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

[Matthew Brost]

On Thu, May 01, 2025 at 12:59:03PM -0700, Kees Cook wrote:
> 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 at 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>

What if we did something like this instead:

diff --git a/drivers/gpu/drm/ttm/ttm_backup.c b/drivers/gpu/drm/ttm/ttm_backup.c
index 93c007f18855..fe936a87c959 100644
--- a/drivers/gpu/drm/ttm/ttm_backup.c
+++ b/drivers/gpu/drm/ttm/ttm_backup.c
@@ -7,18 +7,22 @@
 #include <linux/page-flags.h>
 #include <linux/swap.h>

+struct ttm_backup {
+       struct file file;
+};
+
 /*
  * Casting from randomized struct file * to struct ttm_backup * is fine since
  * struct ttm_backup is never defined nor dereferenced.
  */
 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);
 }

Matt

> 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 | 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
>