Message-ID: <20180406161053.GF7500@magnolia>
Date:   Fri, 6 Apr 2018 09:10:53 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Dave Chinner <david@...morbit.com>,
        syzbot <syzbot+84a67953651a971809ba@...kaller.appspotmail.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-xfs@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: WARNING: bad unlock balance in xfs_iunlock

On Fri, Apr 06, 2018 at 07:38:44AM +1000, Dave Chinner wrote:
> On Thu, Apr 05, 2018 at 08:54:50PM +0200, Dmitry Vyukov wrote:
> > On Tue, Apr 3, 2018 at 6:38 AM, Dave Chinner <david@...morbit.com> wrote:
> > > On Mon, Apr 02, 2018 at 07:01:02PM -0700, syzbot wrote:
> > >> Hello,
> > >>
> > >> syzbot hit the following crash on upstream commit
> > >> 86bbbebac1933e6e95e8234c4f7d220c5ddd38bc (Mon Apr 2 18:47:07 2018 +0000)
> > >> Merge branch 'ras-core-for-linus' of
> > >> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
> > >> syzbot dashboard link:
> > >> https://syzkaller.appspot.com/bug?extid=84a67953651a971809ba
> > >>
> > >> C reproducer: https://syzkaller.appspot.com/x/repro.c?id=5719304272084992
> > >> syzkaller reproducer:
> > >> https://syzkaller.appspot.com/x/repro.syz?id=5767783983874048
> > >
> > > What a mess. A hand built, hopelessly broken filesystem image made
> > > up of hex dumps, written into a mmap()d region of memory, then
> > > copied into a tmpfs file and mounted with the loop device.
> > >
> > > Engineers that can debug broken filesystems don't grow on trees.  If
> > > we are to have any hope of understanding what the hell this test is
> > > doing, the bot needs to supply us with a copy of the built
> > > filesystem image the test uses. We need to be able to point forensic
> > > tools at the image to decode all the structures into human readable
> > > format - if we are forced to do that by hand or jump through hoops
> > > to create our own filesystem image than I'm certainly not going to
> > > waste time looking at these reports...
> > 
> > Hi Dave,
> > 
> > Here is the image:
> > https://drive.google.com/file/d/1jzhGGe5SBJcqfsjxCLHoh4Kazke1oTfC/view
> > (took me about a minute to extract from test by replacing memfd_create
> > with open and running the program).
> 
> Says the expert who knows exactly how it's all put together. It took
> me a couple of hours just to understand WTF the syzbot reproducer
> was actually doing....
> 
> > Then do the following to trigger the bug:
> > losetup /dev/loop0 xfs.repro
> > mkdir xfs
> > mount -t xfs -o nouuid,prjquota,noikeep,quota /dev/loop0 xfs
> > 
> > To answer your more general question: syzbot is not a system to test
> > solely file systems, it finds bugs in hundreds of kernel subsystems.
> > Generating image for file systems, media files for sound and
> > FaceDancer programs that crash host when  FaceDancer device is plugged
> > into USB is not feasible. And in the end it's not even clear what
> 
> I don't care how syzbot generates the filesystem image it uses.
> 
> > kernel subsystem is at fault and even if it somehow figures out that
> > it's a filesystem, it's unclear that it's exactly an image that
> > provokes the bug. syzbot provides C reproducers which is a reasonable
> 
> It doesn't matter *what subsystem breaks*. If syzbot is generating a
> filesystem image and then mounting it, it needs to provide that
> filesystem image to to people who end up having to debug the
> problem. It's a basic "corrupt filesystem" bug triage step.
> 
> > Some bugs are so involved that only an
> > expert in a particular subsystem can figure out what happens there.
> 
> And that's clearly the case here, whether you like it or not.
> 
> You want us to do things that make syzbot more useful as a tool but
> you don't want to do things that make syzbot a useful tool for us.
> It's a two way street....

...here's my take on this whole situation:

So far as I can tell, this syzbot daemon grew the ability to fuzz
filesystems and started emitting bug report after bug report, with
misleading commit ids that have nothing to do with the complaint.  Your
maintainers (Dave, Eric, and me) have spent a few hours trying to figure
out what's going on, to some frustration.  The bug reports themselves
miss the public engagement detail of saying something like "Hey XFS
engineers, if you'd like to talk to the syzbot engineers they can be
reached at <googlegroup address>."  Instead it merely says "direct
questions to <addr>" like this is some press release and the only thing
on the other end of the line will be some disinterested bureaucracy.
Or some robot.

We're willing to take random user reports of corruption and misbehavior
and do all the work to turn that into regression tests and patches, but
that's not the situation here.  You work for a well known engineering
company which (I assume) has decided that fuzz hardening the commons
aligns with its goals.  Collective-you (i.e. your company) could realize
that goal sooner and with a lot less community friction by staffing up
engineers to join our community to help us triage and fix the things
reported by syzbot.  It's much /less/ effective to dump a pile of work
on the people in the community.  We each have our own work-piles and
most likely different priorities.

Hardening XFS to the sorts of things fuzzers find is important to me
(and $employer) as well, but the difference here is that I read every
report that gets generated and start the work to figure out a regression
test and a code fix.  That is what I send to the list for more public
participation and to signal that yes, there's a human behind all this
with some reasonable level of understanding of the problem.

--D

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@...morbit.com
> --
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