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

[Alexander Potapenko]

On Mon, May 2, 2016 at 5:13 PM, Chen Gang <chengang@xxxxxxxxxxxxxxxx> wrote:
> On 5/2/16 22:23, Alexander Potapenko wrote:
>> On Mon, May 2, 2016 at 3:51 PM, Chen Gang <chengang@xxxxxxxxxxxxxxxx> wrote:
>>>
>>> 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.
>>
>
> OK. But it does not look quite easy to use kasan_disable_current() for
> INIT_KASAN which is used in INIT_TASK.
>
> If we have to set "kasan_depth == 1", we have to use kasan_depth-- in
> kasan_enable_current().
Agreed, decrementing the counter in kasan_enable_current() is more natural.
I can fix this together with the comments.
>>>
>>> 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.
>
> If we don't prevent the overflow, it will have negative effect with the
> caller. When we issue an warning, it means the caller's hope fail, but
> can not destroy the caller's original work. In our case:
>
>  - Assume "kasan_depth-- for kasan_enable_current()", the first enable
>    will let kasan_depth be 0.
Sorry, I'm not sure I follow.
If we start with kasan_depth=0 (which is the default case for every
task except for the init, which also gets kasan_depth=0 short after
the kernel starts),
then the first call to kasan_disable_current() will make kasan_depth
nonzero and will disable KASAN.
The subsequent call to kasan_enable_current() will enable KASAN back.

There indeed is a problem when someone calls kasan_enable_current()
without previously calling kasan_disable_current().
In this case we need to check that kasan_depth was zero and print a
warning if it was.
It actually does not matter whether we modify kasan_depth after that
warning or not, because we are already in inconsistent state.
But I think we should modify kasan_depth anyway to ease the debugging.

>  - If we don't prevent the overflow, 2nd enable will cause disable
>    effect, which will destroy the caller's original work.
The subsequent call to kasan_enable_current() will
>  - Enable/disable mismatch is caused by caller, we can issue warnings,
>    and skip it (since it is not caused by us). But we can not generate
>    new issues to the system only because of the caller's issue.
>
>
> Thanks.
> --
> Chen Gang (éå)
>
> Managing Natural Environments is the Duty of Human Beings.



-- 
Alexander Potapenko
Software Engineer

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