Re: [PATCH] kasan: unpoison use memset to init unaligned object size

[Marco Elver <elver@google.com>]

On Mon, 21 Jun 2021 at 17:45, <yee.lee@mediatek.com> wrote:
>
> From: Yee Lee <yee.lee@mediatek.com>
>
> This patch adds a memset to initialize object of unaligned size.

s/This patch adds/Add/

> Duing to the MTE granulrity, the integrated initialization using

s/Duing/Doing/
s/granulrity/granularity/

> hwtag instruction will force clearing out bytes in granular size,
> which may cause undesired effect, such as overwriting to the redzone
> of SLUB debug. In this patch, for the unaligned object size, function

Did you encounter a crash due to this? Was it only SLUB debug that
caused the problem?

Do you have data on what the percentage of allocations are that would
now be treated differently? E.g. what's the percentage of such
odd-sized allocations during a normal boot with SLUB debug off?

We need to know if this change would pessimize a non-debug kernel, and
if so, we'd have to make the below behave differently.

> uses memset to initailize context instead of the hwtag instruction.

s/initailize/initialize/

> Signed-off-by: Yee Lee <yee.lee@mediatek.com>
> ---
>  mm/kasan/kasan.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 8f450bc28045..d8faa64614b7 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -387,8 +387,11 @@ static inline void kasan_unpoison(const void *addr, size_t size, bool init)
>
>         if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
>                 return;
> +       if (init && ((unsigned long)size & KASAN_GRANULE_MASK)) {
> +               init = false;
> +               memset((void *)addr, 0, size);

Should use memzero_explicit().

> +       }
>         size = round_up(size, KASAN_GRANULE_SIZE);
> -

Remove whitespace change.

>         hw_set_mem_tag_range((void *)addr, size, tag, init);
>  }

Thanks,
-- Marco