flag(sk, SOCK_URGINLINE) && tp->copied_seq != tp->rcv_nxt) {
                struct sk_buff *skb = skb_peek(&sk->sk_receive_queue);
                tp->copied_seq++;
                if (skb && !before(tp->copied_seq, TCP_SKB_CB(skb)->end_seq)) {
                        __skb_unlink(skb, &sk->sk_receive_queue);
                        __kfree_skb(skb);
                }
        }

        WRITE_ONCE(tp->urg_data, TCP_URG_NOTYET);
        WRITE_ONCE(tp->urg_seq, ptr);

        /* Disable header prediction. */
        tp->pred_flags = 0;
}

/* This is the 'fast' part of urgent handling. */
static void tcp_urg(struct sock *sk, struct sk_buff *skb, const struct tcphdr *th)
{
        struct tcp_sock *tp = tcp_sk(sk);

        /* Check if we get a new urgent pointer - normally not. */
        if (unlikely(th->urg))
                tcp_check_urg(sk, th);

        /* Do we wait for any urgent data? - normally not... */
        if (unlikely(tp->urg_data == TCP_URG_NOTYET)) {
                u32 ptr = tp->urg_seq - ntohl(th->seq) + (th->doff * 4) -
                          th->syn;

                /* Is the urgent pointer pointing into this packet? */
                if (ptr < skb->len) {
                        u8 tmp;
                        if (skb_copy_bits(skb, ptr, &tmp, 1))
                                BUG();
                        WRITE_ONCE(tp->urg_data, TCP_URG_VALID | tmp);
                        if (!sock_flag(sk, SOCK_DEAD))
                                sk->sk_data_ready(sk);
                }
        }
}

/* Accept RST for rcv_nxt - 1 after a FIN.
 * When tcp connections are abruptly terminated from Mac OSX (via ^C), a
 * FIN is sent followed by a RST packet. The RST is sent with the same
 * sequence number as the FIN, and thus according to RFC 5961 a challenge
 * ACK should be sent. However, Mac OSX rate limits replies to challenge
 * ACKs on the closed socket. In addition middleboxes can drop either the
 * challenge ACK or a subsequent RST.
 */
static bool tcp_reset_check(const struct sock *sk, const struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);

        return unlikely(TCP_SKB_CB(skb)->seq == (tp->rcv_nxt - 1) &&
                        (1 << sk->sk_state) & (TCPF_CLOSE_WAIT | TCPF_LAST_ACK |
                                               TCPF_CLOSING));
}

/* Does PAWS and seqno based validation of an incoming segment, flags will
 * play significant role here.
 */
static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
                                  const struct tcphdr *th, int syn_inerr)
{
        struct tcp_sock *tp = tcp_sk(sk);
        SKB_DR(reason);

        /* RFC1323: H1. Apply PAWS check first. */
        if (tcp_fast_parse_options(sock_net(sk), skb, th, tp) &&
            tp->rx_opt.saw_tstamp &&
            tcp_paws_discard(sk, skb)) {
                if (!th->rst) {
                        NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
                        if (!tcp_oow_rate_limited(sock_net(sk), skb,
                                                  LINUX_MIB_TCPACKSKIPPEDPAWS,
                                                  &tp->last_oow_ack_time))
                                tcp_send_dupack(sk, skb);
                        SKB_DR_SET(reason, TCP_RFC7323_PAWS);
                        goto discard;
                }
                /* Reset is accepted even if it did not pass PAWS. */
        }

        /* Step 1: check sequence number */
        if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) {
                /* RFC793, page 37: "In all states except SYN-SENT, all reset
                 * (RST) segments are validated by checking their SEQ-fields."
                 * And page 69: "If an incoming segment is not acceptable,
                 * an acknowledgment should be sent in reply (unless the RST
                 * bit is set, if so drop the segment and return)".
                 */
                if (!th->rst) {
                        if (th->syn)
                                goto syn_challenge;
                        if (!tcp_oow_rate_limited(sock_net(sk), skb,
                                                  LINUX_MIB_TCPACKSKIPPEDSEQ,
                                                  &tp->last_oow_ack_time))
                                tcp_send_dupack(sk, skb);
                } else if (tcp_reset_check(sk, skb)) {
                        goto reset;
                }
                SKB_DR_SET(reason, TCP_INVALID_SEQUENCE);
                goto discard;
        }

        /* Step 2: check RST bit */
        if (th->rst) {
                /* RFC 5961 3.2 (extend to match against (RCV.NXT - 1) after a
                 * FIN and SACK too if available):
                 * If seq num matches RCV.NXT or (RCV.NXT - 1) after a FIN, or
                 * the right-most SACK block,
                 * then
                 *     RESET the connection
                 * else
                 *     Send a challenge ACK
                 */
                if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt ||
                    tcp_reset_check(sk, skb))
                        goto reset;

                if (tcp_is_sack(tp) && tp->rx_opt.num_sacks > 0) {
                        struct tcp_sack_block *sp = &tp->selective_acks[0];
                        int max_sack = sp[0].end_seq;
                        int this_sack;

                        for (this_sack = 1; this_sack < tp->rx_opt.num_sacks;
                             ++this_sack) {
                                max_sack = after(sp[this_sack].end_seq,
                                                 max_sack) ?
                                        sp[this_sack].end_seq : max_sack;
                        }

                        if (TCP_SKB_CB(skb)->seq == max_sack)
                                goto reset;
                }

                /* Disable TFO if RST is out-of-order
                 * and no data has been received
                 * for current active TFO socket
                 */
                if (tp->syn_fastopen && !tp->data_segs_in &&
                    sk->sk_state == TCP_ESTABLISHED)
                        tcp_fastopen_active_disable(sk);
                tcp_send_challenge_ack(sk);
                SKB_DR_SET(reason, TCP_RESET);
                goto discard;
        }

