linux-kernel - Re: WARNING: bad unlock balance in xfs

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Date:   Tue, 8 May 2018 09:52:36 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Eric Sandeen <sandeen@...deen.net>
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 Mon, Apr 30, 2018 at 5:14 PM, Eric Sandeen <sandeen@...deen.net> wrote:
> On 4/30/18 9:02 AM, Dmitry Vyukov wrote:
>> On Mon, Apr 30, 2018 at 3:49 PM, Eric Sandeen <sandeen@...deen.net> wrote:
>
> ...
>
>>>> 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?
>>
>> Say, a value that we generally pass to close system call is not static
>> and can't be dumped to a static file. It's whatever a previous open
>> system call has returned. Inside of the program we memorize the return
>> value of open in a variable and then pass it to close. This generally
>> stands for all system calls. Say, an image can contain an uid, and
>> that uid can be obtained from a system call too.
>
> Ok, but that's the syscall side.  You are operating on a static xfs image,
> correct?  We're only asking for the actual filesystem you're operating
> against.

Not necessary. Image can be dynamically generate too, all inputs to
kernel are generally dynamically generated.


> (When I say "image" I am talking only about the filesystem itself, not any
> other syzkaller state)

OK, let's do it this way. For the first 10 bugs, ask me, and I will do
it manually.
I am all for automation. And syzbot is already more automated than
most kernel testing systems. But, as I said, this is really
not-trivial, large amount of work, and is specific to one out of
dozens of kernel subsystems.


> Ok, backing up more: When you are testing against an xfs filesystem image, where
> does that image come from?  How is it generated?  A quick look at the syzkaller
> tree didn't make that clear to me.
>
> the xfs.repro file you provided at
> https://drive.google.com/file/d/1jzhGGe5SBJcqfsjxCLHoh4Kazke1oTfC/view
>
> is strange, it doesn't even contain AGF blocks; they aren't fuzzed or corrupted,
> they are completely zeroed out.  I don't know if that's part of the fuzzing,
> or what - what steps led to that image?
>
> Or put another way, how did you arrive at the fs image values in the reproducer,
> i.e.:

Currently they are completely random, nobody taught syzkaller about AGFs, etc.

> oid loop()
> {
>   memcpy((void*)0x20000000, "xfs", 4);
>   memcpy((void*)0x20000100, "./file0", 8);
>   *(uint64_t*)0x20000200 = 0x20010000;
>   memcpy((void*)0x20010000,
>          "\x58\x46\x53\x42\x00\x00\x10\x00\x00\x00\x00\x00\x00\x00\x10\x00\x00"
>          "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x9f\x98"
>          "\x99\xff\xcb\xa1\x4e\xe6\xad\x52\x08\x20\x67\x09\xed\x75\x00\x00\x00"
>          "\x00\x00\x00\x00\x04\x00\x00\x00\x00\x00\x00\x35\xe0\x00\x00\x00\x00"
>          "\x00\x00\x35\xe1\x00\x00\x00\x00\x00\x00\x35\xe2\x00\x00\x00\x01\x00"
>          "\x00\x10\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x03\x55\xb4\xa4"
>          "\x02\x00\x01\x00\x00\x10\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
>          "\x00\x0c\x09\x08\x04\x0c\x00\x00\x19\x00\x00\x00\x00\x00\x00\x00\x40"
>          "\x00\x00\x00\x00\x00\x00\x00\x3d\x00\x00\x00\x00\x00\x00\x0c\xa3\x00"
>          "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
>          "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00"
>          "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x02\x02",
>          204);
>
> ...
>
> The in-memory xfs filesystem it constructs is damaged, is that an intentional
> part of the fuzzing during the test?

Yes, invalid inputs is part of testing.