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

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Date:   Tue, 14 Sep 2021 09:56:21 -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 Tue, 14 Sep 2021 09:32:09 -0700 Eric Dumazet wrote:
> On 9/14/21 7:18 AM, Jakub Kicinski wrote:
> > On Mon, 13 Sep 2021 22:14:00 -0700 Eric Dumazet wrote:  

> >> 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.  
> 
> tcp_disconnect() should probably leave the error queue as is.
> 
> For some reason I thought your report was about inet_sock_destruct()
> 
> tcp_disconnect() has always been full of bugs, it is surprising real applications
> (not fuzzers) are still trying to use it.

I think I hit it because app sets SOCK_LINGER && !sk->sk_lingertime.
I don't think the app disconnects explicitly, but "same difference".

> >> 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")
> >   
> 
> Except that this patch wont prevent a packet being added to sk_error_queue
> right after skb_queue_purge(&sk->sk_error_queue).

Right, but then inet_sock_destruct() also purges the err queue, again.
I was afraid of regressions but we could just remove the purging 
from sk_stream_kill_queues(), and target net-next?

> If you think there is a bug, it must be fixed in another way.
> 
> IMO, preventing err packets from a prior session being queued after a tcp_disconnect()
> is rather hard. We should not even try (packets could be stuck for hours in a qdisc)

Indeed, we could rearrange the SOCK_DEAD check in sock_queue_err_skb()
to skip queuing and put it under the err queue lock (provided we make
sk_stream_kill_queues() take that lock as well). But seems like an
overkill. I'd lean towards the existing patch or removing the purge from
sk_stream_kill_queues(). LMK what you prefer, this is not urgent.