[PATCH] kasan: [v2]unpoison use memzero to init unaligned object

[Marco Elver]

On Tue, 22 Jun 2021 at 10:48, <yee.lee at mediatek.com> wrote:
>
> From: Yee Lee <yee.lee at mediatek.com>
>
> Follows the discussion: https://patchwork.kernel.org/project/linux-mediatek/list/?series=504439

The info about the percentage of how frequent this is could have been
provided as a simple reply to the discussion.

> This patch Add memzero_explict to initialize unaligned object.

This patch does not apply to anything (I see it depends on the previous patch).

What you need to do is modify the original patch, and then send a
[PATCH v2] (git helps with that by passing --reroll-count or -v) that
applies cleanly to your base kernel tree.

The commit message will usually end with '---' and then briefly denote
what changed since the last version.
https://www.kernel.org/doc/html/latest/process/submitting-patches.html#the-canonical-patch-format

> Based on the integrateion of initialization in kasan_unpoison(). The hwtag instructions, constrained with its granularity, has to overwrite the data btyes in unaligned objects. This would cause issue when it works with SLUB debug redzoning.
>
> In this patch, an additional initalizaing path is added for the unaligned objects. It contains memzero_explict() to clear out the data and disables its init flag for the following hwtag actions.
>
> In lab test, this path is executed about 1.1%(941/80854) within the overall kasan_unpoison during a non-debug booting process.

Nice, thanks for the data. If it is somehow doable, however, I'd still
recommend to additionally guard the new code path by a check if
debug-support was requested. Ideally with an IS_ENABLED() config check
so that if it's a production kernel the branch is simply optimized out
by the compiler.

> Lab test: QEMU5.2 (+mte) / linux kernel 5.13-rc7
>
> Signed-off-by: Yee Lee <yee.lee at mediatek.com>
> ---
>  mm/kasan/kasan.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index d8faa64614b7..edc11bcc3ff3 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -389,7 +389,7 @@ static inline void kasan_unpoison(const void *addr, size_t size, bool init)
>                 return;
>         if (init && ((unsigned long)size & KASAN_GRANULE_MASK)) {
>                 init = false;
> -               memset((void *)addr, 0, size);
> +               memzero_explicit((void *)addr, size);
>         }
>         size = round_up(size, KASAN_GRANULE_SIZE);
>         hw_set_mem_tag_range((void *)addr, size, tag, init);
> 2.18.0
>
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