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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 5 Apr 2018 10:52:57 +0200
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Subject: Re: WARNING: kobject bug in sysfs_warn_dup
To:     Steven Whitehouse <swhiteho@redhat.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>, rpeterso@redhat.com,
        cluster-devel@redhat.com,
        syzbot <syzbot+ff87a28e665c163aa7f5@syzkaller.appspotmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller-bugs@googlegroups.com
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On Thu, Apr 5, 2018 at 10:36 AM, Steven Whitehouse <swhiteho@redhat.com> wrote:
> Hi,
>
>
>
> On 05/04/18 09:19, Dmitry Vyukov wrote:
>>
>> On Thu, Apr 5, 2018 at 8:34 AM, Greg KH <gregkh@linuxfoundation.org>
>> 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@syzkaller.appspotmail.com
>>>> 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.