linux-kernel - [PATCH 1/5] rust: transmute: add `from_bytes

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Message-Id: <20250822-nova_firmware-v1-1-ff5633679460@nvidia.com>
Date: Fri, 22 Aug 2025 21:47:16 +0900
From: Alexandre Courbot <acourbot@...dia.com>
To: 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>, Andreas Hindborg <a.hindborg@...nel.org>, 
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>, 
 Simona Vetter <simona@...ll.ch>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
 nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org, 
 Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH 1/5] rust: transmute: add `from_bytes_copy` method to
 `FromBytes` trait

`FromBytes::from_bytes` comes with a few practical limitations:

- It requires the bytes slice to have the same alignment as the returned
  type, which might not be guaranteed in the case of a byte stream,
- It returns a reference, requiring the returned type to implement
  `Clone` if one wants to keep the value for longer than the lifetime of
  the slice.

To overcome these when needed, add a `from_bytes_copy` with a default
implementation in the trait. `from_bytes_copy` returns an owned value
that is populated using an unaligned read, removing the lifetime
constraint and making it usable even on non-aligned byte slices.

Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
---
 rust/kernel/transmute.rs | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index cb33417faa01fd2d87770aac1d2a1539a254cbd6..bea6adaa7e89eb50a03ea1caa0870a8e2e12c461 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -61,6 +61,23 @@ fn from_bytes_mut(bytes: &mut [u8]) -> Option<&mut Self>
             None
         }
     }
+
+    /// Creates an owned instance of `Self` by copying `bytes`.
+    ///
+    /// As the data is copied into a properly-aligned location, this method can be used even if
+    /// [`FromBytes::from_bytes`] would return `None` due to incompatible alignment.
+    fn from_bytes_copy(bytes: &[u8]) -> Option<Self>
+    where
+        Self: Sized,
+    {
+        if bytes.len() == size_of::<Self>() {
+            // SAFETY: `bytes` has the same size as `Self`, and per the invariants of `FromBytes`,
+            // any byte sequence is a valid value for `Self`.
+            Some(unsafe { core::ptr::read_unaligned(bytes.as_ptr().cast::<Self>()) })
+        } else {
+            None
+        }
+    }
 }
 
 macro_rules! impl_frombytes {

-- 
2.50.1