        /* step 3: check security and precedence [ignored] */

        /* step 4: Check for a SYN
         * RFC 5961 4.2 : Send a challenge ack
         */
        if (th->syn) {
syn_challenge:
                if (syn_inerr)
                        TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
                tcp_send_challenge_ack(sk);
                SKB_DR_SET(reason, TCP_INVALID_SYN);
                goto discard;
        }

        bpf_skops_parse_hdr(sk, skb);

        return true;

discard:
        tcp_drop_reason(sk, skb, reason);
        return false;

reset:
        tcp_reset(sk, skb);
        __kfree_skb(skb);
        return false;
}

/*
 *      TCP receive function for the ESTABLISHED state.
 *
 *      It is split into a fast path and a slow path. The fast path is
 *      disabled when:
 *      - A zero window was announced from us - zero window probing
 *        is only handled properly in the slow path.
 *      - Out of order segments arrived.
 *      - Urgent data is expected.
 *      - There is no buffer space left
 *      - Unexpected TCP flags/window values/header lengths are received
 *        (detected by checking the TCP header against pred_flags)
 *      - Data is sent in both directions. Fast path only supports pure senders
 *        or pure receivers (this means either the sequence number or the ack
 *        value must stay constant)
 *      - Unexpected TCP option.
 *
 *      When these conditions are not satisfied it drops into a standard
 *      receive procedure patterned after RFC793 to handle all cases.
 *      The first three cases are guaranteed by proper pred_flags setting,
 *      the rest is checked inline. Fast processing is turned on in
 *      tcp_data_queue when everything is OK.
 */
void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
{
        enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED;
        const struct tcphdr *th = (const struct tcphdr *)skb->data;
        struct tcp_sock *tp = tcp_sk(sk);
        unsigned int len = skb->len;

        /* TCP congestion window tracking */
        trace_tcp_probe(sk, skb);

        tcp_mstamp_refresh(tp);
        if (unlikely(!rcu_access_pointer(sk->sk_rx_dst)))
                inet_csk(sk)->icsk_af_ops->sk_rx_dst_set(sk, skb);
        /*
         *      Header prediction.
         *      The code loosely follows the one in the famous
         *      "30 instruction TCP receive" Van Jacobson mail.
         *
         *      Van's trick is to deposit buffers into socket queue
         *      on a device interrupt, to call tcp_recv function
         *      on the receive process context and checksum and copy
         *      the buffer to user space. smart...
         *
         *      Our current scheme is not silly either but we take the
         *      extra cost of the net_bh soft interrupt processing...
         *      We do checksum and copy also but from device to kernel.
         */

        tp->rx_opt.saw_tstamp = 0;

        /*      pred_flags is 0xS?10 << 16 + snd_wnd
         *      if header_prediction is to be made
         *      'S' will always be tp->tcp_header_len >> 2
         *      '?' will be 0 for the fast path, otherwise pred_flags is 0 to
         *  turn it off (when there are holes in the receive
         *       space for instance)
         *      PSH flag is ignored.
         */

        if ((tcp_flag_word(th) & TCP_HP_BITS) == tp->pred_flags &&
            TCP_SKB_CB(skb)->seq == tp->rcv_nxt &&
            !after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
                int tcp_header_len = tp->tcp_header_len;

                /* Timestamp header prediction: tcp_header_len
                 * is automatically equal to th->doff*4 due to pred_flags
                 * match.
                 */

                /* Check timestamp */
                if (tcp_header_len == sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) {
                        /* No? Slow path! */
                        if (!tcp_parse_aligned_timestamp(tp, th))
                                goto slow_path;

                        /* If PAWS failed, check it more carefully in slow path */
                        if ((s32)(tp->rx_opt.rcv_tsval - tp->rx_opt.ts_recent) < 0)
                                goto slow_path;

                        /* DO NOT update ts_recent here, if checksum fails
                         * and timestamp was corrupted part, it will result
                         * in a hung connection since we will drop all
                         * future packets due to the PAWS test.
                         */
                }

                if (len <= tcp_header_len) {
                        /* Bulk data transfer: sender */
                        if (len == tcp_header_len) {
                                /* Predicted packet is in window by definition.
                                 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
                                 * Hence, check seq<=rcv_wup reduces to:
                                 */
                                if (tcp_header_len ==
                                    (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
                                    tp->rcv_nxt == tp->rcv_wup)
                                        tcp_store_ts_recent(tp);

                                /* We know that such packets are checksummed
                                 * on entry.
                                 */
                                tcp_ack(sk, skb, 0);
                                __kfree_skb(skb);
                                tcp_data_snd_check(sk);
                                /* When receiving pure ack in fast path, update
                                 * last ts ecr directly instead of calling
                                 * tcp_rcv_rtt_measure_ts()
                                 */
                                tp->rcv_rtt_last_tsecr = tp->rx_opt.rcv_tsecr;
                                return;
                        } else { /* Header too small */
                                reason = SKB_DROP_REASON_PKT_TOO_SMALL;
                                TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
                                goto discard;
                        }
                } else {
                        int eaten = 0;
                        bool fragstolen = false;

                        if (tcp_checksum_complete(skb))
                                goto csum_error;

                        if ((int)skb->truesize > sk->sk_forward_alloc)
                                goto step5;

                        /* Predicted packet is in window by definition.
                         * seq == rcv_nxt and rcv_wup <= rcv_nxt.
                         * Hence, check seq<=rcv_wup reduces to:
                         */
                        if (tcp_header_len ==
                            (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
                            tp->rcv_nxt == tp->rcv_wup)
                                tcp_store_ts_recent(tp);

                        tcp_rcv_rtt_measure_ts(sk, skb);

                        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPHITS);

                        /* Bulk data transfer: receiver */
                        skb_dst_drop(skb);
                        __skb_pull(skb, tcp_header_len);
                        eaten = tcp_queue_rcv(sk, skb, &fragstolen);

                        tcp_event_data_recv(sk, skb);

                        if (TCP_SKB_CB(skb)->ack_seq != tp->snd_una) {
                                /* Well, only one small jumplet in fast path... */
                                tcp_ack(sk, skb, FLAG_DATA);
                                tcp_data_snd_check(sk);
                                if (!inet_csk_ack_scheduled(sk))
                                        goto no_ack;
                        } else {
                                tcp_update_wl(tp, TCP_SKB_CB(skb)->seq);
                        }

