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

[Kees Cook]

On Thu, May 01, 2025 at 01:28:52PM -0700, Matthew Brost wrote:
> 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);
>  }

Yup, that works too! I'll send this as a v2.

-- 
Kees Cook