Re: limits->max_sectors is getting set to 0, why/where? [was: Re: dm: kernel oops by divide error on v4.16+]

[Ming Lei <ming.lei@redhat.com>]

On Mon, Apr 09, 2018 at 04:10:17PM -0600, Jens Axboe wrote:
> On 4/9/18 4:05 PM, Kees Cook wrote:
> > On Mon, Apr 9, 2018 at 2:56 PM, Jens Axboe <axboe@kernel.dk> wrote:
> >> On 4/9/18 3:26 PM, Jens Axboe wrote:
> >>> On 4/9/18 1:32 PM, Jens Axboe wrote:
> >>>> On 4/9/18 12:38 PM, Mike Snitzer wrote:
> >>>>> On Mon, Apr 09 2018 at 11:51am -0400,
> >>>>> Mike Snitzer <snitzer@redhat.com> wrote:
> >>>>>
> >>>>>> On Sun, Apr 08 2018 at 12:00am -0400,
> >>>>>> Ming Lei <ming.lei@redhat.com> wrote:
> >>>>>>
> >>>>>>> Hi,
> >>>>>>>
> >>>>>>> The following kernel oops(divide error) is triggered when running
> >>>>>>> xfstest(generic/347) on ext4.
> >>>>>>>
> >>>>>>> [  442.632954] run fstests generic/347 at 2018-04-07 18:06:44
> >>>>>>> [  443.839480] divide error: 0000 [#1] PREEMPT SMP PTI
> >>>>>>> [  443.840201] Dumping ftrace buffer:
> >>>>>>> [  443.840692]    (ftrace buffer empty)
> >>>>> ...
> >>>>>>> [  443.845756] CPU: 1 PID: 29607 Comm: dmsetup Not tainted 4.16.0_f605ba97fb80_master+ #1
> >>>>>>> [  443.846968] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.10.2-2.fc27 04/01/2014
> >>>>>>> [  443.848147] RIP: 0010:pool_io_hints+0x77/0x153 [dm_thin_pool]
> >>>>>
> >>>>> ...
> >>>>>
> >>>>>> I was able to reproduce (in my case RIP was pool_io_hints+0x45)
> >>>>>>
> >>>>>> Which on my kernel, is:
> >>>>>>
> >>>>>> crash> dis -l pool_io_hints+0x45
> >>>>>> /root/snitm/git/linux/drivers/md/dm-thin.c: 2748
> >>>>>> 0xffffffffc0765165 <pool_io_hints+69>:  div    %rdi
> >>>>>>
> >>>>>> Which is drivers/md/dm-thin.c:is_factor()'s return
> >>>>>> !sector_div(block_size, n);
> >>>>>>
> >>>>>> SO looking at pool_io_hints() it would seem limits->max_sectors is 0 for
> >>>>>> this xfstests device... why would that be!?
> >>>>>>
> >>>>>> Clearly pool_io_hints() could stand to be more defensive with a
> >>>>>> !limits->max_sectors negative check but is it ever really valid for
> >>>>>> max_sectors to be 0?
> >>>>>>
> >>>>>> Pretty sure the ultimate bug is outside DM (but not seeing an obvious
> >>>>>> place where block core would set max_sectors to 0, all blk-settings.c
> >>>>>> uses min_not_zero(), etc).
> >>>>>
> >>>>> I successfully ran this test against the linux-dm.git
> >>>>> "for-4.17/dm-changes" tag that Linus merged after the block changes:
> >>>>>  git://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git tags/for-4.17/dm-changes
> >>>>>
> >>>>> # ./check tests/generic/347
> >>>>> FSTYP         -- ext4
> >>>>> PLATFORM      -- Linux/x86_64 thegoat 4.16.0-rc5.snitm
> >>>>> MKFS_OPTIONS  -- /dev/mapper/test-xfstests_scratch
> >>>>> MOUNT_OPTIONS -- -o acl,user_xattr /dev/mapper/test-xfstests_scratch /scratch
> >>>>>
> >>>>> generic/347      65s
> >>>>> Ran: generic/347
> >>>>> Passed all 1 tests
> >>>>>
> >>>>> SO this would seem to implicate some regression in the 4.17 block layer
> >>>>> changes.
> >>>>
> >>>> No immediate ideas come to mind, we didn't have a lot of changes and I
> >>>> don't see anything that looks problematic. Maybe you can try and
> >>>> bisect it and see what you come up with?
> >>>
> >>> I ran it, problematic commit is:
> >>>
> >>> commit 3c8ba0d61d04ced9f8d9ff93977995a9e4e96e91
> >>> Author: Kees Cook <keescook@chromium.org>
> >>> Date:   Fri Mar 30 18:52:36 2018 -0700
> >>>
> >>>     kernel.h: Retain constant expression output for max()/min()
> >>>
> >>
> >> The fun continues. Thinking I'd try a userspace repro and thinking it
> >> would be difficult to reproduce, try the attached min.c that just copies
> >> all the bits from include/linux/kernel.h
> >>
> >> axboe@x1:~ $ gcc -Wall -O2 -o min min.c
> >> axboe@x1:~ $ ./min 128 256
> >> min_not_zero(128, 256) = 0
> > 
> > This should be fixed with e9092d0d9796 ("Fix subtle macro variable
> > shadowing in min_not_zero()").
> 
> Yep that works, which is a relief. Some basic unit testing would have
> been very appropriate in this case, given how fundamentally broken it
> was... It's amazing nothing catastrophic happened.

Actually, there was, :-)

https://lkml.org/lkml/2018/4/9/355

-- 
Ming