                        __tcp_ack_snd_check(sk, 0);
no_ack:
                        if (eaten)
                                kfree_skb_partial(skb, fragstolen);
                        tcp_data_ready(sk);
                        return;
                }
        }

slow_path:
        if (len < (th->doff << 2) || tcp_checksum_complete(skb))
                goto csum_error;

        if (!th->ack && !th->rst && !th->syn) {
                reason = SKB_DROP_REASON_TCP_FLAGS;
                goto discard;
        }

        /*
         *      Standard slow path.
         */

        if (!tcp_validate_incoming(sk, skb, th, 1))
                return;

step5:
        reason = tcp_ack(sk, skb, FLAG_SLOWPATH | FLAG_UPDATE_TS_RECENT);
        if ((int)reason < 0) {
                reason = -reason;
                goto discard;
        }
        tcp_rcv_rtt_measure_ts(sk, skb);

        /* Process urgent data. */
        tcp_urg(sk, skb, th);

        /* step 7: process the segment text */
        tcp_data_queue(sk, skb);

        tcp_data_snd_check(sk);
        tcp_ack_snd_check(sk);
        return;

csum_error:
        reason = SKB_DROP_REASON_TCP_CSUM;
        trace_tcp_bad_csum(skb);
        TCP_INC_STATS(sock_net(sk), TCP_MIB_CSUMERRORS);
        TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);

discard:
        tcp_drop_reason(sk, skb, reason);
}
EXPORT_SYMBOL(tcp_rcv_established);

void tcp_init_transfer(struct sock *sk, int bpf_op, struct sk_buff *skb)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);

        tcp_mtup_init(sk);
        icsk->icsk_af_ops->rebuild_header(sk);
        tcp_init_metrics(sk);

        /* Initialize the congestion window to start the transfer.
         * Cut cwnd down to 1 per RFC5681 if SYN or SYN-ACK has been
         * retransmitted. In light of RFC6298 more aggressive 1sec
         * initRTO, we only reset cwnd when more than 1 SYN/SYN-ACK
         * retransmission has occurred.
         */
        if (tp->total_retrans > 1 && tp->undo_marker)
                tcp_snd_cwnd_set(tp, 1);
        else
                tcp_snd_cwnd_set(tp, tcp_init_cwnd(tp, __sk_dst_get(sk)));
        tp->snd_cwnd_stamp = tcp_jiffies32;

        bpf_skops_established(sk, bpf_op, skb);
        /* Initialize congestion control unless BPF initialized it already: */
        if (!icsk->icsk_ca_initialized)
                tcp_init_congestion_control(sk);
        tcp_init_buffer_space(sk);
}

void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);

        tcp_set_state(sk, TCP_ESTABLISHED);
        icsk->icsk_ack.lrcvtime = tcp_jiffies32;

        if (skb) {
                icsk->icsk_af_ops->sk_rx_dst_set(sk, skb);
                security_inet_conn_established(sk, skb);
                sk_mark_napi_id(sk, skb);
        }

        tcp_init_transfer(sk, BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB, skb);

        /* Prevent spurious tcp_cwnd_restart() on first data
         * packet.
         */
        tp->lsndtime = tcp_jiffies32;

        if (sock_flag(sk, SOCK_KEEPOPEN))
                inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tp));

        if (!tp->rx_opt.snd_wscale)
                __tcp_fast_path_on(tp, tp->snd_wnd);
        else
                tp->pred_flags = 0;
}

static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
                                    struct tcp_fastopen_cookie *cookie)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *data = tp->syn_data ? tcp_rtx_queue_head(sk) : NULL;
        u16 mss = tp->rx_opt.mss_clamp, try_exp = 0;
        bool syn_drop = false;

        if (mss == tp->rx_opt.user_mss) {
                struct tcp_options_received opt;

                /* Get original SYNACK MSS value if user MSS sets mss_clamp */
                tcp_clear_options(&opt);
                opt.user_mss = opt.mss_clamp = 0;
                tcp_parse_options(sock_net(sk), synack, &opt, 0, NULL);
                mss = opt.mss_clamp;
        }

        if (!tp->syn_fastopen) {
                /* Ignore an unsolicited cookie */
                cookie->len = -1;
        } else if (tp->total_retrans) {
                /* SYN timed out and the SYN-ACK neither has a cookie nor
                 * acknowledges data. Presumably the remote received only
                 * the retransmitted (regular) SYNs: either the original
                 * SYN-data or the corresponding SYN-ACK was dropped.
                 */
                syn_drop = (cookie->len < 0 && data);
        } else if (cookie->len < 0 && !tp->syn_data) {
                /* We requested a cookie but didn't get it. If we did not use
                 * the (old) exp opt format then try so next time (try_exp=1).
                 * Otherwise we go back to use the RFC7413 opt (try_exp=2).
                 */
                try_exp = tp->syn_fastopen_exp ? 2 : 1;
        }

        tcp_fastopen_cache_set(sk, mss, cookie, syn_drop, try_exp);

        if (data) { /* Retransmit unacked data in SYN */
                if (tp->total_retrans)
                        tp->fastopen_client_fail = TFO_SYN_RETRANSMITTED;
                else
                        tp->fastopen_client_fail = TFO_DATA_NOT_ACKED;
                skb_rbtree_walk_from(data)
                         tcp_mark_skb_lost(sk, data);
                tcp_xmit_retransmit_queue(sk);
                NET_INC_STATS(sock_net(sk),
                                LINUX_MIB_TCPFASTOPENACTIVEFAIL);
                return true;
        }
        tp->syn_data_acked = tp->syn_data;
        if (tp->syn_data_acked) {
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFASTOPENACTIVE);
                /* SYN-data is counted as two separate packets in tcp_ack() */
                if (tp->delivered > 1)
                        --tp->delivered;
        }

        tcp_fastopen_add_skb(sk, synack);

        return false;
}

static void smc_check_reset_syn(struct tcp_sock *tp)
{
#if IS_ENABLED(CONFIG_SMC)
        if (static_branch_unlikely(&tcp_have_smc)) {
                if (tp->syn_smc && !tp->rx_opt.smc_ok)
                        tp->syn_smc = 0;
        }
#endif
}

static void tcp_try_undo_spurious_syn(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 syn_stamp;

