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

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Message-ID: <CAOssrKcfncAYsQWkfLGFgoOxAQJVT2hYVWdBA6Cw7hhO8RJ_wQ@mail.gmail.com>
Date:   Tue, 30 Aug 2016 11:18:10 +0200
From:   Miklos Szeredi <mszeredi@...hat.com>
To:     Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:     Nikolay Borisov <kernel@...p.com>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        netdev@...r.kernel.org
Subject: Re: kernel BUG at net/unix/garbage.c:149!"

On Tue, Aug 30, 2016 at 12:37 AM, Miklos Szeredi <mszeredi@...hat.com> wrote:
> On Sat, Aug 27, 2016 at 11:55 AM, Miklos Szeredi <mszeredi@...hat.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

View attachment "af_unix-fix-garbage-collect-vs-msg_peek.patch" of type "text/x-patch" (2518 bytes)