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

[Matthew Brost]

On Thu, May 01, 2025 at 05:24:38PM -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;
>       |                ^~~~~~~~~~~~
> 
> Suggested-by: Matthew Brost <matthew.brost at intel.com>

I forgot the policy if suggest-by but will add:
Reviewed-by: Matthew Brost <matthew.brost at intel.com>

Thomas was out today I suspect he will look at this tomorrow when he is
back too.

Matt

> 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
>