        /* undo_marker is set when SYN or SYNACK times out. The timeout is
         * spurious if the ACK's timestamp option echo value matches the
         * original SYN timestamp.
         */
        syn_stamp = tp->retrans_stamp;
        if (tp->undo_marker && syn_stamp && tp->rx_opt.saw_tstamp &&
            syn_stamp == tp->rx_opt.rcv_tsecr)
                tp->undo_marker = 0;
}

static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                                         const struct tcphdr *th)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        struct tcp_fastopen_cookie foc = { .len = -1 };
        int saved_clamp = tp->rx_opt.mss_clamp;
        bool fastopen_fail;
        SKB_DR(reason);

        tcp_parse_options(sock_net(sk), skb, &tp->rx_opt, 0, &foc);
        if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
                tp->rx_opt.rcv_tsecr -= tp->tsoffset;

        if (th->ack) {
                /* rfc793:
                 * "If the state is SYN-SENT then
                 *    first check the ACK bit
                 *      If the ACK bit is set
                 *        If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send
                 *        a reset (unless the RST bit is set, if so drop
                 *        the segment and return)"
                 */
                if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) ||
                    after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
                        /* Previous FIN/ACK or RST/ACK might be ignored. */
                        if (icsk->icsk_retransmits == 0)
                                inet_csk_reset_xmit_timer(sk,
                                                ICSK_TIME_RETRANS,
                                                TCP_TIMEOUT_MIN, TCP_RTO_MAX);
                        goto reset_and_undo;
                }

                if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
                    !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,
                             tcp_time_stamp(tp))) {
                        NET_INC_STATS(sock_net(sk),
                                        LINUX_MIB_PAWSACTIVEREJECTED);
                        goto reset_and_undo;
                }

                /* Now ACK is acceptable.
                 *
                 * "If the RST bit is set
                 *    If the ACK was acceptable then signal the user "error:
                 *    connection reset", drop the segment, enter CLOSED state,
                 *    delete TCB, and return."
                 */

                if (th->rst) {
                        tcp_reset(sk, skb);
consume:
                        __kfree_skb(skb);
                        return 0;
                }

                /* rfc793:
                 *   "fifth, if neither of the SYN or RST bits is set then
                 *    drop the segment and return."
                 *
                 *    See note below!
                 *                                        --ANK(990513)
                 */
                if (!th->syn) {
                        SKB_DR_SET(reason, TCP_FLAGS);
                        goto discard_and_undo;
                }
                /* rfc793:
                 *   "If the SYN bit is on ...
                 *    are acceptable then ...
                 *    (our SYN has been ACKed), change the connection
                 *    state to ESTABLISHED..."
                 */

                tcp_ecn_rcv_synack(tp, th);

                tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
                tcp_try_undo_spurious_syn(sk);
                tcp_ack(sk, skb, FLAG_SLOWPATH);

                /* Ok.. it's good. Set up sequence numbers and
                 * move to established.
                 */
                WRITE_ONCE(tp->rcv_nxt, TCP_SKB_CB(skb)->seq + 1);
                tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;

                /* RFC1323: The window in SYN & SYN/ACK segments is
                 * never scaled.
                 */
                tp->snd_wnd = ntohs(th->window);

                if (!tp->rx_opt.wscale_ok) {
                        tp->rx_opt.snd_wscale = tp->rx_opt.rcv_wscale = 0;
                        tp->window_clamp = min(tp->window_clamp, 65535U);
                }

                if (tp->rx_opt.saw_tstamp) {
                        tp->rx_opt.tstamp_ok       = 1;
                        tp->tcp_header_len =
                                sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
                        tp->advmss          -= TCPOLEN_TSTAMP_ALIGNED;
                        tcp_store_ts_recent(tp);
                } else {
                        tp->tcp_header_len = sizeof(struct tcphdr);
                }

                tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
                tcp_initialize_rcv_mss(sk);

                /* Remember, tcp_poll() does not lock socket!
                 * Change state from SYN-SENT only after copied_seq
                 * is initialized. */
                WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);

                smc_check_reset_syn(tp);

                smp_mb();

                tcp_finish_connect(sk, skb);

                fastopen_fail = (tp->syn_fastopen || tp->syn_data) &&
                                tcp_rcv_fastopen_synack(sk, skb, &foc);

                if (!sock_flag(sk, SOCK_DEAD)) {
                        sk->sk_state_change(sk);
                        sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
                }
                if (fastopen_fail)
                        return -1;
                if (sk->sk_write_pending ||
                    icsk->icsk_accept_queue.rskq_defer_accept ||
                    inet_csk_in_pingpong_mode(sk)) {
                        /* Save one ACK. Data will be ready after
                         * several ticks, if write_pending is set.
                         *
                         * It may be deleted, but with this feature tcpdumps
                         * look so _wonderfully_ clever, that I was not able
                         * to stand against the temptation 8)     --ANK
                         */
                        inet_csk_schedule_ack(sk);
                        tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
                        inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
                                                  TCP_DELACK_MAX, TCP_RTO_MAX);
                        goto consume;
                }
                tcp_send_ack(sk);
                return -1;
        }

        /* No ACK in the segment */

        if (th->rst) {
                /* rfc793:
                 * "If the RST bit is set
                 *
                 *      Otherwise (no ACK) drop the segment and return."
                 */
                SKB_DR_SET(reason, TCP_RESET);
                goto discard_and_undo;
        }

        /* PAWS check. */
        if (tp->rx_opt.ts_recent_stamp && tp->rx_opt.saw_tstamp &&
            tcp_paws_reject(&tp->rx_opt, 0)) {
                SKB_DR_SET(reason, TCP_RFC7323_PAWS);
                goto discard_and_undo;
        }
        if (th->syn) {
                /* We see SYN without ACK. It is attempt of
                 * simultaneous connect with crossed SYNs.
                 * Particularly, it can be connect to self.
                 */
                tcp_set_state(sk, TCP_SYN_RECV);

                if (tp->rx_opt.saw_tstamp) {
                        tp->rx_opt.tstamp_ok = 1;
                        tcp_store_ts_recent(tp);
                        tp->tcp_header_len =
                                sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
                } else {
                        tp->tcp_header_len = sizeof(struct tcphdr);
                }

