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Date:   Thu, 5 Apr 2018 14:43:43 +0100
From:   Steven Whitehouse <>
To:     Greg KH <>
Cc:     Dmitry Vyukov <>,,,
        syzbot <>,
        LKML <>,
Subject: Re: WARNING: kobject bug in sysfs_warn_dup


On 05/04/18 14:34, Greg KH wrote:
> On Thu, Apr 05, 2018 at 10:00:04AM +0100, Steven Whitehouse wrote:
>> Hi,
>> On 05/04/18 09:52, Dmitry Vyukov wrote:
>>> On Thu, Apr 5, 2018 at 10:36 AM, Steven Whitehouse <> wrote:
>>>> Hi,
>>>> On 05/04/18 09:19, Dmitry Vyukov wrote:
>>>>> On Thu, Apr 5, 2018 at 8:34 AM, Greg KH <>
>>>>> 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://
>>>>>>> syzbot dashboard link:
>>>>>>> C reproducer:
>>>>>>> syzkaller reproducer:
>>>>>>> Raw console output:
>>>>>>> Kernel config:
>>>>>>> compiler: gcc (GCC) 7.1.1 20170620
>>>>>>> IMPORTANT: if you fix the bug, please add the following tag to the
>>>>>>> commit:
>>>>>>> Reported-by:
>>>>>>> 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.
> No, it has been there since at least the 3.13 kernel release (back in
> 2013), which is where it got moved to a separate function, but the logic
> has been around in the kernel tree for much longer than that as seen in
> commit d1c1459e4594 ("sysfs: separate out dup filename warning into a
> separate function")
>> 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.
> Don't you have a list of the "names" of the things you are creating
> somewhere?  Or are you relying on sysfs to do your housekeeping for you?
> Also, why did this trigger a syzbot report?  It's only a dump_stack()
> reference, one showing that yes, this is something that should not be
> done, but the kernel keeps on working afterward.
> thanks,
> greg k-h

Unfortunately, no. We don't have the list of names elsewhere. The names 
are used as a cluster-wide ID, so not having duplicates on a single node 
is a good thing :-) The name is effectively the same as the fs label. We 
could create a list - while sysfs does the job for us, we've not needed 
to do that though,


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