Re: [PATCH v6] rust: transmute: Add methods for FromBytes trait

["Alexandre Courbot" <acourbot@nvidia.com>]

On Mon Mar 31, 2025 at 8:40 AM JST, Christian S. Lima wrote:
> Methods receive a slice and perform size check to add a valid way to make
> conversion safe. An Option is used, in error case just return `None`.
>
> The conversion between slices `[T]` is separated from others, because I
> couldn't implement it in the same way as the other conversions.
>
> Link: https://github.com/Rust-for-Linux/linux/issues/1119
> Signed-off-by: Christian S. Lima <christiansantoslima21@gmail.com>
> ---
> Changes in v2:
> - Rollback the implementation for the macro in the repository and implement
>   methods in trait
> - Link to v2: https://lore.kernel.org/rust-for-linux/20241012193657.290cc79c@eugeo/T/#t
>
> Changes in v3:
> - Fix grammar errors
> - Remove repeated tests
> - Fix alignment errors
> - Fix tests not building
> - Link to v3: https://lore.kernel.org/rust-for-linux/20241109055442.85190-1-christiansantoslima21@gmail.com/
>
> Changes in v4:
> - Removed core::simd::ToBytes
> - Changed trait and methods to safe Add
> - Result<&Self, Error> in order to make safe methods
> - Link to v4: https://lore.kernel.org/rust-for-linux/20250314034910.134463-1-christiansantoslima21@gmail.com/
>
> Changes in v5:
> - Changed from Result to Option
> - Removed commentaries
> - Returned trait impl to unsafe
> - Link to v5: https://lore.kernel.org/rust-for-linux/20250320014041.101470-1-christiansantoslima21@gmail.com/
>
> Changes in v6:
> - Add endianess check to doc test and use match to check
> success case
> - Reformulated safety comments
> ---
>  rust/kernel/transmute.rs | 89 +++++++++++++++++++++++++++++++++++++---
>  1 file changed, 83 insertions(+), 6 deletions(-)
>
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index 1c7d43771a37..16dfa5c7d467 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -9,29 +9,106 @@
>  ///
>  /// It's okay for the type to have padding, as initializing those bytes has no effect.
>  ///
> +/// # Example
> +/// ```
> +/// let foo = &[1, 2, 3, 4];
> +///
> +/// let result = u32::from_bytes(foo);
> +///
> +/// #[cfg(target_endian = "little")]
> +/// match result {
> +///     Some(x) => assert_eq!(*x, 0x4030201),
> +///     None => unreachable!()
> +/// }
> +///
> +/// #[cfg(target_endian = "big")]
> +/// match result {
> +///     Some(x) => assert_eq!(*x, 0x1020304),
> +///     None => unreachable!()
> +/// }
> +/// ```
> +///
>  /// # Safety
>  ///
>  /// All bit-patterns must be valid for this type. This type must not have interior mutability.
> -pub unsafe trait FromBytes {}
> +pub unsafe trait FromBytes {
> +    /// Converts a slice of bytes to a reference to `Self` when possible.
> +    fn from_bytes(bytes: &[u8]) -> Option<&Self>;
> +
> +    /// Converts a mutable slice of bytes to a reference to `Self` when possible.
> +    fn from_bytes_mut(bytes: &mut [u8]) -> Option<&mut Self>
> +    where
> +        Self: AsBytes;
> +}
>  
>  macro_rules! impl_frombytes {
>      ($($({$($generics:tt)*})? $t:ty, )*) => {
>          // SAFETY: Safety comments written in the macro invocation.
> -        $(unsafe impl$($($generics)*)? FromBytes for $t {})*
> +        $(unsafe impl$($($generics)*)? FromBytes for $t {
> +            fn from_bytes(bytes: &[u8]) -> Option<&$t> {
> +                if bytes.len() == core::mem::size_of::<$t>() {
> +                    let slice_ptr = bytes.as_ptr().cast::<$t>();
> +                    unsafe { Some(&*slice_ptr) }
> +                } else {
> +                    None
> +                }
> +            }

This is probably a naive question, but is there a reason why this cannot
be the default implementation of `from_bytes`? IIUC other implementors
would have to come with their own implementation, which adds an extra
burden.