Re: WARNING: kobject bug in sysfs_warn_dup

[Dmitry Vyukov]

On Thu, Apr 5, 2018 at 11:00 AM, Steven Whitehouse <swhiteho@xxxxxxxxxx> wrote:
> Hi,
>
>
>
> On 05/04/18 09:52, Dmitry Vyukov wrote:
>>
>> On Thu, Apr 5, 2018 at 10:36 AM, Steven Whitehouse <swhiteho@xxxxxxxxxx>
>> wrote:
>>>
>>> Hi,
>>>
>>>
>>>
>>> On 05/04/18 09:19, Dmitry Vyukov wrote:
>>>>
>>>> On Thu, Apr 5, 2018 at 8:34 AM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
>>>> wrote:
>>>>>
>>>>> On Wed, Apr 04, 2018 at 07:02:01PM -0700, syzbot wrote:
>>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> syzbot hit the following crash on upstream commit
>>>>>> 3e968c9f1401088abc9a19ae6ff571644d37a355 (Wed Apr 4 21:19:24 2018
>>>>>> +0000)
>>>>>> Merge tag 'ext4_for_linus' of
>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4
>>>>>> syzbot dashboard link:
>>>>>> https://syzkaller.appspot.com/bug?extid=ff87a28e665c163aa7f5
>>>>>>
>>>>>> C reproducer:
>>>>>> https://syzkaller.appspot.com/x/repro.c?id=5104666266304512
>>>>>> syzkaller reproducer:
>>>>>> https://syzkaller.appspot.com/x/repro.syz?id=5683447737614336
>>>>>> Raw console output:
>>>>>> https://syzkaller.appspot.com/x/log.txt?id=5104818200772608
>>>>>> Kernel config:
>>>>>> https://syzkaller.appspot.com/x/.config?id=9118669095563550941
>>>>>> compiler: gcc (GCC) 7.1.1 20170620
>>>>>>
>>>>>> IMPORTANT: if you fix the bug, please add the following tag to the
>>>>>> commit:
>>>>>> Reported-by: syzbot+ff87a28e665c163aa7f5@xxxxxxxxxxxxxxxxxxxxxxxxx
>>>>>> It will help syzbot understand when the bug is fixed. See footer for
>>>>>> details.
>>>>>> If you forward the report, please keep this part and the footer.
>>>>>>
>>>>>> R10: 0000000000000000 R11: 0000000000000286 R12: 0000000000000003
>>>>>> R13: 0000000000000004 R14: 0000000000000000 R15: 0000000000000000
>>>>>> ------------[ cut here ]------------
>>>>>> kobject_add_internal failed for nodev( with -EEXIST, don't try to
>>>>>> register
>>>>>> things with the same name in the same directory.
>>>>>> sysfs: cannot create duplicate filename '/fs/gfs2/nodev('
>>>>>> WARNING: CPU: 1 PID: 4473 at lib/kobject.c:238
>>>>>> kobject_add_internal+0x8d4/0xbc0 lib/kobject.c:235
>>>>>> CPU: 0 PID: 4474 Comm: syzkaller533472 Not tainted 4.16.0+ #15
>>>>>> Kernel panic - not syncing: panic_on_warn set ...
>>>>>>
>>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine,
>>>>>> BIOS
>>>>>> Google 01/01/2011
>>>>>> Call Trace:
>>>>>>    __dump_stack lib/dump_stack.c:17 [inline]
>>>>>>    dump_stack+0x1a7/0x27d lib/dump_stack.c:53
>>>>>>    sysfs_warn_dup+0x83/0xa0 fs/sysfs/dir.c:30
>>>>>>    sysfs_create_dir_ns+0x178/0x1d0 fs/sysfs/dir.c:58
>>>>>>    create_dir lib/kobject.c:69 [inline]
>>>>>>    kobject_add_internal+0x335/0xbc0 lib/kobject.c:227
>>>>>>    kobject_add_varg lib/kobject.c:364 [inline]
>>>>>>    kobject_init_and_add+0xf9/0x150 lib/kobject.c:436
>>>>>>    gfs2_sys_fs_add+0x1ff/0x580 fs/gfs2/sys.c:652
>>>>>>    fill_super+0x86f/0x1d70 fs/gfs2/ops_fstype.c:1118
>>>>>>    gfs2_mount+0x587/0x6e0 fs/gfs2/ops_fstype.c:1321
>>>>>
>>>>> gfs2 bug, not a sysfs bug, we are correctly warning about an incorrect
>>>>> usage of the api.
>>>>
>>>> Then +gfs2 maintainers.
>>>>
>>>>> Now if we should turn this into a non-WARN message, that's a different
>>>>> thing, I'll gladly take a patch for that.
>>>>
>>>> If it's API usage bug in higher level code, then I think WARN is a
>>>> proper thing. We already had similar ones and they were fixed.
>>>
>>>
>>> I'm trying to figure out what the test is doing, but it is not very
>>> clear.
>>> At a guess I'd say that perhaps it is trying to mount multiple
>>> filesystems
>>> with the same label? If that is the case then it is not allowed, and it
>>> should be caught be the sysfs code and result in a refusal to mount,
>>> which
>>> is what I think I see here. Knowing which sysfs directory is involved
>>> would
>>> allow us to confirm, but I suspect that the test needs altering to give
>>> each
>>> gfs2 mount a different label at an initial guess,
>>
>>
>> Hi Steve,
>>
>> But Greg claims that this is incorrect usage of sysfs API:
>>
>>> gfs2 bug, not a sysfs bug, we are correctly warning about an incorrect
>>> usage of the api.
>>
>> I think this means that sysfs callers must not try to create the same
>> thing twice.
>>
>> Either way user-space code must not be able to triggers WARNINGs in
>> kernel. If it does than this is something to fix in kernel.
>
>
> I guess that this warning was added more recently as I've not seen it
> before. My expectation is that it will return -EEXIST and not print a
> warning there. To avoid that we would have to create a new list of GFS2
> superblocks, and check the list for each mount I think. We could do that,
> but it seems a bit odd to duplicate code that is already there and working.
>
> So it sounds like a case of differing assumptions about what is a valid use
> of the sysfs api. Shouldn't be too hard to fix though,


Greg?

Should we go with your other option of demoting WARNING to pr_err
then? I don't how many real bugs that warning caught versus callers
just properly handle EEXIST.