how can KAsan support arm32?

[Dmitry Vyukov]

On Thu, Jun 1, 2017 at 6:42 PM, Russell King - ARM Linux
<linux at armlinux.org.uk> wrote:
> On Thu, Jun 01, 2017 at 07:37:26PM +0300, Andrey Ryabinin wrote:
>> kasan needs at least 1/8 of the total memory, so it will be 32MB in
>> your case.
>
> Is that really the case?  It is if we want to account for every byte,
> but I don't think that's necessary.
>
> Memory is allocated not on a per-byte basis, but on a larger granule,
> mainly determined by L1_CACHE_BYTES / ARCH_SLAB_MINALIGN etc.  While
> it means that we would lose the ability to capture accesses that
> happen strictly over the size requested from kmalloc() etc, if we
> implemented this, it would at least allow the more seriously wrong
> accesses to be trapped, while being cheaper.
>
> So, for 1GB of kernel virtual space, we'd only need 4MB for the bitmap
> for a 32-byte cache line system, which is much less of a problem.


KASAN shadow is not bit-per-byte, it's more elaborate. We can increase
shadow scale from 1:8 to 1:32 without loss of precision. This was
recently discussed here:
https://groups.google.com/d/msg/kasan-dev/JKYk2uUBriY/s5aH0bCaAwAJ