tcp: TCP Fast Open Server - take SYNACK RTT after completing 3WHS

When taking SYNACK RTT samples for servers using TCP Fast Open, fix
the code to ensure that we only call tcp_valid_rtt_meas() after we
receive the ACK that completes the 3-way handshake.

Previously we were always taking an RTT sample in
tcp_v4_syn_recv_sock(). However, for TCP Fast Open connections
tcp_v4_conn_req_fastopen() calls tcp_v4_syn_recv_sock() at the time we
receive the SYN. So for TFO we must wait until tcp_rcv_state_process()
to take the RTT sample.

To fix this, we wait until after TFO calls tcp_v4_syn_recv_sock()
before we set the snt_synack timestamp, since tcp_synack_rtt_meas()
already ensures that we only take a SYNACK RTT sample if snt_synack is
non-zero. To be careful, we only take a snt_synack timestamp when
a SYNACK transmit or retransmit succeeds.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1 insertions, 1 deletions

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index cfeeeb7fcf54..d6212d6bc8d8 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1169,7 +1169,6 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
         }
 have_isn:
         tcp_rsk(req)->snt_isn = isn;
-        tcp_rsk(req)->snt_synack = tcp_time_stamp;
 
         if (security_inet_conn_request(sk, skb, req))
                 goto drop_and_release;
@@ -1180,6 +1179,7 @@ have_isn:
             want_cookie)
                 goto drop_and_free;
 
+        tcp_rsk(req)->snt_synack = tcp_time_stamp;
         tcp_rsk(req)->listener = NULL;
         inet6_csk_reqsk_queue_hash_add(sk, req, TCP_TIMEOUT_INIT);
         return 0;