Re: WARNING: kobject bug in sysfs_warn_dup

[Greg KH]

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 <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.

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