Re: [PATCH v1 00/11] mm/kasan: support per-page shadow memory to reduce memory consumption

[Andrey Ryabinin]

On 05/31/2017 08:50 AM, Joonsoo Kim wrote:
>>> But the main win as I see it is that that's basically complete support
>>> for 32-bit arches. People do ask about arm32 support:
>>> https://groups.google.com/d/msg/kasan-dev/Sk6BsSPMRRc/Gqh4oD_wAAAJ
>>> https://groups.google.com/d/msg/kasan-dev/B22vOFp-QWg/EVJPbrsgAgAJ
>>> and probably mips32 is relevant as well.
>>
>> I don't see how above is relevant for 32-bit arches. Current design
>> is perfectly fine for 32-bit arches. I did some POC arm32 port couple years
>> ago - https://github.com/aryabinin/linux/commits/kasan/arm_v0_1
>> It has some ugly hacks and non-critical bugs. AFAIR it also super-slow because I (mistakenly) 
>> made shadow memory uncached. But otherwise it works.
> 
> Could you explain that where is the code to map shadow memory uncached?
> I don't find anything related to it.
> 

I didn't set set any cache policy (L_PTE_MT_*) on shadow mapping (see set_pte_at() calls )
which means it's L_PTE_MT_UNCACHED 

>>> Such mode does not require a huge continuous address space range, has
>>> minimal memory consumption and requires minimal arch-dependent code.
>>> Works only with outline instrumentation, but I think that's a
>>> reasonable compromise.
>>>
>>> What do you think?
>>  
>> I don't understand why we trying to invent some hacky/complex schemes when we already have
>> a simple one - scaling shadow to 1/32. It's easy to implement and should be more performant comparing
>> to suggested schemes.
> 
> My approach can co-exist with changing scaling approach. It has it's
> own benefit.
> 
> And, as Dmitry mentioned before, scaling shadow to 1/32 also has downsides,
> expecially for inline instrumentation. And, it requires compiler
> modification and user needs to update their compiler to newer version
> which is not so simple in terms of the user's usability
> 
> Thanks.
>