[PATCH v2] drm/ttm: Silence randstruct warning about casting struct file

[Kees Cook]

Casting through a "void *" isn't sufficient to convince the randstruct
GCC plugin that the result is intentional. Instead operate through an
explicit union to silence the warning:

drivers/gpu/drm/ttm/ttm_backup.c: In function 'ttm_file_to_backup':
drivers/gpu/drm/ttm/ttm_backup.c:21:16: note: randstruct: casting between randomized structure pointer types (ssa): 'struct ttm_backup' and 'struct file'
   21 |         return (void *)file;
      |                ^~~~~~~~~~~~

Suggested-by: Matthew Brost <matthew.brost at intel.com>
Fixes: e7b5d23e5d47 ("drm/ttm: Provide a shmem backup implementation")
Signed-off-by: Kees Cook <kees at kernel.org>
---
 v2: use struct and container_of (matthew)
 v1: https://lore.kernel.org/all/20250501195859.work.107-kees@kernel.org/
Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
Cc: Christian Koenig <christian.koenig at amd.com>
Cc: Somalapuram Amaranath <Amaranath.Somalapuram at amd.com>
Cc: Matthew Brost <matthew.brost at intel.com>
Cc: Huang Rui <ray.huang at amd.com>
Cc: Matthew Auld <matthew.auld at intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Maxime Ripard <mripard at kernel.org>
Cc: Thomas Zimmermann <tzimmermann at suse.de>
Cc: David Airlie <airlied at gmail.com>
Cc: Simona Vetter <simona at ffwll.ch>
Cc: <dri-devel at lists.freedesktop.org>
---
 drivers/gpu/drm/ttm/ttm_backup.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_backup.c b/drivers/gpu/drm/ttm/ttm_backup.c
index 93c007f18855..60cff6c60db4 100644
--- a/drivers/gpu/drm/ttm/ttm_backup.c
+++ b/drivers/gpu/drm/ttm/ttm_backup.c
@@ -9,16 +9,21 @@
 
 /*
  * Casting from randomized struct file * to struct ttm_backup * is fine since
- * struct ttm_backup is never defined nor dereferenced.
+ * struct ttm_backup is never defined nor dereferenced. Use a single-member
+ * struct to avoid cast warnings.
  */
+struct ttm_backup {
+	struct file file;
+};
+
 static struct file *ttm_backup_to_file(struct ttm_backup *backup)
 {
-	return (void *)backup;
+	return &backup->file;
 }
 
 static struct ttm_backup *ttm_file_to_backup(struct file *file)
 {
-	return (void *)file;
+	return container_of(file, struct ttm_backup, file);
 }
 
 /*
-- 
2.34.1