netdev - Re: [PATCH net-next v4 3/3] vsock/test: Expand linger test to ensure close() does not misbehave

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Message-ID: <CAGxU2F7z-Fj4-_154KeYYb6YeBe1uZCc52Nq0afYAR0ViVv0Ug@mail.gmail.com>
Date: Tue, 20 May 2025 12:20:07 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Michal Luczaj <mhal@...x.co>
Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez <eperezma@...hat.com>, 
	Stefan Hajnoczi <stefanha@...hat.com>, virtualization@...ts.linux.dev, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH net-next v4 3/3] vsock/test: Expand linger test to ensure
 close() does not misbehave

On Tue, 20 May 2025 at 11:01, Stefano Garzarella <sgarzare@...hat.com> wrote:
>
> On Tue, 20 May 2025 at 10:54, Stefano Garzarella <sgarzare@...hat.com> wrote:
> >
> > On Mon, May 12, 2025 at 02:23:12PM +0200, Michal Luczaj wrote:
> > >On 5/7/25 10:26, Stefano Garzarella wrote:
> > >> On Wed, 7 May 2025 at 00:47, Michal Luczaj <mhal@...x.co> wrote:
> > >>>
> > >>> On 5/6/25 11:46, Stefano Garzarella wrote:
> > >>>> On Tue, 6 May 2025 at 11:43, Stefano Garzarella <sgarzare@...hat.com> wrote:
> > >>>>>
> > >>>>> On Thu, May 01, 2025 at 10:05:24AM +0200, Michal Luczaj wrote:
> > >>>>>> There was an issue with SO_LINGER: instead of blocking until all queued
> > >>>>>> messages for the socket have been successfully sent (or the linger timeout
> > >>>>>> has been reached), close() would block until packets were handled by the
> > >>>>>> peer.
> > >>>>>
> > >>>>> This is a new behaviour that only new kernels will follow, so I think
> > >>>>> it is better to add a new test instead of extending a pre-existing test
> > >>>>> that we described as "SOCK_STREAM SO_LINGER null-ptr-deref".
> > >>>>>
> > >>>>> The old test should continue to check the null-ptr-deref also for old
> > >>>>> kernels, while the new test will check the new behaviour, so we can skip
> > >>>>> the new test while testing an old kernel.
> > >>>
> > >>> Right, I'll split it.
> > >>>
> > >>>> I also saw that we don't have any test to verify that actually the
> > >>>> lingering is working, should we add it since we are touching it?
> > >>>
> > >>> Yeah, I agree we should. Do you have any suggestion how this could be done
> > >>> reliably?
> > >>
> > >> Can we play with SO_VM_SOCKETS_BUFFER_SIZE like in credit-update tests?
> > >>
> > >> One peer can set it (e.g. to 1k), accept the connection, but without
> > >> read anything. The other peer can set the linger timeout, send more
> > >> bytes than the buffer size set by the receiver.
> > >> At this point the extra bytes should stay on the sender socket buffer,
> > >> so we can do the close() and it should time out, and we can check if
> > >> it happens.
> > >>
> > >> WDYT?
> > >
> > >Haven't we discussed this approach in [1]? I've reported that I can't make
> >
> > Sorry, I forgot. What was the conclusion? Why this can't work?
> >
> > >it work. But maybe I'm misunderstanding something, please see the code
> > >below.
> >
> > What I should check in the code below?
>
> Okay, I see the send() is blocking (please next time explain better
> the issue, etc.)
>
> I don't want to block this series, so feel free to investigate that
> later if we have a way to test it. If I'll find some time, I'll try to
> check if we have a way.

I've tried to take a look, and no, there's no easy way except to
somehow slow down the receiver, but I don't think we have a reliable
way to do that, so I can't think of anything for now. Let's skip it
(I'll try to remember ;-)

Thanks,
Stefano