                WRITE_ONCE(tp->rcv_nxt, TCP_SKB_CB(skb)->seq + 1);
                WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
                tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;

                /* RFC1323: The window in SYN & SYN/ACK segments is
                 * never scaled.
                 */
                tp->snd_wnd    = ntohs(th->window);
                tp->snd_wl1    = TCP_SKB_CB(skb)->seq;
                tp->max_window = tp->snd_wnd;

                tcp_ecn_rcv_syn(tp, th);

                tcp_mtup_init(sk);
                tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
                tcp_initialize_rcv_mss(sk);

                tcp_send_synack(sk);
#if 0
                /* Note, we could accept data and URG from this segment.
                 * There are no obstacles to make this (except that we must
                 * either change tcp_recvmsg() to prevent it from returning data
                 * before 3WHS completes per RFC793, or employ TCP Fast Open).
                 *
                 * However, if we ignore data in ACKless segments sometimes,
                 * we have no reasons to accept it sometimes.
                 * Also, seems the code doing it in step6 of tcp_rcv_state_process
                 * is not flawless. So, discard packet for sanity.
                 * Uncomment this return to process the data.
                 */
                return -1;
#else
                goto consume;
#endif
        }
        /* "fifth, if neither of the SYN or RST bits is set then
         * drop the segment and return."
         */

discard_and_undo:
        tcp_clear_options(&tp->rx_opt);
        tp->rx_opt.mss_clamp = saved_clamp;
        tcp_drop_reason(sk, skb, reason);
        return 0;

reset_and_undo:
        tcp_clear_options(&tp->rx_opt);
        tp->rx_opt.mss_clamp = saved_clamp;
        return 1;
}

static void tcp_rcv_synrecv_state_fastopen(struct sock *sk)
{
        struct request_sock *req;

        /* If we are still handling the SYNACK RTO, see if timestamp ECR allows
         * undo. If peer SACKs triggered fast recovery, we can't undo here.
         */
        if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss)
                tcp_try_undo_loss(sk, false);

        /* Reset rtx states to prevent spurious retransmits_timed_out() */
        tcp_sk(sk)->retrans_stamp = 0;
        inet_csk(sk)->icsk_retransmits = 0;

        /* Once we leave TCP_SYN_RECV or TCP_FIN_WAIT_1,
         * we no longer need req so release it.
         */
        req = rcu_dereference_protected(tcp_sk(sk)->fastopen_rsk,
                                        lockdep_sock_is_held(sk));
        reqsk_fastopen_remove(sk, req, false);

        /* Re-arm the timer because data may have been sent out.
         * This is similar to the regular data transmission case
         * when new data has just been ack'ed.
         *
         * (TFO) - we could try to be more aggressive and
         * retransmitting any data sooner based on when they
         * are sent out.
         */
        tcp_rearm_rto(sk);
}

/*
 *      This function implements the receiving procedure of RFC 793 for
 *      all states except ESTABLISHED and TIME_WAIT.
 *      It's called from both tcp_v4_rcv and tcp_v6_rcv and should be
 *      address independent.
 */

int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);
        const struct tcphdr *th = tcp_hdr(skb);
        struct request_sock *req;
        int queued = 0;
        bool acceptable;
        SKB_DR(reason);

        switch (sk->sk_state) {
        case TCP_CLOSE:
                SKB_DR_SET(reason, TCP_CLOSE);
                goto discard;

        case TCP_LISTEN:
                if (th->ack)
                        return 1;

                if (th->rst) {
                        SKB_DR_SET(reason, TCP_RESET);
                        goto discard;
                }
                if (th->syn) {
                        if (th->fin) {
                                SKB_DR_SET(reason, TCP_FLAGS);
                                goto discard;
                        }
                        /* It is possible that we process SYN packets from backlog,
                         * so we need to make sure to disable BH and RCU right there.
                         */
                        rcu_read_lock();
                        local_bh_disable();
                        acceptable = icsk->icsk_af_ops->conn_request(sk, skb) >= 0;
                        local_bh_enable();
                        rcu_read_unlock();

                        if (!acceptable)
                                return 1;
                        consume_skb(skb);
                        return 0;
                }
                SKB_DR_SET(reason, TCP_FLAGS);
                goto discard;

        case TCP_SYN_SENT:
                tp->rx_opt.saw_tstamp = 0;
                tcp_mstamp_refresh(tp);
                queued = tcp_rcv_synsent_state_process(sk, skb, th);
                if (queued >= 0)
                        return queued;

                /* Do step6 onward by hand. */
                tcp_urg(sk, skb, th);
                __kfree_skb(skb);
                tcp_data_snd_check(sk);
                return 0;
        }

        tcp_mstamp_refresh(tp);
        tp->rx_opt.saw_tstamp = 0;
        req = rcu_dereference_protected(tp->fastopen_rsk,
                                        lockdep_sock_is_held(sk));
        if (req) {
                bool req_stolen;

                WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
                    sk->sk_state != TCP_FIN_WAIT1);

                if (!tcp_check_req(sk, skb, req, true, &req_stolen)) {
                        SKB_DR_SET(reason, TCP_FASTOPEN);
                        goto discard;
                }
        }

        if (!th->ack && !th->rst && !th->syn) {
                SKB_DR_SET(reason, TCP_FLAGS);
                goto discard;
        }
        if (!tcp_validate_incoming(sk, skb, th, 0))
                return 0;

