linux-kernel - [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait

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Message-Id: <20250726-as_bytes-v3-1-eb7514faab28@nvidia.com>
Date: Sat, 26 Jul 2025 11:47:25 +0900
From: Alexandre Courbot <acourbot@...dia.com>
To: Abdiel Janulgue <abdiel.janulgue@...il.com>, 
 Danilo Krummrich <dakr@...nel.org>, 
 Daniel Almeida <daniel.almeida@...labora.com>, 
 Robin Murphy <robin.murphy@....com>, 
 Andreas Hindborg <a.hindborg@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
 Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
 Gary Guo <gary@...yguo.net>, 
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
 Benno Lossin <lossin@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
 Trevor Gross <tmgross@...ch.edu>
Cc: "Christian S. Lima" <christiansantoslima21@...il.com>, 
 rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes`
 trait

Every type that implements `AsBytes` should be able to provide its byte
representation. Introduce the `as_bytes` method that returns the
implementer as a stream of bytes, and provide a default implementation
that should be suitable for any type that satisfies `AsBytes`'s safety
requirements.

Reviewed-by: Danilo Krummrich <dakr@...nel.org>
Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
---
This is the sister patch of [1], providing an `as_bytes` method for
`AsBytes`.

It is going to be used in Nova, but should also be universally useful -
if anything, it felt a bit strange that `AsBytes` did not provide this
method so far.

[1] https://lore.kernel.org/rust-for-linux/20250624042802.105623-1-christiansantoslima21@gmail.com/
---
Changes in v3:
- Use `ptr::from_ref` instead of `as *const T`.
- Link to v2: https://lore.kernel.org/r/20250725-as_bytes-v2-1-c6584c211a6c@nvidia.com

Changes in v2:
- Use `size_of_val` to provide a default implementation for both `Sized`
  and non-`Sized` types, and remove `AsBytesSized`. (thanks Alice!)
- Link to v1: https://lore.kernel.org/r/20250725-as_bytes-v1-1-6f06a3744f69@nvidia.com
---
 rust/kernel/transmute.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index 1c7d43771a37b90150de86699f114a2ffb84db91..69c46c19a89191d8a2abc5801564cacda232218c 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
 ///
 /// Values of this type may not contain any uninitialized bytes. This type must not have interior
 /// mutability.
-pub unsafe trait AsBytes {}
+pub unsafe trait AsBytes {
+    /// Returns `self` as a slice of bytes.
+    fn as_bytes(&self) -> &[u8] {
+        let data = core::ptr::from_ref(self).cast::<u8>();
+        let len = size_of_val(self);
+
+        // SAFETY: `data` is non-null and valid for `len * sizeof::<u8>()` bytes.
+        unsafe { core::slice::from_raw_parts(data, len) }
+    }
+}

 macro_rules! impl_asbytes {
     ($($({$($generics:tt)*})? $t:ty, )*) => {

---
base-commit: 14ae91a81ec8fa0bc23170d4aa16dd2a20d54105
change-id: 20250725-as_bytes-6cbc11f2e8c3

Best regards,
-- 
Alexandre Courbot <acourbot@...dia.com>