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

[Kees Cook <kees@kernel.org>]

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;
      |                ^~~~~~~~~~~~

Fixes: e7b5d23e5d47 ("drm/ttm: Provide a shmem backup implementation")
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Somalapuram Amaranath <Amaranath.Somalapuram@amd.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: <dri-devel@lists.freedesktop.org>
---
 drivers/gpu/drm/ttm/ttm_backup.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/ttm/ttm_backup.c b/drivers/gpu/drm/ttm/ttm_backup.c
index 93c007f18855..626af1de562f 100644
--- a/drivers/gpu/drm/ttm/ttm_backup.c
+++ b/drivers/gpu/drm/ttm/ttm_backup.c
@@ -18,7 +18,13 @@ static struct file *ttm_backup_to_file(struct ttm_backup *backup)
 
 static struct ttm_backup *ttm_file_to_backup(struct file *file)
 {
-	return (void *)file;
+	/* Explicit union instead of a cast to make randstruct ignore us. */
+	union {
+		struct file *file;
+		struct ttm_backup *backup;
+	} u;
+	u.file = file;
+	return u.backup;
 }
 
 /*
-- 
2.34.1