Re: kernel BUG at net/unix/garbage.c:149!"

From	Miklos Szeredi <mszeredi@redhat.com>
Newsgroups	linux.kernel
Subject	Re: kernel BUG at net/unix/garbage.c:149!"
Date	2016-08-30 11:20 +0200
Message-ID	<sbNn3-2ST-7@gated-at.bofh.it> (permalink)
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On Tue, Aug 30, 2016 at 12:37 AM, Miklos Szeredi <mszeredi@redhat.com> wrote:
> On Sat, Aug 27, 2016 at 11:55 AM, Miklos Szeredi <mszeredi@redhat.com> wrote:

> crash> list -H gc_inflight_list unix_sock.link -s unix_sock.inflight |
> grep counter | cut -d= -f2 | awk '{s+=$1} END {print s}'
> 130
> crash> p unix_tot_inflight
> unix_tot_inflight = $2 = 135
>
> We've lost track of a total of five inflight sockets, so it's not a
> one-off thing.  Really weird...  Now off to sleep, maybe I'll dream of
> the solution.

Okay, found one bug: gc assumes that in-flight sockets that don't have
an external ref can't gain one while unix_gc_lock is held.  That is
true because unix_notinflight() will be called before detaching fds,
which takes unix_gc_lock.  Only MSG_PEEK was somehow overlooked.  That
one also clones the fds, also keeping them in the skb.  But through
MSG_PEEK an external reference can definitely be gained without ever
touching unix_gc_lock.

Not sure whether the reported bug can be explained by this.  Can you
confirm the MSG_PEEK was used in the setup?

Does someone want to write a stress test for SCM_RIGHTS + MSG_PEEK?

Anyway, attaching a fix that works by acquiring unix_gc_lock in case
of MSG_PEEK also.  It is trivially correct, but I haven't tested it.

Thanks,
Miklos

Thread

Re: kernel BUG at net/unix/garbage.c:149!" Miklos Szeredi <mszeredi@redhat.com> - 2016-08-27 12:00 +0200
  Re: kernel BUG at net/unix/garbage.c:149!" Miklos Szeredi <mszeredi@redhat.com> - 2016-08-30 00:40 +0200
    Re: kernel BUG at net/unix/garbage.c:149!" Miklos Szeredi <mszeredi@redhat.com> - 2016-08-30 11:20 +0200
      Re: kernel BUG at net/unix/garbage.c:149!" Nikolay Borisov <kernel@kyup.com> - 2016-08-30 11:40 +0200
        Re: kernel BUG at net/unix/garbage.c:149!" Miklos Szeredi <mszeredi@redhat.com> - 2016-08-30 11:50 +0200
      Re: kernel BUG at net/unix/garbage.c:149!" Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-09-01 11:20 +0200

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