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

[Chen Gang]

On 5/3/16 00:23, Chen Gang wrote:
> On 5/2/16 23:35, Alexander Potapenko wrote:
>> On Mon, May 2, 2016 at 5:13 PM, Chen Gang <chengang@xxxxxxxxxxxxxxxx> wrote:
>>>
>>> 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. And need I also send patch v2 for include/linux/kasan.h? (or
> you will fix them together).
> 
>>>
>>> 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.
>>

Oh, sorry, I forgot one of our original discussing content:

 - If we use signed int kasan_depth, and kasan_depth <= 0 means enable, I
   guess, we can always modify kasan_depth.

 - When overflow/underflow (singed int overflow), we can use BUG_ON(),
   since it should be rarely happen.

Thanks.

> 
> For me, BUG_ON() will be better for debugging, but it is really not well
> for using.  For WARN_ON(), it already print warnings, so I am not quite
> sure "always modifying kasan_depth will be ease the debugging".
> 
> When we are in inconsistent state, for me, what we can do is:
> 
>  - Still try to do correct things within our control: "when the caller
>    make a mistake, if kasan_enable_current() notices about it, it need
>    issue warning, and prevent itself to make mistake (causing disable).
> 
>  - "try to let negative effect smaller to user", e.g. let users "loose
>    hope" (call enable has no effect) instead of destroying users'
>    original work (call enable, but get disable).
> 
> Thanks.
> 

-- 
Chen Gang (éå)

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