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Date:   Fri, 1 Jun 2018 12:30:02 +1000
From:   Dave Chinner <david@fromorbit.com>
To:     Jan Kara <jack@suse.cz>
Cc:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        syzbot <syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com>,
        syzkaller-bugs@googlegroups.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, viro@zeniv.linux.org.uk,
        axboe@kernel.dk, tj@kernel.org, linux-block@vger.kernel.org,
        Dmitry Vyukov <dvyukov@google.com>
Subject: Re: general protection fault in wb_workfn (2)
Message-ID: <20180601023002.GA10972@dastard>
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On Thu, May 31, 2018 at 03:42:12PM +0200, Jan Kara wrote:
> On Thu 31-05-18 22:19:44, Tetsuo Handa wrote:
> > On 2018/05/31 20:42, Jan Kara wrote:
> > > On Thu 31-05-18 01:00:08, Tetsuo Handa wrote:
> > >> So, we have no idea what is happening...
> > >> Then, what about starting from temporary debug printk() patch shown below?
> > >>
> > >> >From 4f70f72ad3c9ae6ce1678024ef740aca4958e5b0 Mon Sep 17 00:00:00 2001
> > >> From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > >> Date: Wed, 30 May 2018 09:57:10 +0900
> > >> Subject: [PATCH] bdi: Add temporary config for debugging wb_workfn() versus
> > >>  bdi_unregister() race bug.
> > >>
> > >> syzbot is hitting NULL pointer dereference at wb_workfn() [1]. But due to
> > >> limitations that syzbot cannot find reproducer for this bug (frequency is
> > >> once or twice per a day) nor we can't capture vmcore in the environment
> > >> which syzbot is using, for now we need to rely on printk() debugging.
> > >>
> > >> [1] https://syzkaller.appspot.com/bug?id=e0818ccb7e46190b3f1038b0c794299208ed4206
> > >>
> > >> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > > 
> > > Hum a bit ugly solution but if others are fine with this, I can live with
> > > it for a while as well. Or would it be possible for syzkaller to just test
> > > some git tree where this patch is included? Then we would not even have to
> > > have the extra config option...
> > 
> > If syzbot can reproduce this bug that way. While it is possible to add/remove
> > git trees syzbot tests, frequently adding/removing trees is bothering.
> > 
> > syzbot can enable extra config option. Maybe the config name should be
> > something like CONFIG_DEBUG_FOR_SYZBOT rather than individual topic.
> > 
> > I think that syzbot is using many VM instances. I don't know how many
> > instances will be needed for reproducing this bug within reasonable period.
> > More git trees syzbot tests, (I assume that) longer period will be needed
> > for reproducing this bug. The most reliable way is to use the shared part
> > of all trees (i.e. linux.git).
> 
> I understand this, I'd be just a bit reluctant to merge temporary debug
> patches like this to Linus' tree only to revert them later just because
> syzkaller... What do others think?

Don't commit temporary debug crap to the mainline kernel. Enable
syzkaller to run against a user supplied git tree specification if
it needs special debug to track down a problem.

-Dave.
-- 
Dave Chinner
david@fromorbit.com