tcp: better SYNACK sent timestamp

Detecting spurious SYNACK timeout using timestamp option requires
recording the exact SYNACK skb timestamp. Previously the SYNACK
sent timestamp was stamped slightly earlier before the skb
was transmitted. This patch uses the SYNACK skb transmission
timestamp directly.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 0 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 32061928b054..0c4ed66dc1bf 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3247,7 +3247,11 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
                 skb->skb_mstamp_ns = cookie_init_timestamp(req);
         else
 #endif
+        {
                 skb->skb_mstamp_ns = tcp_clock_ns();
+                if (!tcp_rsk(req)->snt_synack) /* Timestamp first SYNACK */
+                        tcp_rsk(req)->snt_synack = tcp_skb_timestamp_us(skb);
+        }
 
 #ifdef CONFIG_TCP_MD5SIG
         rcu_read_lock();