tcp: fix wrong checksum calculation on MTU probing

With TCP MTU probing enabled and offload TX checksumming disabled,
tcp_mtu_probe() calculated the wrong checksum when a fragment being copied
into the probe's SKB had an odd length. This was caused by the direct use
of skb_copy_and_csum_bits() to calculate the checksum, as it pads the
fragment being copied, if needed. When this fragment was not the last, a
subsequent call used the previous checksum without considering this
padding.

The effect was a stale connection in one way, as even retransmissions
wouldn't solve the problem, because the checksum was never recalculated for
the full SKB length.

Signed-off-by: Douglas Caetano dos Santos <douglascs@taghos.com.br>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 5288cec..d48d557 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1966,12 +1966,14 @@ static int tcp_mtu_probe(struct sock *sk)
        len = 0;
        tcp_for_write_queue_from_safe(skb, next, sk) {
                copy = min_t(int, skb->len, probe_size - len);
-               if (nskb->ip_summed)
+               if (nskb->ip_summed) {
                        skb_copy_bits(skb, 0, skb_put(nskb, copy), copy);
-               else
-                       nskb->csum = skb_copy_and_csum_bits(skb, 0,
-                                                           skb_put(nskb, copy),
-                                                           copy, nskb->csum);
+               } else {
+                       __wsum csum = skb_copy_and_csum_bits(skb, 0,
+                                                            skb_put(nskb, copy),
+                                                            copy, 0);
+                       nskb->csum = csum_block_add(nskb->csum, csum, len);
+               }
 
                if (skb->len <= copy) {
                        /* We've eaten all the data from this skb.