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>

2 files changed, 10 insertions, 3 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e2bec815ff23..bb326684495b 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5983,6 +5983,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
                                  * need req so release it.
                                  */
                                 if (req) {
+                                        tcp_synack_rtt_meas(sk, req);
                                         reqsk_fastopen_remove(sk, req, false);
                                 } else {
                                         /* Make sure socket is routed, for
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 1e66f7fb4fe6..0a7e020f16b5 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -868,6 +868,8 @@ static int tcp_v4_send_synack(struct sock *sk, struct dst_entry *dst,
                                             ireq->rmt_addr,
                                             ireq->opt);
                 err = net_xmit_eval(err);
+                if (!tcp_rsk(req)->snt_synack && !err)
+                        tcp_rsk(req)->snt_synack = tcp_time_stamp;
         }
 
         return err;
@@ -1382,6 +1384,7 @@ static int tcp_v4_conn_req_fastopen(struct sock *sk,
         struct request_sock_queue *queue = &inet_csk(sk)->icsk_accept_queue;
         const struct inet_request_sock *ireq = inet_rsk(req);
         struct sock *child;
+        int err;
 
         req->retrans = 0;
         req->sk = NULL;
@@ -1393,8 +1396,11 @@ static int tcp_v4_conn_req_fastopen(struct sock *sk,
                 kfree_skb(skb_synack);
                 return -1;
         }
-        ip_build_and_send_pkt(skb_synack, sk, ireq->loc_addr,
-                        ireq->rmt_addr, ireq->opt);
+        err = ip_build_and_send_pkt(skb_synack, sk, ireq->loc_addr,
+                                    ireq->rmt_addr, ireq->opt);
+        err = net_xmit_eval(err);
+        if (!err)
+                tcp_rsk(req)->snt_synack = tcp_time_stamp;
         /* XXX (TFO) - is it ok to ignore error and continue? */
 
         spin_lock(&queue->fastopenq->lock);
@@ -1612,7 +1618,6 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
                 isn = tcp_v4_init_sequence(skb);
         }
         tcp_rsk(req)->snt_isn = isn;
-        tcp_rsk(req)->snt_synack = tcp_time_stamp;
 
         if (dst == NULL) {
                 dst = inet_csk_route_req(sk, &fl4, req);
@@ -1650,6 +1655,7 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
                 if (err || want_cookie)
                         goto drop_and_free;
 
+                tcp_rsk(req)->snt_synack = tcp_time_stamp;
                 tcp_rsk(req)->listener = NULL;
                 /* Add the request_sock to the SYN table */
                 inet_csk_reqsk_queue_hash_add(sk, req, TCP_TIMEOUT_INIT);