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SubjectRe: [net-next PATCH v3 4/8] net: Change return type of sk_busy_loop from bool to void
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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@gmail.com> wrote:
> > From: Alexander Duyck <alexander.h.duyck@intel.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@intel.com>
> > Acked-by: Eric Dumazet <edumazet@google.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@google.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.

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.

Could you please test it?

Any feedback welcome!


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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