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.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250826-nova_firmware-v2-1-93566252fe3a@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index 5fc7d4ed76b4280fb58d6d19507efc2ce1f73ae3..cfc37d81adf2ed01271412bf3569c0a22d87fde5 100644 (file)
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -79,6 +79,24 @@ pub unsafe trait FromBytes {
             None
         }
     }
+
+    /// Creates an owned instance of `Self` by copying `bytes`.
+    ///
+    /// Unlike [`FromBytes::from_bytes`], which requires aligned input, this method can be used on
+    /// non-aligned data at the cost of a copy.
+    fn from_bytes_copy(bytes: &[u8]) -> Option<Self>
+    where
+        Self: Sized,
+    {
+        if bytes.len() == size_of::<Self>() {
+            // SAFETY: we just verified that `bytes` has the same size as `Self`, and per the
+            // invariants of `FromBytes`, any byte sequence of the correct length is a valid value
+            // for `Self`.
+            Some(unsafe { core::ptr::read_unaligned(bytes.as_ptr().cast::<Self>()) })
+        } else {
+            None
+        }
+    }
 }
 
 macro_rules! impl_frombytes {