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

[Kees Cook <kees@kernel.org>]

On Fri, May 02, 2025 at 03:34:47AM +0100, Al Viro wrote:
> On Thu, May 01, 2025 at 07:13:12PM -0700, Matthew Brost wrote:
> > 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@intel.com>
> > 
> > I forgot the policy if suggest-by but will add:
> > Reviewed-by: Matthew Brost <matthew.brost@intel.com>
> > 
> > Thomas was out today I suspect he will look at this tomorrow when he is
> > back too.
> 
> [fsdevel and the rest of VFS maintainers Cc'd]
> 
> NAKed-by: Al Viro <viro@zeniv.linux.org.uk>
> 
> Reason: struct file should *NOT* be embedded into other objects without
> the core VFS being very explicitly aware of those.  The only such case
> is outright local to fs/file_table.c, and breeding more of those is
> a really bad idea.
> 
> Don't do that.  Same goes for struct {dentry, super_block, mount}
> in case anyone gets similar ideas.

Well, in that case, the NAK should be against commit e7b5d23e5d47
("drm/ttm: Provide a shmem backup implementation"), but that looks to
have had 15 revisions already...

But I will just back away slowly now. I was just fixing a warning
introduced by it. :)

-- 
Kees Cook