netdev - Re: [net-next PATCH v3 4/8] net: Change return type of sk_busy

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Date:   Fri, 22 Mar 2019 05:59:03 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Christoph Paasch <christoph.paasch@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        David Miller <davem@...emloft.net>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [net-next PATCH v3 4/8] net: Change return type of sk_busy_loop
 from bool to void

On Fri, Mar 22, 2019 at 3:33 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> Hi,
>
> On Thu, 2019-03-21 at 23:05 -0400, Christoph Paasch wrote:
> > On Thu, Mar 21, 2019 at 12:43 PM Alexander Duyck
> > <alexander.duyck@...il.com> wrote:
> > > On Thu, Mar 21, 2019 at 2:45 AM Paolo Abeni <pabeni@...hat.com> wrote:
> > > > The following - completely untested - should avoid the unbounded loop,
> > > > but it's not a complete fix, I *think* we should also change
> > > > sk_busy_loop_end() in a similar way, but that is a little more complex
> > > > due to the additional indirections.
> > >
> > > As far as sk_busy_loop_end we could look at just forking sk_busy_loop
> > > and writing a separate implementation for datagram sockets that uses a
> > > different loop_end function. It shouldn't take much to change since
> > > all we would need to do is pass a structure containing the sk and last
> > > pointers instead of just passing the sk directly as the loop_end
> > > argument.
> > >
> > > > Could you please test it?
> > > >
> > > > Any feedback welcome!
> > >
> > > The change below looks good to me.
> >
> > I just tried it out. Worked for me!
> >
> > You can add my Tested-by if you do a formal patch-submission:
> >
> > Tested-by: Christoph Paasch <cpaasch@...le.com>
>
> Thanks for testing!
>
> I'm trying to reproduce the issue locally, but I'm unable. I think that
> the current UDP implementation is not affected, as we always ensure
> sk_receive_queue is empty before busy polling.

But right after check is done we release the queue lock, so a packet might
come right after the test has been done.

> Unix sockets should not
> be affected, too, as busy polling should not have any effect there
> (sk_napi_id should be never >= MIN_NAPI_ID). Can you reproduce the
> issue on an unpatched, recent, upstream kernel?



>
> Can you please provide the syzkaller repro?
>
> Thanks,
>
> Paolo
>
>
>
>
>