net: tcp: dctcp_update_alpha() fixes.

dctcp_alpha can be read by from dctcp_get_info() without
synchro, so use WRITE_ONCE() to prevent compiler from using
dctcp_alpha as a temporary variable.

Also, playing with small dctcp_shift_g (like 1), can expose
an overflow with 32bit values shifted 9 times before divide.

Use an u64 field to avoid this problem, and perform the divide
only if acked_bytes_ecn is not zero.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_dctcp.c b/net/ipv4/tcp_dctcp.c
index 4c41c12..7092a61 100644 (file)
--- a/net/ipv4/tcp_dctcp.c
+++ b/net/ipv4/tcp_dctcp.c
@@ -204,20 +204,26 @@ static void dctcp_update_alpha(struct sock *sk, u32 flags)
 
        /* Expired RTT */
        if (!before(tp->snd_una, ca->next_seq)) {
-               /* For avoiding denominator == 1. */
-               if (ca->acked_bytes_total == 0)
-                       ca->acked_bytes_total = 1;
+               u64 bytes_ecn = ca->acked_bytes_ecn;
+               u32 alpha = ca->dctcp_alpha;
 
                /* alpha = (1 - g) * alpha + g * F */
-               ca->dctcp_alpha = ca->dctcp_alpha -
-                                 (ca->dctcp_alpha >> dctcp_shift_g) +
-                                 (ca->acked_bytes_ecn << (10U - dctcp_shift_g)) /
-                                 ca->acked_bytes_total;
 
-               if (ca->dctcp_alpha > DCTCP_MAX_ALPHA)
-                       /* Clamp dctcp_alpha to max. */
-                       ca->dctcp_alpha = DCTCP_MAX_ALPHA;
+               alpha -= alpha >> dctcp_shift_g;
+               if (bytes_ecn) {
+                       /* If dctcp_shift_g == 1, a 32bit value would overflow
+                        * after 8 Mbytes.
+                        */
+                       bytes_ecn <<= (10 - dctcp_shift_g);
+                       do_div(bytes_ecn, max(1U, ca->acked_bytes_total));
 
+                       alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA);
+               }
+               /* dctcp_alpha can be read from dctcp_get_info() without
+                * synchro, so we ask compiler to not use dctcp_alpha
+                * as a temporary variable in prior operations.
+                */
+               WRITE_ONCE(ca->dctcp_alpha, alpha);
                dctcp_reset(tp, ca);
        }
 }