Date:   Mon, 30 Apr 2018 08:49:15 -0500
From:   Eric Sandeen <sandeen@...deen.net>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dave Chinner <david@...morbit.com>,
        syzbot <syzbot+84a67953651a971809ba@...kaller.appspotmail.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-xfs@...r.kernel.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Theodore Ts'o <tytso@....edu>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: WARNING: bad unlock balance in xfs_iunlock

On 4/30/18 8:23 AM, Dmitry Vyukov wrote:
> On Mon, Apr 16, 2018 at 9:22 PM, Eric Sandeen <sandeen@...deen.net> wrote:

...

>> It sure /seems/ to have a notion of images: what else is syz_mount_image()?
>>
>> i.e. you are mounting an image to reproduce the problem, correct?
>> And the system is "smart" enough to fire off an email to a filesystem list;
>> if it does so, add a link to the image itself, as you already have already done
>> for the C reproducer.
>>
>> Filesystem images are common parlance for filesystem engineers.  When
>> you engage with them you'll have better results if you  provide them with
>> inputs they can use directly instead of requiring them to reverse-engineer
>> your custom test harness.
> 
> 
> Well, yes and no.
> syz_mount_image() is the only part of a large system that knows about
> images. For the rest of the system it's just a syscall like any other
> syscall. And the part that sends emails is far away from
> syz_mount_image().
> syzbot does not know per se that it sends an email to filesystems
> list.

I am asking it to learn this trick as an enhancement.  The MAINTAINERS
file contains big clues about which subsystems are filesystems, for example:

$ grep FILESYSTEM$ MAINTAINERS 
AFS FILESYSTEM
CRAMFS FILESYSTEM
EFI VARIABLE FILESYSTEM
EFS FILESYSTEM
FREEVXFS FILESYSTEM
...

> It just extracted kernel source file name that looked relevant
> to this crash and run get_maintainers.pl on it.
> Also the image can contain dynamically generated data, which makes it
> impossible to have as a file at all.

I guess I'm not sure what this means, can you explain?

> Thinking of this, what should be reasonably easy to do and may be a
> compromise for near future is the following. We could insert code into
> syz_mount_image() which dump the image if you build a program with a
> special define (e.g. -DSYZ_DUMP_IMAGE). Will this work for you?

If this is possible, I guess I still don't understand why you can't dump the
image and provide link.  You have fast, efficient robots.  We have slow,
busy humans.

>>> Please elaborate re commits. It's a basic rule of any good bug report
>>> -- communicate exact state of source code when the bug was hit, i.e.
>>> provide the commit hash.
>>
>> Further best practice is to provide the /correct/ commit hash.

....

>> I can't imagine these are right...
> 
> 
> As I said, bisection is on our plate:
> https://github.com/google/syzkaller/issues/501
> Though, we will see how well it works, because it's not trivial (see
> the issue for details).

Oh I see. I had misunderstood; so:

> 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

doesn't mean you bisected to that commit, or that this was the first bad commit,
it just means you happened to have a tree at this commit when you hit the problem.

That was not at all clear to me.  I thought when syzkaller was telling us
"on upstream commit XYZ," it meant that it had identified commit XYZ as bad.
I'm not sure if anyone else made that mistake, but  perhaps you could also clarify
the bug report text in this regard?

...

>> I think that in this case, what we are asking for is a fine tuning of the
>> testing and reporting so that we can more efficiently address these issues.
>> Off the top of my head, and there may be more items:
>>
>> 1) Add a human contact to the emails, start an IRC channel, etc, for better
>>    two-way communication.  (it wasn't clear that syzkaller@ reached humans,
>>    tbh.)
> 
> There is a human contact at syzkaller@...glegroups.com. Report footer
> will be improved to make it more clear:
> https://github.com/google/syzkaller/issues/565#issuecomment-380793620
> 
>> 2) _Properly_ identify the regressing commit, if any.  If it doesn't look like
>>    a recent regression, you can state that too.
> 
> Bisection is on our plate.
> 
>> 3) If you're reporting a filesystem bug that arose from using a filesystem
>>    image, provide a URL to that filesystem image directly in the report.
> 
> See above. It may not be necessary representable as a static file at all.

Can you explain this?  Do you mean that the mounted image is changing while the
tool runs, while the filesystem is mounted?
 
>> 4) Create a filesystem image that can be more easily debugged by the experts,
>>    i.e. one with > 1 allocation group, so standard repair & analysis tools can
>>    be used with it.
> 
> What is "> 1 allocation group"?

Maybe I should back up; how are the xfs images created?  I had assumed that
surely you start with a base image of some sort, and start fuzzing it from there.
Is this correct?

If so, allocation groups are a fundamental geometry of the filesystem; from
man mkfs.xfs.8:

                   agcount=value
                          This is used to specify  the  number  of  allocation
                          groups.  The  data  section  of  the  filesystem  is
                          divided into allocation groups to improve  the  per‐
                          formance  of  XFS. More allocation groups imply that
                          more parallelism can  be  achieved  when  allocating
                          blocks and inodes. The minimum allocation group size
                          is 16 MiB; the maximum size is  just  under  1  TiB.
                          The  data  section of the filesystem is divided into
                          value allocation groups  (default  value  is  scaled
                          automatically based on the underlying device size).

If the base image only has one allocation group, it makes it more difficult for
some tools to work with the image, because there is no redundancy.  1 AG is
not a supported or recommended geometry for any real-life use of xfs.

If I am correct that you start with a base image w/ a certain geometry or
set of mkfs options, starting with >= 2 AGs would improve the usefulness of the
filesystem image.

Thanks,
-Eric

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