Commit 09f90256 authored Aug 26, 2025 by Alexandre Courbot

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>

parent 72031905

rust/kernel/transmute.rs

+18 −0

Original line number	Diff line number	Diff line
		@@ -79,6 +79,24 @@ fn from_bytes_mut(bytes: &mut [u8]) -> Option<&mut Self>
		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 {