[PATCH net] tcp: fix TFO max_qlen accounting across reuseport migration

[PATCH net] tcp: fix TFO max_qlen accounting across reuseport migration

[Jiayuan Chen]

A listener's TCP_FASTOPEN max_qlen stops being accurate and lets through
far more pending Fast Open requests than it was configured for.

This only shows up with SO_REUSEPORT listener migration, where closing a
listener hands its still-pending TFO children over to a surviving one.

fastopenq.qlen is charged in tcp_fastopen_create_child() when the child
is created and uncharged in reqsk_fastopen_remove() when the handshake
completes.  The uncharge follows rsk_listener of the request the child
points at, and inet_reqsk_clone() has repointed the child at a new
request owned by the new listener, so the ++ and the -- land on two
different sockets.  The new listener's qlen drifts negative and its
limit no longer binds.

Charge the new listener during migration, like reqsk_queue_migrated()
already does for queue->young and queue->qlen.

Fixes: 54b92e841937 ("tcp: Migrate TCP_ESTABLISHED/TCP_SYN_RECV sockets in accept queues.")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
fastopenq.qlen is read without the lock in tcp_fastopen_queue_check() and
has been since long before this patch, but as this is meant to be
backported I left the READ_ONCE()/WRITE_ONCE() conversion to a separate
patch.
---
 net/ipv4/inet_connection_sock.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 56902bba5483..6257459bcee2 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -943,11 +943,23 @@ static struct request_sock *inet_reqsk_clone(struct request_sock *req,
 
 	nreq->rsk_listener = sk;
 
-	/* We need not acquire fastopenq->lock
-	 * because the child socket is locked in inet_csk_listen_stop().
-	 */
-	if (sk->sk_protocol == IPPROTO_TCP && tcp_rsk(nreq)->tfo_listener)
+	if (sk->sk_protocol == IPPROTO_TCP && tcp_rsk(nreq)->tfo_listener) {
+		struct fastopen_queue *fastopenq;
+
+		/* reqsk_fastopen_remove() will uncharge nreq->rsk_listener,
+		 * that is @sk, so charge it here.  Unlike the listener
+		 * being closed, @sk is live and needs its lock.
+		 */
+		fastopenq = &inet_csk(sk)->icsk_accept_queue.fastopenq;
+		spin_lock_bh(&fastopenq->lock);
+		fastopenq->qlen++;
+		spin_unlock_bh(&fastopenq->lock);
+
+		/* We need not acquire fastopenq->lock
+		 * because the child socket is locked in inet_csk_listen_stop().
+		 */
 		rcu_assign_pointer(tcp_sk(nreq->sk)->fastopen_rsk, nreq);
+	}
 
 	return nreq;
 }
-- 
2.43.0

Re: [PATCH net] tcp: fix TFO max_qlen accounting across reuseport migration

[Kuniyuki Iwashima]

On Sun, Aug 2, 2026 at 11:18 PM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
>
> A listener's TCP_FASTOPEN max_qlen stops being accurate and lets through
> far more pending Fast Open requests than it was configured for.
>
> This only shows up with SO_REUSEPORT listener migration, where closing a
> listener hands its still-pending TFO children over to a surviving one.
>
> fastopenq.qlen is charged in tcp_fastopen_create_child() when the child
> is created and uncharged in reqsk_fastopen_remove() when the handshake
> completes.  The uncharge follows rsk_listener of the request the child
> points at, and inet_reqsk_clone() has repointed the child at a new
> request owned by the new listener, so the ++ and the -- land on two
> different sockets.  The new listener's qlen drifts negative and its
> limit no longer binds.
>
> Charge the new listener during migration, like reqsk_queue_migrated()
> already does for queue->young and queue->qlen.
>
> Fixes: 54b92e841937 ("tcp: Migrate TCP_ESTABLISHED/TCP_SYN_RECV sockets in accept queues.")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

Re: [PATCH net] tcp: fix TFO max_qlen accounting across reuseport migration

[Eric Dumazet]

On Wed, Aug 5, 2026 at 7:15 AM Kuniyuki Iwashima <kuniyu@google.com> wrote:
>
> On Sun, Aug 2, 2026 at 11:18 PM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
> >
> > A listener's TCP_FASTOPEN max_qlen stops being accurate and lets through
> > far more pending Fast Open requests than it was configured for.
> >
> > This only shows up with SO_REUSEPORT listener migration, where closing a
> > listener hands its still-pending TFO children over to a surviving one.
> >
> > fastopenq.qlen is charged in tcp_fastopen_create_child() when the child
> > is created and uncharged in reqsk_fastopen_remove() when the handshake
> > completes.  The uncharge follows rsk_listener of the request the child
> > points at, and inet_reqsk_clone() has repointed the child at a new
> > request owned by the new listener, so the ++ and the -- land on two
> > different sockets.  The new listener's qlen drifts negative and its
> > limit no longer binds.
> >
> > Charge the new listener during migration, like reqsk_queue_migrated()
> > already does for queue->young and queue->qlen.
> >
> > Fixes: 54b92e841937 ("tcp: Migrate TCP_ESTABLISHED/TCP_SYN_RECV sockets in accept queues.")
> > Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
>
> Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

Reviewed-by: Eric Dumazet <edumazet@google.com>

Re: [PATCH net] tcp: fix TFO max_qlen accounting across reuseport migration

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Mon,  3 Aug 2026 14:17:38 +0800 you wrote:
> A listener's TCP_FASTOPEN max_qlen stops being accurate and lets through
> far more pending Fast Open requests than it was configured for.
> 
> This only shows up with SO_REUSEPORT listener migration, where closing a
> listener hands its still-pending TFO children over to a surviving one.
> 
> fastopenq.qlen is charged in tcp_fastopen_create_child() when the child
> is created and uncharged in reqsk_fastopen_remove() when the handshake
> completes.  The uncharge follows rsk_listener of the request the child
> points at, and inet_reqsk_clone() has repointed the child at a new
> request owned by the new listener, so the ++ and the -- land on two
> different sockets.  The new listener's qlen drifts negative and its
> limit no longer binds.
> 
> [...]

Here is the summary with links:
  - [net] tcp: fix TFO max_qlen accounting across reuseport migration
    https://git.kernel.org/netdev/net/c/a0ab2ba83e35

You are awesome, thank you!
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