Re: [PATCH v2 2/3] slab: don't put freepointer outside of object if only orig_size

[Vlastimil Babka <vbabka@suse.cz>]

On 6/5/24 9:13 AM, Chengming Zhou wrote:
> The commit 946fa0dbf2d8 ("mm/slub: extend redzone check to extra
> allocated kmalloc space than requested") will extend right redzone
> when allocating for orig_size < object_size. So we can't overlay the
> freepointer in the object space in this case.
> 
> But the code looks like it forgot to check SLAB_RED_ZONE, since there
> won't be extended right redzone if only orig_size enabled.
> 
> As we are here, make this complex conditional expressions a little
> prettier and add some comments about extending right redzone when
> slub_debug_orig_size() enabled.
> 
> Reviewed-by: Feng Tang <feng.tang@intel.com>
> Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/slub.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 7fbd5ce4320a..704c662227e6 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5152,10 +5152,9 @@ static int calculate_sizes(struct kmem_cache *s)
>  	 */
>  	s->inuse = size;
>  
> -	if (slub_debug_orig_size(s) ||
> -	    (flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
> -	    ((flags & SLAB_RED_ZONE) && s->object_size < sizeof(void *)) ||
> -	    s->ctor) {
> +	if ((flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) || s->ctor ||
> +	    ((flags & SLAB_RED_ZONE) &&
> +	     (s->object_size < sizeof(void *) || slub_debug_orig_size(s)))) {
>  		/*
>  		 * Relocate free pointer after the object if it is not
>  		 * permitted to overwrite the first word of the object on
> @@ -5163,7 +5162,9 @@ static int calculate_sizes(struct kmem_cache *s)
>  		 *
>  		 * This is the case if we do RCU, have a constructor or
>  		 * destructor, are poisoning the objects, or are
> -		 * redzoning an object smaller than sizeof(void *).
> +		 * redzoning an object smaller than sizeof(void *) or are
> +		 * redzoning an object with slub_debug_orig_size() enabled,
> +		 * in which case the right redzone may be extended.
>  		 *
>  		 * The assumption that s->offset >= s->inuse means free
>  		 * pointer is outside of the object is used in the
>