linux-kernel - [PATCH v5 2/2] rust: transmute: add `as_bytes

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Message-Id: <20250801-as_bytes-v5-2-975f87d5dc85@nvidia.com>
Date: Fri, 01 Aug 2025 22:24:10 +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 v5 2/2] rust: transmute: add `as_bytes_mut` method to
 `AsBytes` trait

Types that implement both `AsBytes` and `FromBytes` can be safely
modified as a slice of bytes. Add a `as_bytes_mut` method for that
purpose.

Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
Reviewed-by: Danilo Krummrich <dakr@...nel.org>
Reviewed-by: Gary Guo <gary@...yguo.net>
Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
---
 rust/kernel/transmute.rs | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index 08e6f0ff92a563e90895efa31f31f887aa05a0ae..5b2a53aa5df041e56be0c23993f684aa803551ae 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -57,6 +57,21 @@ fn as_bytes(&self) -> &[u8] {
         // SAFETY: `data` is non-null and valid for reads of `len * sizeof::<u8>()` bytes.
         unsafe { core::slice::from_raw_parts(data, len) }
     }
+
+    /// Returns `self` as a mutable slice of bytes.
+    fn as_bytes_mut(&mut self) -> &mut [u8]
+    where
+        Self: FromBytes,
+    {
+        // CAST: `Self` implements both `AsBytes` and `FromBytes` thus making `Self`
+        // bi-directionally transmutable to `[u8; size_of_val(self)]`.
+        let data = core::ptr::from_mut(self).cast::<u8>();
+        let len = size_of_val(self);
+
+        // SAFETY: `data` is non-null and valid for read and writes of `len * sizeof::<u8>()`
+        // bytes.
+        unsafe { core::slice::from_raw_parts_mut(data, len) }
+    }
 }

 macro_rules! impl_asbytes {

-- 
2.50.1