Re: [PATCH] mm: Handle ksize() vs __alloc_size by forgetting size

[Nick Desaulniers <ndesaulniers@google.com>]

On Fri, Feb 25, 2022 at 2:16 PM Kees Cook <keescook@chromium.org> wrote:
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 37bde99b74af..a14f3bfa2f44 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -182,8 +182,32 @@ int kmem_cache_shrink(struct kmem_cache *s);
>  void * __must_check krealloc(const void *objp, size_t new_size, gfp_t flags) __alloc_size(2);
>  void kfree(const void *objp);
>  void kfree_sensitive(const void *objp);
> +
> +/**
> + * ksize - get the actual amount of memory allocated for a given object
> + * @objp: Pointer to the object
> + *
> + * kmalloc may internally round up allocations and return more memory
> + * than requested. ksize() can be used to determine the actual amount of
> + * memory allocated. The caller may use this additional memory, even though
> + * a smaller amount of memory was initially specified with the kmalloc call.
> + * The caller must guarantee that objp points to a valid object previously
> + * allocated with either kmalloc() or kmem_cache_alloc(). The object
> + * must not be freed during the duration of the call.
> + *
> + * Return: size of the actual memory used by @objp in bytes
> + */
> +#define ksize(objp) ({                                                 \
> +       /*                                                              \
> +        * Getting the actual allocation size means the __alloc_size    \
> +        * hints are no longer valid, and the compiler needs to         \
> +        * forget about them.                                           \
> +        */                                                             \
> +       OPTIMIZER_HIDE_VAR(objp);                                       \
> +       _ksize(objp);                                                   \
> +})
>  size_t __ksize(const void *objp);
> -size_t ksize(const void *objp);
> +size_t _ksize(const void *objp);

If you wanted to discourage others from calling _ksize, you could hide
its declaration within the scope of statement expression within ksize:
https://godbolt.org/z/e4sd4nE6q
-- 
Thanks,
~Nick Desaulniers