Re: [PATCH] mm/kasan/kasan.h: Fix boolean checking issue for kasan_report_enabled()

[Alexander Potapenko]

On Mon, May 2, 2016 at 3:51 PM, Chen Gang <chengang@xxxxxxxxxxxxxxxx> wrote:
> On 5/2/16 20:42, Alexander Potapenko wrote:
>> On Mon, May 2, 2016 at 2:27 PM, Chen Gang <chengang@xxxxxxxxxxxxxxxx> wrote:
>>> On 5/2/16 19:21, Dmitry Vyukov wrote:
>>>>
>>>> Signed counter looks good to me.
>>>
>>> Oh, sorry, it seems a little mess (originally, I need let the 2 patches
>>> in one patch set).
>>>
>>> If what Alexander's idea is OK (if I did not misunderstand), I guess,
>>> unsigned counter is still necessary.
>> I don't think it's critical for us to use an unsigned counter.
>> If we increment the counter in kasan_disable_current() and decrement
>> it in kasan_enable_current(), as Dmitry suggested, we'll be naturally
>> using only positive integers for the counter.
>> If the counter drops below zero, or exceeds a certain number (say,
>> 20), we can immediately issue a warning.
>>
>
> OK, thanks.
>
> And for "kasan_depth == 1", I guess, its meaning is related with
> kasan_depth[++|--] in kasan_[en|dis]able_current():
Assuming you are talking about the assignment of 1 to kasan_depth in
/include/linux/init_task.h,
it's somewhat counterintuitive. I think we just need to replace it
with kasan_disable_current(), and add a corresponding
kasan_enable_current() to the end of kasan_init.

>  - If kasan_depth++ in kasan_enable_current() with preventing overflow/
>    underflow, it means "we always want to disable KASAN, if CONFIG_KASAN
>    is not under arm64 or x86_64".
>
>  - If kasan_depth-- in kasan_enable_current() with preventing overflow/
>    underflow, it means "we can enable KASAN if CONFIG_KASAN, but firstly
>    we disable it, if it is not under arm64 or x86_64".
>
> For me, I don't know which one is correct (or my whole 'guess' is
> incorrect). Could any members provide your ideas?
>
>>>> We can both issue a WARNING and prevent the actual overflow/underflow.
>>>> But I don't think that there is any sane way to handle the bug (other
>>>> than just fixing the unmatched disable/enable). If we ignore an
>>>> excessive disable, then we can end up with ignores enabled
>>>> permanently. If we ignore an excessive enable, then we can end up with
>>>> ignores enabled when they should not be enabled. The main point here
>>>> is to bark loudly, so that the unmatched annotations are noticed and
>>>> fixed.
>>>>
>>>
>>> How about BUG_ON()?
>> As noted by Dmitry in an offline discussion, we shouldn't bail out as
>> long as it's possible to proceed, otherwise the kernel may become very
>> hard to debug.
>> A mismatching annotation isn't a case in which we can't proceed with
>> the execution.
>
> OK, thanks.
>
> I guess, we are agree with each other: "We can both issue a WARNING and
> prevent the actual overflow/underflow.".
No, I am not sure think that we need to prevent the overflow.
As I showed before, this may result in kasan_depth being off even in
the case kasan_enable_current()/kasan_disable_current() are used
consistently.
> Thanks.
> --
> Chen Gang (éå)
>
> Managing Natural Environments is the Duty of Human Beings.



-- 
Alexander Potapenko
Software Engineer

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