Re: [PATCH 3/4] arm64/kasan: don't allocate extra shadow memory

[Dmitry Vyukov]

On Thu, Jun 1, 2017 at 6:34 PM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> On Thu, Jun 01, 2017 at 07:23:37PM +0300, Andrey Ryabinin wrote:
>> We used to read several bytes of the shadow memory in advance.
>> Therefore additional shadow memory mapped to prevent crash if
>> speculative load would happen near the end of the mapped shadow memory.
>>
>> Now we don't have such speculative loads, so we no longer need to map
>> additional shadow memory.
>
> I see that patch 1 fixed up the Linux helpers for outline
> instrumentation.
>
> Just to check, is it also true that the inline instrumentation never
> performs unaligned accesses to the shadow memory?

Inline instrumentation generally accesses only a single byte.

> If so, this looks good to me; it also avoids a potential fencepost issue
> when memory exists right at the end of the linear map. Assuming that
> holds:
>
> Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
>
> Thanks,
> Mark.
>
>>
>> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>> Cc: Will Deacon <will.deacon@xxxxxxx>
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> ---
>>  arch/arm64/mm/kasan_init.c | 8 +-------
>>  1 file changed, 1 insertion(+), 7 deletions(-)
>>
>> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
>> index 687a358a3733..81f03959a4ab 100644
>> --- a/arch/arm64/mm/kasan_init.c
>> +++ b/arch/arm64/mm/kasan_init.c
>> @@ -191,14 +191,8 @@ void __init kasan_init(void)
>>               if (start >= end)
>>                       break;
>>
>> -             /*
>> -              * end + 1 here is intentional. We check several shadow bytes in
>> -              * advance to slightly speed up fastpath. In some rare cases
>> -              * we could cross boundary of mapped shadow, so we just map
>> -              * some more here.
>> -              */
>>               vmemmap_populate((unsigned long)kasan_mem_to_shadow(start),
>> -                             (unsigned long)kasan_mem_to_shadow(end) + 1,
>> +                             (unsigned long)kasan_mem_to_shadow(end),
>>                               pfn_to_nid(virt_to_pfn(start)));
>>       }
>>
>> --
>> 2.13.0
>>
>>
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