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

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Message-ID: <20250502023447.GV2023217@ZenIV>
Date: Fri, 2 May 2025 03:34:47 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Matthew Brost <matthew.brost@...el.com>
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 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
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.