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

[Dmitry Vyukov]

On Thu, Jun 1, 2017 at 7:00 PM, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
>
>
> On 06/01/2017 07:59 PM, Andrey Ryabinin wrote:
>>
>>
>> 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
>
> s/Outline/inline  of course.


I suspect that actual implementations have diverged from that
description. Trying to follow asan_expand_check_ifn in:
https://gcc.gnu.org/viewcvs/gcc/trunk/gcc/asan.c?revision=246703&view=markup
but it's not trivial.

+Yuri, maybe you know off the top of your head if asan instrumentation
in gcc ever accesses off-by-one shadow byte (i.e. 1 byte after actual
object end)?