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

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

[Bill Wendling]

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>
---
 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.
-- 
2.56.0.rc1.315.gc6ed9934b7-goog

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

[Matthew Auld]

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.

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

[Bill Wendling]

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?

-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.
>

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

[Matthew Auld]

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.
>>