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

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Message-ID: <aBRJcXfBuK29mVP+@lstrano-desk.jf.intel.com>
Date: Thu, 1 May 2025 21:26:25 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: Al Viro <viro@...iv.linux.org.uk>
CC: Kees Cook <kees@...nel.org>, Thomas Hellström
	<thomas.hellstrom@...ux.intel.com>, Christian Koenig
	<christian.koenig@....com>, Somalapuram Amaranath
	<Amaranath.Somalapuram@....com>, Huang Rui <ray.huang@....com>, Matthew Auld
	<matthew.auld@...el.com>, Maarten Lankhorst
	<maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>, <dri-devel@...ts.freedesktop.org>,
	<linux-kernel@...r.kernel.org>, <linux-hardening@...r.kernel.org>,
	<linux-fsdevel@...r.kernel.org>, Christian Brauner <brauner@...nel.org>, "Jan
 Kara" <jack@...e.cz>
Subject: Re: [PATCH v2] drm/ttm: Silence randstruct warning about casting
 struct file

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@...el.com>
> > 
> > I forgot the policy if suggest-by but will add:
> > Reviewed-by: Matthew Brost <matthew.brost@...el.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@...iv.linux.org.uk>
> 
> Reason: struct file should *NOT* be embedded into other objects without

I;m fairly certain is just aliasing... but I do understand a file cannot
be embedded. Would comment help here indicating no other fields should
be added to ttm_backup without struct file be converted to pointer or
that just to risky?

Matt

> 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.