On 8/25/25 9:07 PM, Alexandre Courbot wrote:
> `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 <alicer...@google.com>
> Signed-off-by: Alexandre Courbot <acour...@nvidia.com>
> ---
> rust/kernel/transmute.rs | 17 +++++++++++++++++
> 1 file changed, 17 insertions(+)
>
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index
> 494bb3b1d059337520efef694fc8952972d44fbf..721dd8254dcedd71ed7c1fc0ee9292950c16c89e
> 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -78,6 +78,23 @@ 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.
Some very minor suggestions:
This wording less precise than it could be: "as the data is copied" can mean
either "while it is being copied", or "because it is copied". Also, there
should not be a hyphen in "properly aligned".
I'd suggest something like this instead:
/// Unlike [`FromBytes::from_bytes`], which requires aligned input, this
/// method can be used on non-aligned data.
> + 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`.
More wording suggestions. How about this:
// 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 {
>
I'm unable to find anything wrong with the code itself, and the above are just
tiny nits, so whether or not you apply either or both of the above suggestions,
please feel free to add:
Reviewed-by: John Hubbard <jhubb...@nvidia.com>
thanks,
--
John Hubbard
