Re: WARNING in input_alloc_absinfo

[Dmitry Vyukov]

On Mon, Jul 2, 2018 at 11:30 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Fri, Jun 29, 2018 at 11:59 PM,  <dmitry.torokhov@xxxxxxxxx> wrote:
>> On Monday, June 25, 2018 at 5:43:02 AM UTC-7, Dmitry Vyukov wrote:
>>>
>>> On Tue, Jun 19, 2018 at 8:51 PM, Dmitry Torokhov
>>> <dmitry....@xxxxxxxxx> wrote:
>>> > On Thu, Jun 14, 2018 at 05:47:03AM -0700, syzbot wrote:
>>> >> Hello,
>>> >>
>>> >> syzbot found the following crash on:
>>> >>
>>> >> HEAD commit:    d2d741e5d189 kmsan: add initialization for shmem pages
>>> >> git tree:       https://github.com/google/kmsan.git/master
>>> >> console output:
>>> >> https://syzkaller.appspot.com/x/log.txt?x=1775bae7800000
>>> >> kernel config:
>>> >> https://syzkaller.appspot.com/x/.config?x=48f9de3384bcd0f
>>> >> dashboard link:
>>> >> https://syzkaller.appspot.com/bug?extid=c382812c78d98ecd9fb8
>>> >> compiler:       clang version 7.0.0 (trunk 329391)
>>> >> syzkaller
>>> >> repro:https://syzkaller.appspot.com/x/repro.syz?x=13b31ae7800000
>>> >> C reproducer:
>>> >> https://syzkaller.appspot.com/x/repro.c?x=1733255b800000
>>> >>
>>> >> IMPORTANT: if you fix the bug, please add the following tag to the
>>> >> commit:
>>> >> Reported-by: syzbot+c38281...@xxxxxxxxxxxxxxxxxxxxxxxxx
>>> >>
>>> >> RBP: 00000000006cb018 R08: 0000000000000001 R09: 00007ffe93080031
>>> >> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000004
>>> >> R13: ffffffffffffffff R14: 0000000000000000 R15: 0000000000000000
>>> >> ------------[ cut here ]------------
>>> >> input_alloc_absinfo(): kcalloc() failed?
>>> >> WARNING: CPU: 1 PID: 4498 at drivers/input/input.c:487
>>> >> input_alloc_absinfo+0x183/0x190 drivers/input/input.c:487
>>> >> Kernel panic - not syncing: panic_on_warn set ...
>>> >
>>> > Hmm, so there is not really a problem as far as I am concerned. We do
>>> > generate a warning if we can't allocate memory for absinfo,  as this is
>>> > really unexpected, but the case is handled properly by the callers so
>>> > there is no reason for us to go belly up here.
>>>
>>> Note to myself: ping this bug when "include/asm-generic/bug.h: clarify
>>> valid uses of WARN()" is fully merged.
>>
>>
>> No, this warning will still be there even after the "clarifying" patch is
>> merged. It does not check user inputs, it warns that the system is so low on
>> memory that we could not allocate measly amount needed for absinfo. Treat it
>> as you treat OOM warnings from kmalloc() itself.
>
> Hi Dmitry,
>
> kmalloc does not produce WARNING on OOM. The rule is not only about
> invalid inputs, it's also about any transient conditions and "WARNING
> only for kernel bugs".
>
> To put this in larger context, being able to distinguish kernel bugs
> from non-bugs is a very important and practically useful capability,
> which in particular enables systematic testing, but also makes things
> simpler for all kernel users. There must be a very significant reason
> to abandon this capability. What is that reason in this case?
>
> I also don't understand what is so special about this case. If we want
> user message for kmalloc failures, kmalloc is the right place for such
> warning, rather than a random call site out of thousands. Consider,
> the allocation failure can happen on the very next or previous kmalloc
> call, and user won't be warned. The rest of the kernel (including the
> rest of input sybsystem) does not warn on allocation failures, so that
> looks like what we need to do here as well. Or, if there is something
> very special about this particular kmalloc call site, something that
> makes it different from thousands of other call sites, why don't you
> want to replace it with pr_err which would both give the diagnostics
> but also not block systematic testing? Which looks like a win-win to
> me.
>
> Thanks


So, Dmitry, do you mind fixing this in the name of unblocking kernel testing?