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

[Andrey Ryabinin]

On 06/01/2017 07:52 PM, Mark Rutland wrote:
> On Thu, Jun 01, 2017 at 06:45:32PM +0200, Dmitry Vyukov wrote:
>> 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?
>>

Correct, inline instrumentation assumes that all accesses are properly aligned as it
required by C standard. I knew that the kernel violates this rule in many places,
therefore I decided to add checks for unaligned accesses in outline case.


>> Inline instrumentation generally accesses only a single byte.
> 
> Sorry to be a little pedantic, but does that mean we'll never access the
> additional shadow, or does that mean it's very unlikely that we will?
> 
> I'm guessing/hoping it's the former!
> 

Outline will never access additional shadow byte: https://github.com/google/sanitizers/wiki/AddressSanitizerAlgorithm#unaligned-accesses

> Thanks,
> Mark.
>