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@xxxxxxxxx> 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@xxxxxxxxx>

Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>