On Tue, 12 Jul 2022, Hyeonggon Yoo wrote:
> __ksize() returns size of objects allocated from slab allocator.
> When invalid object is passed to __ksize(), returning zero
> prevents further memory corruption and makes caller be able to
> check if there is an error.
>
> If address of large object is not beginning of folio or size of
> the folio is too small, it must be invalid. Return zero in such cases.
Why return 0 if there is an error and why bother the callers with these
checks. BUG()?
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 1f8db7959366..0d6cbe9d7ad0 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1013,8 +1013,12 @@ size_t __ksize(const void *object)
>
> folio = virt_to_folio(object);
>
> - if (unlikely(!folio_test_slab(folio)))
> + if (unlikely(!folio_test_slab(folio))) {
> + if (WARN_ON(object != folio_address(folio) ||
> + folio_size(folio) <= KMALLOC_MAX_CACHE_SIZE))
Hmmm... This may change things a bit. Before this patch it was possible to
determine the storage size of order-0 pages using ksize(). Now this
returns 0?
I guess this is an error since the order-0 page cannot come from slab
allocations.
