netdev - Re: [RFC net] net: stream: don't purge sk_error

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Message-ID: <20210914071836.46813650@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 14 Sep 2021 07:18:36 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     willemb@...gle.com, netdev@...r.kernel.org
Subject: Re: [RFC net] net: stream: don't purge sk_error_queue without
 holding its lock

On Mon, 13 Sep 2021 22:14:00 -0700 Eric Dumazet wrote:
> On 9/13/21 3:38 PM, Jakub Kicinski wrote:
> > sk_stream_kill_queues() can be called when there are still
> > outstanding skbs to transmit. Those skbs may try to queue
> > notifications to the error queue (e.g. timestamps).
> > If sk_stream_kill_queues() purges the queue without taking
> > its lock the queue may get corrupted.
> > 
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> > ---
> > Sending as an RFC for review, compile-tested only.
> > 
> > Seems far more likely that I'm missing something than that
> > this has been broken forever and nobody noticed :S
> > ---
> >  net/core/stream.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/core/stream.c b/net/core/stream.c
> > index 4f1d4aa5fb38..7c585088f394 100644
> > --- a/net/core/stream.c
> > +++ b/net/core/stream.c
> > @@ -196,7 +196,7 @@ void sk_stream_kill_queues(struct sock *sk)
> >  	__skb_queue_purge(&sk->sk_receive_queue);
> >  
> >  	/* Next, the error queue. */
> > -	__skb_queue_purge(&sk->sk_error_queue);
> > +	skb_queue_purge(&sk->sk_error_queue);
> >  
> >  	/* Next, the write queue. */
> >  	WARN_ON(!skb_queue_empty(&sk->sk_write_queue));
> 
> This should not be needed.
> 
> By definition, sk_stream_kill_queues() is only called when there is no
> more references on the sockets.
> 
> So all outstanding packets must have been orphaned or freed.

I don't see the wait anywhere, would you mind spelling it out?
My (likely flawed) understanding is that inet_sock_destruct() gets
called when refs are gone (via sk->sk_destruct).

But tcp_disconnect() + tcp_close() seem to happily call
inet_csk_destroy_sock() -> sk_stream_kill_queues() with outstanding
sk_wmem_alloc refs.

> Anyway, Linux-2.6.12-rc2 had no timestamps yet.

I see, thanks, if some form of the patch stands perhaps:

Fixes: cb9eff097831 ("net: new user space API for time stamping of incoming and outgoing packets")