netdev - Re: [PATCH net-next v4] vsock/test: Add test for null ptr deref when transport changes

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Message-ID: <1915e11f-9513-4d9a-a46a-3f97ac152a3b@linux.ibm.com>
Date: Wed, 9 Jul 2025 14:03:02 -0500
From: Konstantin Shkolnyy <kshk@...ux.ibm.com>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: mhal@...x.co, virtualization@...ts.linux.dev, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, v4bel@...ori.io, leonardi@...hat.com
Subject: Re: [PATCH net-next v4] vsock/test: Add test for null ptr deref when
 transport changes

On 09-Jul-25 10:41, Stefano Garzarella wrote:
> On Wed, 9 Jul 2025 at 17:26, Stefano Garzarella <sgarzare@...hat.com> wrote:
>>
>> On Wed, 9 Jul 2025 at 16:54, Konstantin Shkolnyy <kshk@...ux.ibm.com> wrote:
>>>
>>> I'm seeing a problem on s390 with the new "SOCK_STREAM transport change
>>> null-ptr-deref" test. Here is how it appears to happen:
>>>
>>> test_stream_transport_change_client() spins for 2s and sends 70K+
>>> CONTROL_CONTINUE messages to the "control" socket.
>>>
>>> test_stream_transport_change_server() spins calling accept() because it
>>> keeps receiving CONTROL_CONTINUE.
>>>
>>> When the client exits, the server has received just under 1K of those
>>> 70K CONTROL_CONTINUE, so it calls accept() again but the client has
>>> exited, so accept() never returns and the server never exits.
> 
> Just to be clear, I was seeing something a bit different.
> The accept() in the server is no-blocking, since we set O_NONBLOCK on
> the socket, so I see the server looping around a failing accept()
> (errno == EAGAIN) while dequeueing the CONTROL_CONTINUE messages, so
> after 10/15 seconds the server ends on my case.
> 
> It seems strange that in your case it blocks, since it should be a
> no-blocking call.

It was my mistake. The accept() doesn't block. I've retested it more 
carefully and it keeps returning and the loop eventually consumes all 
queued CONTROL_CONTINUE messages and quits, as you described.