tcp: remove poll() flakes with FastOpen

When using TCP FastOpen for an active session, we send one wakeup event
from tcp_finish_connect(), right before the data eventually contained in
the received SYNACK is queued to sk->sk_receive_queue.

This means that depending on machine load or luck, poll() users
might receive POLLOUT events instead of POLLIN|POLLOUT

To fix this, we need to move the call to sk->sk_state_change()
after the (optional) call to tcp_rcv_fastopen_synack()

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

1 files changed, 9 insertions, 7 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 37e2aa925f62..341f021f02a2 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5580,10 +5580,6 @@ void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
         else
                 tp->pred_flags = 0;
 
-        if (!sock_flag(sk, SOCK_DEAD)) {
-                sk->sk_state_change(sk);
-                sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
-        }
 }
 
 static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
@@ -5652,6 +5648,7 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
         struct tcp_sock *tp = tcp_sk(sk);
         struct tcp_fastopen_cookie foc = { .len = -1 };
         int saved_clamp = tp->rx_opt.mss_clamp;
+        bool fastopen_fail;
 
         tcp_parse_options(skb, &tp->rx_opt, 0, &foc);
         if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
@@ -5755,10 +5752,15 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 
                 tcp_finish_connect(sk, skb);
 
-                if ((tp->syn_fastopen || tp->syn_data) &&
-                    tcp_rcv_fastopen_synack(sk, skb, &foc))
-                        return -1;
+                fastopen_fail = (tp->syn_fastopen || tp->syn_data) &&
+                                tcp_rcv_fastopen_synack(sk, skb, &foc);
 
+                if (!sock_flag(sk, SOCK_DEAD)) {
+                        sk->sk_state_change(sk);
+                        sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
+                }
+                if (fastopen_fail)
+                        return -1;
                 if (sk->sk_write_pending ||
                     icsk->icsk_accept_queue.rskq_defer_accept ||
                     icsk->icsk_ack.pingpong) {