        /* step 5: check the ACK field */
        acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH |
                                      FLAG_UPDATE_TS_RECENT |
                                      FLAG_NO_CHALLENGE_ACK) > 0;

        if (!acceptable) {
                if (sk->sk_state == TCP_SYN_RECV)
                        return 1;       /* send one RST */
                tcp_send_challenge_ack(sk);
                SKB_DR_SET(reason, TCP_OLD_ACK);
                goto discard;
        }
        switch (sk->sk_state) {
        case TCP_SYN_RECV:
                tp->delivered++; /* SYN-ACK delivery isn't tracked in tcp_ack */
                if (!tp->srtt_us)
                        tcp_synack_rtt_meas(sk, req);

                if (req) {
                        tcp_rcv_synrecv_state_fastopen(sk);
                } else {
                        tcp_try_undo_spurious_syn(sk);
                        tp->retrans_stamp = 0;
                        tcp_init_transfer(sk, BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB,
                                          skb);
                        WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
                }
                smp_mb();
                tcp_set_state(sk, TCP_ESTABLISHED);
                sk->sk_state_change(sk);

                /* Note, that this wakeup is only for marginal crossed SYN case.
                 * Passively open sockets are not waked up, because
                 * sk->sk_sleep == NULL and sk->sk_socket == NULL.
                 */
                if (sk->sk_socket)
                        sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);

                tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
                tp->snd_wnd = ntohs(th->window) << tp->rx_opt.snd_wscale;
                tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);

                if (tp->rx_opt.tstamp_ok)
                        tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;

                if (!inet_csk(sk)->icsk_ca_ops->cong_control)
                        tcp_update_pacing_rate(sk);

                /* Prevent spurious tcp_cwnd_restart() on first data packet */
                tp->lsndtime = tcp_jiffies32;

                tcp_initialize_rcv_mss(sk);
                tcp_fast_path_on(tp);
                break;

        case TCP_FIN_WAIT1: {
                int tmo;

                if (req)
                        tcp_rcv_synrecv_state_fastopen(sk);

                if (tp->snd_una != tp->write_seq)
                        break;

                tcp_set_state(sk, TCP_FIN_WAIT2);
                sk->sk_shutdown |= SEND_SHUTDOWN;

                sk_dst_confirm(sk);

                if (!sock_flag(sk, SOCK_DEAD)) {
                        /* Wake up lingering close() */
                        sk->sk_state_change(sk);
                        break;
                }

                if (tp->linger2 < 0) {
                        tcp_done(sk);
                        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
                        return 1;
                }
                if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
                    after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
                        /* Receive out of order FIN after close() */
                        if (tp->syn_fastopen && th->fin)
                                tcp_fastopen_active_disable(sk);
                        tcp_done(sk);
                        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
                        return 1;
                }

                tmo = tcp_fin_time(sk);
                if (tmo > TCP_TIMEWAIT_LEN) {
                        inet_csk_reset_keepalive_timer(sk, tmo - TCP_TIMEWAIT_LEN);
                } else if (th->fin || sock_owned_by_user(sk)) {
                        /* Bad case. We could lose such FIN otherwise.
                         * It is not a big problem, but it looks confusing
                         * and not so rare event. We still can lose it now,
                         * if it spins in bh_lock_sock(), but it is really
                         * marginal case.
                         */
                        inet_csk_reset_keepalive_timer(sk, tmo);
                } else {
                        tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
                        goto consume;
                }
                break;
        }

        case TCP_CLOSING:
                if (tp->snd_una == tp->write_seq) {
                        tcp_time_wait(sk, TCP_TIME_WAIT, 0);
                        goto consume;
                }
                break;

        case TCP_LAST_ACK:
                if (tp->snd_una == tp->write_seq) {
                        tcp_update_metrics(sk);
                        tcp_done(sk);
                        goto consume;
                }
                break;
        }

        /* step 6: check the URG bit */
        tcp_urg(sk, skb, th);

        /* step 7: process the segment text */
        switch (sk->sk_state) {
        case TCP_CLOSE_WAIT:
        case TCP_CLOSING:
        case TCP_LAST_ACK:
                if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
                        /* If a subflow has been reset, the packet should not
                         * continue to be processed, drop the packet.
                         */
                        if (sk_is_mptcp(sk) && !mptcp_incoming_options(sk, skb))
                                goto discard;
                        break;
                }
                fallthrough;
        case TCP_FIN_WAIT1:
        case TCP_FIN_WAIT2:
                /* RFC 793 says to queue data in these states,
                 * RFC 1122 says we MUST send a reset.
                 * BSD 4.4 also does reset.
                 */
                if (sk->sk_shutdown & RCV_SHUTDOWN) {
                        if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
                            after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
                                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
                                tcp_reset(sk, skb);
                                return 1;
                        }
                }
                fallthrough;
        case TCP_ESTABLISHED:
                tcp_data_queue(sk, skb);
                queued = 1;
                break;
        }

        /* tcp_data could move socket to TIME-WAIT */
        if (sk->sk_state != TCP_CLOSE) {
                tcp_data_snd_check(sk);
                tcp_ack_snd_check(sk);
        }

        if (!queued) {
discard:
                tcp_drop_reason(sk, skb, reason);
        }
        return 0;

consume:
        __kfree_skb(skb);
        return 0;
}
EXPORT_SYMBOL(tcp_rcv_state_process);

static inline void pr_drop_req(struct request_sock *req, __u16 port, int family)
{
        struct inet_request_sock *ireq = inet_rsk(req);

        if (family == AF_INET)
                net_dbg_ratelimited("drop open request from %pI4/%u\n",
                                    &ireq->ir_rmt_addr, port);
#if IS_ENABLED(CONFIG_IPV6)
        else if (family == AF_INET6)
                net_dbg_ratelimited("drop open request from %pI6/%u\n",
                                    &ireq->ir_v6_rmt_addr, port);
#endif
}

