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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe