Re: WARNING: bad unlock balance in xfs_iunlock

[Eric Sandeen]

On 4/13/18 5:03 AM, Dmitry Vyukov wrote:
> On Fri, Apr 6, 2018 at 6:10 PM, Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote:
>> 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@xxxxxxxxxxxxx> 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....

*nod* more on this below.

>>>> 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.

*nod*

>>>> 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....
> 
> Hi Dave, Darrick,
> 
> It's not that we don't want to make the system more useful, it's just
> that we don't see what reasonably can be done here. The system does
> not have notion of images, sound files, USB devices. It feeds data
> into system calls, and that's what it provides as reproducers. Also
> see the last paragraph.

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.

>> ...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.
> 
> 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.

syzbot has identified commits which seem almost certainly incorrect.

For example,

KASAN: use-after-free Read in radix_tree_next_chunk

identified:

9dd2326890d89a5179967c947dab2bab34d7ddee (Fri Mar 30 17:29:47 2018 +0000)
Merge tag 'ceph-for-4.16-rc8' of git://github.com/ceph/ceph-client

and:

WARNING: bad unlock balance in xfs_iunlock

identified:

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

I can't imagine these are right...

>>  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.
> 
> This is quite subjective and we hear opinions all possible ways. I
> don't think there is one size fits all. E.g. +Ted argued in exactly
> the opposite direction: make reports more laconic, formal,
> table-formatted with dry information. There is also a tradeoff between
> describing each detail in proper, friendly English and being more
> laconic. If we increase everything 4x and post a wall of text with
> each report, lots of people won't read anything of it just because
> it's a wall of text. I am also not a native English speaker, so
> providing simple formal text is a safer option than writing something
> potentially clumsy.

What Darrick is asking for, I think, is a human on the other end to talk
to if there are any issues or concerns about the reports.  (For example,
"hey that commit looks wrong, are you sure?)  We are not asking for a
long narrative with each report.  We're asking for a dialogue about this
framework and the reporting, so it can improve.

> Having said that, I am collecting proposals for report format
> improvements. Here is fortunately slightly better wording for footer
> based on your idea:
> https://github.com/google/syzkaller/issues/565#issuecomment-380793620

...

> Well, I guess nobody has infinite engineering resources. If we would
> have them we would also fix all these bug ourselves and don't bother
> you at all. Just as any other company could invest in writing bug
> detection tools, fuzzers, deploy them and report bugs, which would
> relief us from this.
> We have limited resources and do what's possible within these
> resources. Unfortunately providing individual handling of each of the
> thousands of bugs is not possible within these resources. I know that
> what we are doing is useful for kernel overall because lots of
> hundreds of bugs get fixed due to this effort. If you, as xfs
> maintainers, think that these reports are net negative for xfs and xfs
> should not be tested, say so and we will figure out how to make it
> happen.

We've been inundated lately with results of scripts which generate bug reports
quickly but do not provide enough information to quickly triage, reproduce, and
fix those reports.

(For example, another fuzzer is using the same "poc.c" to exercise a fuzzed
image; it might be the 1st operation or the 50th that causes the issue, but
the harness doesn't bother to find out or report it, it just sends all 50
potential operations to us, and assumes we'll take the time to figure it out.
It's laziness on the script/reporting side, resulting in extra work on the
human/debugging side.)

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.)
2) _Properly_ identify the regressing commit, if any.  If it doesn't look like
   a recent regression, you can state that too.
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.
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.

Hopefully this sort of feedback will result in better bug reports from you,
and faster (and more) bugfixes from us.

Personally, I'm certainly not asking you to stop sending reports of bugs you
find, but I /am/ asking that they be as refined, specific, and useful as possible.

Thanks,
-Eric