/* RFC3168 : 6.1.1 SYN packets must not have ECT/ECN bits set
 *
 * If we receive a SYN packet with these bits set, it means a
 * network is playing bad games with TOS bits. In order to
 * avoid possible false congestion notifications, we disable
 * TCP ECN negotiation.
 *
 * Exception: tcp_ca wants ECN. This is required for DCTCP
 * congestion control: Linux DCTCP asserts ECT on all packets,
 * including SYN, which is most optimal solution; however,
 * others, such as FreeBSD do not.
 *
 * Exception: At least one of the reserved bits of the TCP header (th->res1) is
 * set, indicating the use of a future TCP extension (such as AccECN). See
 * RFC8311 §4.3 which updates RFC3168 to allow the development of such
 * extensions.
 */
static void tcp_ecn_create_request(struct request_sock *req,
                                   const struct sk_buff *skb,
                                   const struct sock *listen_sk,
                                   const struct dst_entry *dst)
{
        const struct tcphdr *th = tcp_hdr(skb);
        const struct net *net = sock_net(listen_sk);
        bool th_ecn = th->ece && th->cwr;
        bool ect, ecn_ok;
        u32 ecn_ok_dst;

        if (!th_ecn)
                return;

        ect = !INET_ECN_is_not_ect(TCP_SKB_CB(skb)->ip_dsfield);
        ecn_ok_dst = dst_feature(dst, DST_FEATURE_ECN_MASK);
        ecn_ok = net->ipv4.sysctl_tcp_ecn || ecn_ok_dst;

        if (((!ect || th->res1) && ecn_ok) || tcp_ca_needs_ecn(listen_sk) ||
            (ecn_ok_dst & DST_FEATURE_ECN_CA) ||
            tcp_bpf_ca_needs_ecn((struct sock *)req))
                inet_rsk(req)->ecn_ok = 1;
}

static void tcp_openreq_init(struct request_sock *req,
                             const struct tcp_options_received *rx_opt,
                             struct sk_buff *skb, const struct sock *sk)
{
        struct inet_request_sock *ireq = inet_rsk(req);

        req->rsk_rcv_wnd = 0;           /* So that tcp_send_synack() knows! */
        tcp_rsk(req)->rcv_isn = TCP_SKB_CB(skb)->seq;
        tcp_rsk(req)->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
        tcp_rsk(req)->snt_synack = 0;
        tcp_rsk(req)->last_oow_ack_time = 0;
        req->mss = rx_opt->mss_clamp;
        req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0;
        ireq->tstamp_ok = rx_opt->tstamp_ok;
        ireq->sack_ok = rx_opt->sack_ok;
        ireq->snd_wscale = rx_opt->snd_wscale;
        ireq->wscale_ok = rx_opt->wscale_ok;
        ireq->acked = 0;
        ireq->ecn_ok = 0;
        ireq->ir_rmt_port = tcp_hdr(skb)->source;
        ireq->ir_num = ntohs(tcp_hdr(skb)->dest);
        ireq->ir_mark = inet_request_mark(sk, skb);
#if IS_ENABLED(CONFIG_SMC)
        ireq->smc_ok = rx_opt->smc_ok && !(tcp_sk(sk)->smc_hs_congested &&
                        tcp_sk(sk)->smc_hs_congested(sk));
#endif
}

struct request_sock *inet_reqsk_alloc(const struct request_sock_ops *ops,
                                      struct sock *sk_listener,
                                      bool attach_listener)
{
        struct request_sock *req = reqsk_alloc(ops, sk_listener,
                                               attach_listener);

        if (req) {
                struct inet_request_sock *ireq = inet_rsk(req);

                ireq->ireq_opt = NULL;
#if IS_ENABLED(CONFIG_IPV6)
                ireq->pktopts = NULL;
#endif
                atomic64_set(&ireq->ir_cookie, 0);
                ireq->ireq_state = TCP_NEW_SYN_RECV;
                write_pnet(&ireq->ireq_net, sock_net(sk_listener));
                ireq->ireq_family = sk_listener->sk_family;
                req->timeout = TCP_TIMEOUT_INIT;
        }

        return req;
}
EXPORT_SYMBOL(inet_reqsk_alloc);

/*
 * Return true if a syncookie should be sent
 */
static bool tcp_syn_flood_action(const struct sock *sk, const char *proto)
{
        struct request_sock_queue *queue = &inet_csk(sk)->icsk_accept_queue;
        const char *msg = "Dropping request";
        bool want_cookie = false;
        struct net *net = sock_net(sk);

#ifdef CONFIG_SYN_COOKIES
        if (net->ipv4.sysctl_tcp_syncookies) {
                msg = "Sending cookies";
                want_cookie = true;
                __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDOCOOKIES);
        } else
#endif
                __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDROP);

        if (!queue->synflood_warned &&
            net->ipv4.sysctl_tcp_syncookies != 2 &&
            xchg(&queue->synflood_warned, 1) == 0)
                net_info_ratelimited("%s: Possible SYN flooding on port %d. %s.  Check SNMP counters.\n",
                                     proto, sk->sk_num, msg);

        return want_cookie;
}

static void tcp_reqsk_record_syn(const struct sock *sk,
                                 struct request_sock *req,
                                 const struct sk_buff *skb)
{
        if (tcp_sk(sk)->save_syn) {
                u32 len = skb_network_header_len(skb) + tcp_hdrlen(skb);
                struct saved_syn *saved_syn;
                u32 mac_hdrlen;
                void *base;

                if (tcp_sk(sk)->save_syn == 2) {  /* Save full header. */
                        base = skb_mac_header(skb);
                        mac_hdrlen = skb_mac_header_len(skb);
                        len += mac_hdrlen;
                } else {
                        base = skb_network_header(skb);
                        mac_hdrlen = 0;
                }

                saved_syn = kmalloc(struct_size(saved_syn, data, len),
                                    GFP_ATOMIC);
                if (saved_syn) {
                        saved_syn->mac_hdrlen = mac_hdrlen;
                        saved_syn->network_hdrlen = skb_network_header_len(skb);
                        saved_syn->tcp_hdrlen = tcp_hdrlen(skb);
                        memcpy(saved_syn->data, base, len);
                        req->saved_syn = saved_syn;
                }
        }
}

