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charset=UTF-8 Date: Fri, 09 Oct 2026 22:24:31 +0900 Message-Id: Cc: "Thorsten Blum" , "Miguel Ojeda" , "Boqun Feng" , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , "Benno Lossin" , "Andreas Hindborg" , "Alice Ryhl" , "Trevor Gross" , "Danilo Krummrich" , "Daniel Almeida" , "Tamir Duberstein" , =?utf-8?q?Onur_=C3=96zkan?= , "Greg Kroah-Hartman" , "Timur Tabi" , "Alistair Popple" , , Subject: Re: [PATCH] rust: uaccess: avoid unsafe unwrap_unchecked() in strcpy_into_buf() From: "Alexandre Courbot" To: "Gary Guo" References: <20261008061326.177841-2-blum@kernel.org> In-Reply-To: X-ClientProxiedBy: TY4P301CA0052.JPNP301.PROD.OUTLOOK.COM (2603:1096:405:36b::15) To MW4PR12MB6873.namprd12.prod.outlook.com (2603:10b6:303:20c::17) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MW4PR12MB6873:EE_|DM6PR12MB4042:EE_ X-MS-Office365-Filtering-Correlation-Id: f26bf428-2bf2-4a0e-92e4-08df2608abbc X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|23010399003|10070799003|1800799024|7416014|366016|6133799003|10067099003|18002099003|22082099003|4143699003|11063799006|56012099006; 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In this= case, we add a NUL-terminator >>> > // and return it. Unlike the `len < dst.len()` branch, d= on't modify `len` because it >>> > // already represents the length including the NUL-termi= nator. >>> > - // >>> > - // SAFETY: Due to the check at the beginning, the buffer= is not empty. >>> > - unsafe { *buf.last_mut().unwrap_unchecked() =3D 0 }; >>> > + *buf.last_mut().ok_or(EINVAL)? =3D 0; >>>=20 >>> I am not sure this gives us much - we are trading an unsafe statement >>> that is well-controlled (enforced by the first two lines of the method) >>> for a runtime check. I'd say this is working as intended here. >> >> I checked the generated code before and after the patch and it is >> identical since the compiler is able to remove the additional check. >> Therefore, this removes an unsafe block without adding runtime cost. >> >> It also avoids relying on the buf.is_empty() check to prevent undefined >> behavior. > > Adding an error returning path is worse for something that cannot happen = is > worse than invoking unsafe in my opinion. > > Why not just unwrap? > > *buf.last_mut().unwrap() =3D 0; I guess the author of the code decided to avoid `unwrap` for the same reason they avoided a runtime error: the first two lines of the method guarantee that the access is valid. Now I wish we could keep the enforcing statement closer to the unsafe block relying it, but I cannot find a better way to write that method. `unwrap` would just switch the `SAFETY` statement for a `PANIC` one. Honestly I think this code is fine as it is.