Re: [PATCH] gpu/buddy: add __counted_by_ptr attribute to roots

[Matthew Auld <matthew.auld@intel.com>]

On 25/09/2026 21:44, Bill Wendling wrote:
> In 'struct gpu_buddy', the 'roots' field points to an array of pointers
> to 'struct gpu_buddy_block'. The number of allocated roots is tracked
> by the 'n_roots' field.
> 
> Annotate the 'roots' pointer with the '__counted_by_ptr' attribute
> referencing 'n_roots' to enable compile-time and runtime bounds-checking
> via KASAN and '__builtin_dynamic_object_size'.
> 
> In 'gpu_buddy_init', 'mm->n_roots' is initialized first, and 'mm->roots' is
> subsequently allocated with 'kmalloc_objs(struct gpu_buddy_block *,
> mm->n_roots)'.
> 
> Since the bounds associated with 'roots' ('n_roots') are fully set prior
> to any array allocation or access and remain invariant, this annotation
> will not cause runtime panics or false-positive bounds checks.
> 
> Cc: codemender-patching+linux@google.com
> Assisted-by: LLM
> Signed-off-by: Bill Wendling <morbo@google.com>

Reviewed-by: Matthew Auld <matthew.auld@intel.com>

Would it make sense to also do this for *_scoreboard and free_tree, if 
we add a new field n_orders?

> ---
>   include/linux/gpu_buddy.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/gpu_buddy.h b/include/linux/gpu_buddy.h
> index 2c36124bb696..d7249e500ab6 100644
> --- a/include/linux/gpu_buddy.h
> +++ b/include/linux/gpu_buddy.h
> @@ -172,7 +172,7 @@ struct gpu_buddy {
>   	 * a power of two, with each root being the largest power-of-two
>   	 * that fits in the remaining space.
>   	 */
> -	struct gpu_buddy_block **roots;
> +	struct gpu_buddy_block **roots __counted_by_ptr(n_roots);
>   	/*
>   	 * Per-order free block scoreboard: free_scoreboard[order] holds the
>   	 * number of blocks of that order currently in the free state.