/* If a SYN cookie is required and supported, returns a clamped MSS value to be
 * used for SYN cookie generation.
 */
u16 tcp_get_syncookie_mss(struct request_sock_ops *rsk_ops,
                          const struct tcp_request_sock_ops *af_ops,
                          struct sock *sk, struct tcphdr *th)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u16 mss;

        if (sock_net(sk)->ipv4.sysctl_tcp_syncookies != 2 &&
            !inet_csk_reqsk_queue_is_full(sk))
                return 0;

        if (!tcp_syn_flood_action(sk, rsk_ops->slab_name))
                return 0;

        if (sk_acceptq_is_full(sk)) {
                NET_INC_STATS(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
                return 0;
        }

        mss = tcp_parse_mss_option(th, tp->rx_opt.user_mss);
        if (!mss)
                mss = af_ops->mss_clamp;

        return mss;
}
EXPORT_SYMBOL_GPL(tcp_get_syncookie_mss);

int tcp_conn_request(struct request_sock_ops *rsk_ops,
                     const struct tcp_request_sock_ops *af_ops,
                     struct sock *sk, struct sk_buff *skb)
{
        struct tcp_fastopen_cookie foc = { .len = -1 };
        __u32 isn = TCP_SKB_CB(skb)->tcp_tw_isn;
        struct tcp_options_received tmp_opt;
        struct tcp_sock *tp = tcp_sk(sk);
        struct net *net = sock_net(sk);
        struct sock *fastopen_sk = NULL;
        struct request_sock *req;
        bool want_cookie = false;
        struct dst_entry *dst;
        struct flowi fl;

        /* TW buckets are converted to open requests without
         * limitations, they conserve resources and peer is
         * evidently real one.
         */
        if ((net->ipv4.sysctl_tcp_syncookies == 2 ||
             inet_csk_reqsk_queue_is_full(sk)) && !isn) {
                want_cookie = tcp_syn_flood_action(sk, rsk_ops->slab_name);
                if (!want_cookie)
                        goto drop;
        }

        if (sk_acceptq_is_full(sk)) {
                NET_INC_STATS(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
                goto drop;
        }

        req = inet_reqsk_alloc(rsk_ops, sk, !want_cookie);
        if (!req)
                goto drop;

        req->syncookie = want_cookie;
        tcp_rsk(req)->af_specific = af_ops;
        tcp_rsk(req)->ts_off = 0;
#if IS_ENABLED(CONFIG_MPTCP)
        tcp_rsk(req)->is_mptcp = 0;
#endif

        tcp_clear_options(&tmp_opt);
        tmp_opt.mss_clamp = af_ops->mss_clamp;
        tmp_opt.user_mss  = tp->rx_opt.user_mss;
        tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0,
                          want_cookie ? NULL : &foc);

        if (want_cookie && !tmp_opt.saw_tstamp)
                tcp_clear_options(&tmp_opt);

        if (IS_ENABLED(CONFIG_SMC) && want_cookie)
                tmp_opt.smc_ok = 0;

        tmp_opt.tstamp_ok = tmp_opt.saw_tstamp;
        tcp_openreq_init(req, &tmp_opt, skb, sk);
        inet_rsk(req)->no_srccheck = inet_sk(sk)->transparent;

        /* Note: tcp_v6_init_req() might override ir_iif for link locals */
        inet_rsk(req)->ir_iif = inet_request_bound_dev_if(sk, skb);

        dst = af_ops->route_req(sk, skb, &fl, req);
        if (!dst)
                goto drop_and_free;

        if (tmp_opt.tstamp_ok)
                tcp_rsk(req)->ts_off = af_ops->init_ts_off(net, skb);

        if (!want_cookie && !isn) {
                /* Kill the following clause, if you dislike this way. */
                if (!net->ipv4.sysctl_tcp_syncookies &&
                    (net->ipv4.sysctl_max_syn_backlog - inet_csk_reqsk_queue_len(sk) <
                     (net->ipv4.sysctl_max_syn_backlog >> 2)) &&
                    !tcp_peer_is_proven(req, dst)) {
                        /* Without syncookies last quarter of
                         * backlog is filled with destinations,
                         * proven to be alive.
                         * It means that we continue to communicate
                         * to destinations, already remembered
                         * to the moment of synflood.
                         */
                        pr_drop_req(req, ntohs(tcp_hdr(skb)->source),
                                    rsk_ops->family);
                        goto drop_and_release;
                }

                isn = af_ops->init_seq(skb);
        }

        tcp_ecn_create_request(req, skb, sk, dst);

        if (want_cookie) {
                isn = cookie_init_sequence(af_ops, sk, skb, &req->mss);
                if (!tmp_opt.tstamp_ok)
                        inet_rsk(req)->ecn_ok = 0;
        }

        tcp_rsk(req)->snt_isn = isn;
        tcp_rsk(req)->txhash = net_tx_rndhash();
        tcp_rsk(req)->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
        tcp_openreq_init_rwin(req, sk, dst);
        sk_rx_queue_set(req_to_sk(req), skb);
        if (!want_cookie) {
                tcp_reqsk_record_syn(sk, req, skb);
                fastopen_sk = tcp_try_fastopen(sk, skb, req, &foc, dst);
        }
        if (fastopen_sk) {
                af_ops->send_synack(fastopen_sk, dst, &fl, req,
                                    &foc, TCP_SYNACK_FASTOPEN, skb);
                /* Add the child socket directly into the accept queue */
                if (!inet_csk_reqsk_queue_add(sk, req, fastopen_sk)) {
                        reqsk_fastopen_remove(fastopen_sk, req, false);
                        bh_unlock_sock(fastopen_sk);
                        sock_put(fastopen_sk);
                        goto drop_and_free;
                }
                sk->sk_data_ready(sk);
                bh_unlock_sock(fastopen_sk);
                sock_put(fastopen_sk);
        } else {
                tcp_rsk(req)->tfo_listener = false;
                if (!want_cookie) {
                        req->timeout = tcp_timeout_init((struct sock *)req);
                        inet_csk_reqsk_queue_hash_add(sk, req, req->timeout);
                }
                af_ops->send_synack(sk, dst, &fl, req, &foc,
                                    !want_cookie ? TCP_SYNACK_NORMAL :
                                                   TCP_SYNACK_COOKIE,
                                    skb);
                if (want_cookie) {
                        reqsk_free(req);
                        return 0;
                }
        }
        reqsk_put(req);
        return 0;

drop_and_release:
        dst_release(dst);
drop_and_free:
        __reqsk_free(req);
drop:
        tcp_listendrop(sk);
        return 0;
}
EXPORT_SYMBOL(tcp_conn_request);