linux-kernel - [PATCH net v2 2/2] tcp: limit wake-up for crossed SYN cases to SYN-ACK

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Message-Id: <20240718-upstream-net-next-20240716-tcp-3rd-ack-consume-sk_socket-v2-2-d653f85639f6@kernel.org>
Date: Thu, 18 Jul 2024 12:33:57 +0200
From: "Matthieu Baerts (NGI0)" <matttbe@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>, 
 "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Kuniyuki Iwashima <kuniyu@...zon.com>, Jerry Chu <hkchu@...gle.com>
Cc: netdev@...r.kernel.org, mptcp@...ts.linux.dev, 
 linux-kernel@...r.kernel.org, "Matthieu Baerts (NGI0)" <matttbe@...nel.org>
Subject: [PATCH net v2 2/2] tcp: limit wake-up for crossed SYN cases to
 SYN-ACK

sk->sk_socket will be assigned in case of marginal crossed SYN, but also
in other cases, e.g.

 - With TCP Fast Open, if the connection got accept()'ed before
   receiving the 3rd ACK ;

 - With MPTCP, when accepting additional subflows to an existing MPTCP
   connection.

In these cases, the switch to TCP_ESTABLISHED is done when receiving the
3rd ACK, without the SYN flag then.

To properly restrict the wake-up to crossed SYN cases, it is then
required to also limit the check to packets containing the SYN-ACK
flags.

While at it, also update the attached comment: sk->sk_sleep has been
removed in 2010, and replaced by sk->sk_wq in commit 43815482370c ("net:
sock_def_readable() and friends RCU conversion").

Fixes: 168a8f58059a ("tcp: TCP Fast Open Server - main code path")
Suggested-by: Kuniyuki Iwashima <kuniyu@...zon.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>
---
Notes:
 - The above 'Fixes' tag should correspond to the commit introducing the
   possibility to have sk->sk_socket being set there in other cases than
   the crossed SYN one. But I might have missed other cases. Maybe
   1da177e4c3f4 ("Linux-2.6.12-rc2") might be safer? On the other hand,
   I don't think this wake-up was causing any visible issue, apart from
   not being needed.
---
 net/ipv4/tcp_input.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index bfe1bc69dc3e..5cebb389bf71 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6797,9 +6797,9 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)

 		/* Note, that this wakeup is only for marginal crossed SYN case.
 		 * Passively open sockets are not waked up, because
-		 * sk->sk_sleep == NULL and sk->sk_socket == NULL.
+		 * sk->sk_wq == NULL and sk->sk_socket == NULL.
 		 */
-		if (sk->sk_socket)
+		if (sk->sk_socket && th->syn)
 			sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);

 		tp->snd_una = TCP_SKB_CB(skb)->ack_seq;

-- 
2.45.2