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

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

On 28/09/2026 19:44, Bill Wendling wrote:
> On Mon, Sep 28, 2026 at 5:20 AM Matthew Auld <matthew.auld@intel.com> wrote:
>>
>> 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?
>>
> Sure! There's already a 'max_order'. I assume that 'n_orders' would be
> more dynamic?

Yeah, I think just mm.n_orders = max_order + 1. I assume the 
__counted_by_ptr() only works with a field?

> 
> -bw
> 
>>> ---
>>>    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.
>>