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Wed, 31 Dec 2025 04:22:40 -0800 (PST) Date: Wed, 31 Dec 2025 12:22:27 +0000 In-Reply-To: <20251231-rwonce-v1-0-702a10b85278@google.com> Mime-Version: 1.0 References: <20251231-rwonce-v1-0-702a10b85278@google.com> X-Developer-Key: i=aliceryhl@google.com; a=openpgp; fpr=49F6C1FAA74960F43A5B86A1EE7A392FDE96209F X-Developer-Signature: v=1; a=openpgp-sha256; l=2011; i=aliceryhl@google.com; h=from:subject:message-id; bh=f+ElsItk3DSISt1bsxPjTx60qDfLwGd+MdLyPyzjyQI=; b=owEBbQKS/ZANAwAKAQRYvu5YxjlGAcsmYgBpVRWL4oDnxIVIQOubpJwcZRLcyzDfMlz7wIFAM FONlx/oCYKJAjMEAAEKAB0WIQSDkqKUTWQHCvFIvbIEWL7uWMY5RgUCaVUViwAKCRAEWL7uWMY5 RmH6D/46KKFKWvICb3h6YXEsXSSffucVzMDtCNxV/nON8HqUTVC9QRwy+QA5BR0hrfvViIl1dKx QFW5mE6zsjqHKs9m3qdyng4MUI8hLxBMk3ZddZ5++5KMoowDj7M9YW79F70Q5JL8cPNiGTxEn8T VOpmnKD0W0uqGzPVPn23nyrMXrCuGOSOlv1Gd7VlP14rVqitTFZNDbO/h3Jxy6lNTukLQ+koVaY IfTia3CyF5IhIS2g511uFdNOICKH6TDOYX8thm7ndly6HHx+6ARDy7WILR47BIDGvXOQdbt1kLi V6/5WiEP3hVfSFSXEsvYYHcIWFmsvvPJOUUlHKqLTHmMiG5YzhOcIMdWR2xCLaCeAb147Thitc8 +Ze9StLM0Z7stUstGbYjIUZFQ+V4Rndjy2ocRJH5Ed52rnZN9G1Iy0NTMogqlQ8hkUiNoISzYer 7JORef2rWkONYm1I2EyDd+XsVaBfYDD7SBf7gj1LjEzjn/cJZqNNetKASeZnYDGe8oD8X/hfN41 /K11v2L2aZsQ/aZx/TvPkBaD5UlBxpRXYfQLDLE0Xgz2TNev4vNJ6gzjLvwaHsk+5wi29msQ69Q ZqqGYocGcZHMGcf7aalYz96ESMDQx6plGafNctmZtxnbAR6RDeb4Gt0M3Bf/FonllPV4dkLOmiO +fYoPkvnNt3oL+w== X-Mailer: b4 0.14.2 Message-ID: <20251231-rwonce-v1-3-702a10b85278@google.com> Subject: [PATCH 3/5] rust: sync: support using bool with READ_ONCE From: Alice Ryhl To: Boqun Feng , Will Deacon , Peter Zijlstra Cc: Richard Henderson , Matt Turner , Magnus Lindholm , Catalin Marinas , Miguel Ojeda , Gary Guo , "=?utf-8?q?Bj=C3=B6rn_Roy_Baron?=" , Benno Lossin , Andreas Hindborg , Trevor Gross , Danilo Krummrich , Mark Rutland , FUJITA Tomonori , Frederic Weisbecker , Lyude Paul , Thomas Gleixner , Anna-Maria Behnsen , John Stultz , Stephen Boyd , Alexander Viro , Christian Brauner , Jan Kara , linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org, Alice Ryhl Content-Type: text/plain; charset="utf-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251231_042242_961945_4143D757 X-CRM114-Status: GOOD ( 12.56 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Normally it is undefined behavior for a bool to take any value other than 0 or 1. However, in the case of READ_ONCE(some_bool) is used, this UB seems dangerous and unnecessary. I can easily imagine some Rust code that looks like this: if READ_ONCE(&raw const (*my_c_struct).my_bool_field) { ... } And by making an analogy to what the equivalent C code is, anyone writing this probably just meant to treat any non-zero value as true. For WRITE_ONCE no special logic is required. Signed-off-by: Alice Ryhl --- rust/kernel/sync/rwonce.rs | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/rust/kernel/sync/rwonce.rs b/rust/kernel/sync/rwonce.rs index a1660e43c9ef94011812d1816713cf031a73de1d..73477f53131926996614df573b2d50fff98e624f 100644 --- a/rust/kernel/sync/rwonce.rs +++ b/rust/kernel/sync/rwonce.rs @@ -163,6 +163,7 @@ unsafe fn write_once(ptr: *mut Self, val: Self) { // sizes, so picking the wrong helper should lead to a build error. impl_rw_once_type! { + bool, read_once_bool, write_once_1; u8, read_once_1, write_once_1; i8, read_once_1, write_once_1; u16, read_once_2, write_once_2; @@ -186,3 +187,21 @@ unsafe fn write_once(ptr: *mut Self, val: Self) { usize, read_once_8, write_once_8; isize, read_once_8, write_once_8; } + +/// Read an integer as a boolean once. +/// +/// Returns `true` if the value behind the pointer is non-zero. Otherwise returns `false`. +/// +/// # Safety +/// +/// It must be safe to `READ_ONCE` the `ptr` with type `u8`. +#[inline(always)] +#[track_caller] +unsafe fn read_once_bool(ptr: *const bool) -> bool { + // Implement `read_once_bool` in terms of `read_once_1`. The arch-specific logic is inside + // of `read_once_1`. + // + // SAFETY: It is safe to `READ_ONCE` the `ptr` with type `u8`. + let byte = unsafe { read_once_1(ptr.cast::()) }; + byte != 0u8 +} -- 2.52.0.351.gbe84eed79e-goog