Re: [PATCH v5 17/37] mm: slub: Disable KMSAN when checking the padding bytes

Alexander Potapenko
2024-06-20T09:01:16Z

On Wed, Jun 19, 2024 at 5:45 PM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> Even though the KMSAN warnings generated by memchr_inv() are suppressed
> by metadata_access_enable(), its return value may still be poisoned.
>
> The reason is that the last iteration of memchr_inv() returns
> `*start != value ? start : NULL`, where *start is poisoned. Because of
> this, somewhat counterintuitively, the shadow value computed by
> visitSelectInst() is equal to `(uintptr_t)start`.
>
> One possibility to fix this, since the intention behind guarding
> memchr_inv() behind metadata_access_enable() is to touch poisoned
> metadata without triggering KMSAN, is to unpoison its return value.
> However, this approach is too fragile. So simply disable the KMSAN
> checks in the respective functions.
>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>