Re: [PATCH] x86/process: Silence KASAN warnings in get_wchan()

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 10/06/2015 10:26 AM, Ingo Molnar wrote:
> 
> * Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> 
>> On 10/05/2015 07:39 PM, Andi Kleen wrote:
>>>> But, I think I have the solution.
>>>> We could have some blacklist - list of function names which we should be ignored.
>>>> In kasan_report() we could resolve return address to function name and compare it with name in list.
>>>> If name in list -> ignore report.
>>>
>>> I think annotating statements is cleaner than functions, even if it
>>> is more code. Much better documentation
>>>
>>
>> I agree with that, that's why I suggested to add READ_ONCE_NOCHECK():
>> 	READ_ONCE_NOCHECK()
>> 	{
>> 		kasan_disable_current();
>> 		READ_ONCE();
>> 		kasan_enable_current();
>> 	}
>>
>> Anywone objects?
> 
> Sounds good to me! As long as it's hidden from plain .c files I'm a happy camper.
> 
> This should probably also be faster for KASAN than triggering a warning and having 
> to parse a blacklist, right?
> 

Sure.

>>> If disabling with an attribute doesn't work, you could put it into a special 
>>> section with __attribute__((section ...)) and check the start/end symbol 
>>> before reporting. That's how kprobes solves similar issues. It also has the 
>>> advantage that it stops inlining.
>>
>> Yes, it might be better. Although, because of broken -fconserve-stack, this may 
>> not work in some cases - https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63533 
>> Function splitter may split original function into two parts and it always puts 
>> one split part in default .text section.
> 
> We do a _ton_ of such section tricks in the kernel (all of exception handling is 
> based on that) - if that's broken by -fconserve-stack then the kernel is broken 
> much more widely.
> 

I'm mistaken here. It was broken once, at some point of development of gcc 5, but this was fixed
eventually. I just checked gcc 5.2, 4.9.2, 4.8.4, all of them are ok.

> So unless KASAN wants to do something special here you can rely on sections just 
> fine.
> 
> Thanks,
> 
> 	Ingo
>