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Date:   Thu, 21 Mar 2019 23:05:27 -0400
From:   Christoph Paasch <christoph.paasch@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Eric Dumazet <edumazet@...gle.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

Hi,

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:
> >
> > Hi,
> >
> > On Wed, 2019-03-20 at 11:35 -0700, Christoph Paasch wrote:
> > > Hello,
> > >
> > > On Fri, Mar 24, 2017 at 3:23 PM Alexander Duyck
> > > <alexander.duyck@...il.com> wrote:
> > > > From: Alexander Duyck <alexander.h.duyck@...el.com>
> > > >
> > > > > From what I can tell there is only a couple spots where we are actually
> > > > checking the return value of sk_busy_loop. As there are only a few
> > > > consumers of that data, and the data being checked for can be replaced
> > > > with a check for !skb_queue_empty() we might as well just pull the code
> > > > out of sk_busy_loop and place it in the spots that actually need it.
> > > >
> > > > Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> > > > Acked-by: Eric Dumazet <edumazet@...gle.com>
> > > > ---
> > > >  include/net/busy_poll.h |    5 ++---
> > > >  net/core/datagram.c     |    8 ++++++--
> > > >  net/core/dev.c          |   25 +++++++++++--------------
> > > >  net/sctp/socket.c       |    9 ++++++---
> > > >  4 files changed, 25 insertions(+), 22 deletions(-)
> > > >
> > > > diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
> > > > index b82d6ba70a14..c55760f4820f 100644
> > > > --- a/include/net/busy_poll.h
> > > > +++ b/include/net/busy_poll.h
> > > > @@ -74,7 +74,7 @@ static inline bool busy_loop_timeout(unsigned long end_time)
> > > >         return time_after(now, end_time);
> > > >  }
> > > >
> > > > -bool sk_busy_loop(struct sock *sk, int nonblock);
> > > > +void sk_busy_loop(struct sock *sk, int nonblock);
> > > >
> > > >  #else /* CONFIG_NET_RX_BUSY_POLL */
> > > >  static inline unsigned long net_busy_loop_on(void)
> > > > @@ -97,9 +97,8 @@ static inline bool busy_loop_timeout(unsigned long end_time)
> > > >         return true;
> > > >  }
> > > >
> > > > -static inline bool sk_busy_loop(struct sock *sk, int nonblock)
> > > > +static inline void sk_busy_loop(struct sock *sk, int nonblock)
> > > >  {
> > > > -       return false;
> > > >  }
> > > >
> > > >  #endif /* CONFIG_NET_RX_BUSY_POLL */
> > > > diff --git a/net/core/datagram.c b/net/core/datagram.c
> > > > index ea633342ab0d..4608aa245410 100644
> > > > --- a/net/core/datagram.c
> > > > +++ b/net/core/datagram.c
> > > > @@ -256,8 +256,12 @@ struct sk_buff *__skb_try_recv_datagram(struct
> > > > sock *sk, unsigned int flags,
> > > >                 }
> > > >
> > > >                 spin_unlock_irqrestore(&queue->lock, cpu_flags);
> > > > -       } while (sk_can_busy_loop(sk) &&
> > > > -                sk_busy_loop(sk, flags & MSG_DONTWAIT));
> > > > +
> > > > +               if (!sk_can_busy_loop(sk))
> > > > +                       break;
> > > > +
> > > > +               sk_busy_loop(sk, flags & MSG_DONTWAIT);
> > > > +       } while (!skb_queue_empty(&sk->sk_receive_queue));
> > >
> > > since this change I am hitting stalls where it's looping in this
> > > while-loop with syzkaller.
> > >
> > > It worked prior to this change because sk->sk_napi_id was not set thus
> > > sk_busy_loop would make us get out of the loop.
> > >
> > > Now, it keeps on looping because there is an skb in the queue with
> > > skb->len == 0 and we are peeking with an offset, thus
> > > __skb_try_recv_from_queue will return NULL and thus we have no way of
> > > getting out of the loop.
> > >
> > > I'm not sure what would be the best way to fix it. I don't see why we
> > > end up with an skb in the list with skb->len == 0. So, shooting a
> > > quick e-mail, maybe somebody has an idea :-)
> > >
> > > I have the syzkaller-reproducer if needed.
> >
> > IIRC we can have 0 len UDP packet sitting on sk_receive_queue since:
> >
> > commit e6afc8ace6dd5cef5e812f26c72579da8806f5ac
> > Author: samanthakumar <samanthakumar@...gle.com>
> > Date:   Tue Apr 5 12:41:15 2016 -0400
> >
> >     udp: remove headers from UDP packets before queueing
> >
> > Both __skb_try_recv_datagram() and napi_busy_loop() assume that we
> > received some packets if the queue is not empty. When peeking such
> > assumption is not true, we should check if the last packet is changed,
> > as __skb_recv_datagram() already does. So I *think* the root cause of
> > this issue is older than Alex's patch.
>
> I agree.
>
> > 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>


Christoph

>
> > Could you please test it?
> >
> > Paolo
> > ---
> > diff --git a/net/core/datagram.c b/net/core/datagram.c
> > index b2651bb6d2a3..e657289db4ac 100644
> > --- a/net/core/datagram.c
> > +++ b/net/core/datagram.c
> > @@ -279,7 +279,7 @@ struct sk_buff *__skb_try_recv_datagram(struct sock
> > *sk, unsigned int flags,
> >                         break;
> >
> >                 sk_busy_loop(sk, flags & MSG_DONTWAIT);
> > -       } while (!skb_queue_empty(&sk->sk_receive_queue));
> > +       } while (sk->sk_receive_queue.prev != *last);
> >
> >         error = -EAGAIN;
> >
> >

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