Re: [PATCH v2 13/16] mempool: Use kmalloc_size_roundup() to match ksize() usage

[Vlastimil Babka]

On 9/23/22 22:28, Kees Cook wrote:
> Round up allocations with kmalloc_size_roundup() so that mempool's use
> of ksize() is always accurate and no special handling of the memory is
> needed by KASAN, UBSAN_BOUNDS, nor FORTIFY_SOURCE.
>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>   mm/mempool.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/mempool.c b/mm/mempool.c
> index 96488b13a1ef..0f3107b28e6b 100644
> --- a/mm/mempool.c
> +++ b/mm/mempool.c
> @@ -526,7 +526,7 @@ EXPORT_SYMBOL(mempool_free_slab);
>    */
>   void *mempool_kmalloc(gfp_t gfp_mask, void *pool_data)
>   {
> -	size_t size = (size_t)pool_data;
> +	size_t size = kmalloc_size_roundup((size_t)pool_data);

Hm it is kinda wasteful to call into kmalloc_size_roundup for every 
allocation that has the same input. We could do it just once in 
mempool_init_node() for adjusting pool->pool_data ?

But looking more closely, I wonder why poison_element() and 
kasan_unpoison_element() in mm/mempool.c even have to use 
ksize()/__ksize() and not just operate on the requested size (again, 
pool->pool_data). If no kmalloc mempool's users use ksize() to write 
beyond requested size, then we don't have to unpoison/poison that area 
either?

>   	return kmalloc(size, gfp_mask);
>   }
>   EXPORT_SYMBOL(mempool_kmalloc);