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Fri, 2 May 2025 04:25:02 +0000 Date: Thu, 1 May 2025 21:26:25 -0700 From: Matthew Brost To: Al Viro CC: Kees Cook , Thomas =?iso-8859-1?Q?Hellstr=F6m?= , Christian Koenig , Somalapuram Amaranath , Huang Rui , Matthew Auld , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , , , , , Christian Brauner , "Jan Kara" Subject: Re: [PATCH v2] drm/ttm: Silence randstruct warning about casting struct file Message-ID: References: <20250502002437.it.851-kees@kernel.org> <20250502023447.GV2023217@ZenIV> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20250502023447.GV2023217@ZenIV> X-ClientProxiedBy: SJ0PR13CA0001.namprd13.prod.outlook.com (2603:10b6:a03:2c0::6) To PH7PR11MB6522.namprd11.prod.outlook.com (2603:10b6:510:212::12) Precedence: bulk X-Mailing-List: linux-hardening@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR11MB6522:EE_|IA4PR11MB9299:EE_ X-MS-Office365-Filtering-Correlation-Id: 2616f0ae-ff66-4d2e-ef3a-08dd89314ebd X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|1800799024|7416014|7053199007; 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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 > > > > I forgot the policy if suggest-by but will add: > > Reviewed-by: Matthew Brost > > > > 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 > > 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.