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

[Bill Wendling <morbo@google.com>]

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