From mboxrd@z Thu Jan 1 00:00:00 1970 From: Willem de Bruijn Subject: Re: [net-next PATCH v3 4/8] net: Change return type of sk_busy_loop from bool to void Date: Thu, 21 Mar 2019 10:28:45 -0400 Message-ID: References: <20170324164902.15226.48358.stgit@localhost.localdomain> <20170324170812.15226.97497.stgit@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Paolo Abeni Cc: Christoph Paasch , Alexander Duyck , netdev , LKML , "Samudrala, Sridhar" , Eric Dumazet , David Miller , Linux API , Alexei Starovoitov List-Id: linux-api@vger.kernel.org On Thu, Mar 21, 2019 at 5:46 AM Paolo Abeni 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 > > wrote: > > > From: Alexander Duyck > > > > > > > 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 > > > Acked-by: Eric Dumazet > > > --- > > > 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: Yes, as of header before enqueue pulling zero byte datagrams may be queued. And these need to be delivered, among other reason for their cmsg metadata. > commit e6afc8ace6dd5cef5e812f26c72579da8806f5ac > Author: samanthakumar > 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. Good catch. The condition in sk_busy_loop_end is not easy to address. Since busy poll is an optimization and poll at offset rare, one way out may be to amend the __sk_can_busy_loop test in __skb_recv_udp to disallow busy polling together with peek at offset. The difference in behavior betwee __skb_try_recv_datagram and __skb_recv_datagram also reminds of Alexei's earlier report (without busy polling, seemingly with a list corruption introduced elsewhere) in [net-next,1/3] net/sock: factor out dequeue/peek with offset code https://patchwork.ozlabs.org/patch/762327/ > 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; > >