Re: [PATCH v8 2/3] mm, kasan: align free_meta_offset on sizeof(void*)

[Andrey Ryabinin]

On 07/28/2016 06:31 PM, Alexander Potapenko wrote:
> When free_meta_offset is not zero, it is usually aligned on 4 bytes,
> because the size of preceding kasan_alloc_meta is aligned on 4 bytes.
> As a result, accesses to kasan_free_meta fields may be misaligned.
> 
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> ---
>  mm/kasan/kasan.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> index 6845f92..0379551 100644
> --- a/mm/kasan/kasan.c
> +++ b/mm/kasan/kasan.c
> @@ -390,7 +390,8 @@ void kasan_cache_create(struct kmem_cache *cache, size_t *size,
>  	/* Add free meta. */
>  	if (cache->flags & SLAB_DESTROY_BY_RCU || cache->ctor ||
>  	    cache->object_size < sizeof(struct kasan_free_meta)) {
> -		cache->kasan_info.free_meta_offset = *size;
> +		cache->kasan_info.free_meta_offset =
> +			ALIGN(*size, sizeof(void *));

This cannot work.

I slightly changed metadata layout in http://lkml.kernel.org/g/<1470062715-14077-5-git-send-email-aryabinin@xxxxxxxxxxxxx>
which should also fix UBSAN's complain.

>  		*size += sizeof(struct kasan_free_meta);
>  	}
>  	redzone_adjust = optimal_redzone(cache->object_size) -
>