drm/amdgpu: Annotate struct amdgpu_bo_list with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct amdgpu_bo_list.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Luben Tuikov <luben.tuikov@amd.com>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-hardening@vger.kernel.org
Link: https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci [1]
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c

@@ -84,6 +84,7 @@ int amdgpu_bo_list_create(struct amdgpu_device *adev, struct drm_file *filp,
 
 	kref_init(&list->refcount);
 
+	list->num_entries = num_entries;
 	array = list->entries;
 
 	for (i = 0; i < num_entries; ++i) {
@@ -129,7 +130,6 @@ int amdgpu_bo_list_create(struct amdgpu_device *adev, struct drm_file *filp,
 	}
 
 	list->first_userptr = first_userptr;
-	list->num_entries = num_entries;
 	sort(array, last_entry, sizeof(struct amdgpu_bo_list_entry),
 	     amdgpu_bo_list_entry_cmp, NULL);
 

drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.h

@@ -56,7 +56,7 @@ struct amdgpu_bo_list {
 	 */
 	struct mutex bo_list_mutex;
 
-	struct amdgpu_bo_list_entry entries[];
+	struct amdgpu_bo_list_entry entries[] __counted_by(num_entries);
 };
 
 int amdgpu_bo_list_get(struct amdgpu_fpriv *fpriv, int id,