linux-kernel - Re: [PATCH v2 1/3] rust: transmute: Support transmuting slices of AsBytes/FromBytes types

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Message-Id: <0448F279-3759-423C-86B9-7A92137A28E7@collabora.com>
Date: Wed, 17 Dec 2025 13:51:46 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Matthew Maurer <mmaurer@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>,
 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>,
 rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] rust: transmute: Support transmuting slices of
 AsBytes/FromBytes types

Hi Matthew,

> On 15 Dec 2025, at 21:44, Matthew Maurer <mmaurer@...gle.com> wrote:
> 
> Currently, we support either transmuting a byte slice of the exact size
> of the target type or a single element from a prefix of a byte slice.
> 
> This adds support for transmuting from a byte slice to a slice of
> elements, assuming appropriate traits are implemented.
> 
> Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
> ---
> rust/kernel/transmute.rs | 72 ++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 72 insertions(+)
> 
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index be5dbf3829e25c92f85083cc0f4940f35bb7baec..5cc5f4fde4c31cea3c51ab078b4d68f3a0a2caa1 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -122,6 +122,78 @@ fn from_bytes_mut_prefix(bytes: &mut [u8]) -> Option<(&mut Self, &mut [u8])>
>         }
>     }
> 
> +    /// Converts a slice of bytes to a slice of `Self`.
> +    ///
> +    /// Succeeds if the reference is properly aligned, and the size of `bytes` is a multiple of that
> +    /// of `Self`.
> +    ///
> +    /// Otherwise, returns [`None`].
> +    fn from_bytes_to_slice(bytes: &[u8]) -> Option<&[Self]>
> +    where
> +        Self: Sized,
> +    {
> +        let size = size_of::<Self>();
> +        if size == 0 {
> +            return None;
> +        }
> +        if bytes.len() % size != 0 {
> +            return None;
> +        }
> +        let len = bytes.len() / size;
> +        let ptr = bytes.as_ptr().cast::<Self>();
> +
> +        #[allow(clippy::incompatible_msrv)]
> +        if ptr.is_aligned() {
> +            // SAFETY:
> +            // - `ptr` is valid for reads of `bytes.len()` bytes, which is
> +            //   `len * size_of::<Self>()`.
> +            // - `ptr` is aligned for `Self`.
> +            // - `ptr` points to `len` consecutive properly initialized values of type `Self`
> +            //   because `Self` implements `FromBytes` (any bit pattern is valid).
> +            // - The lifetime of the returned slice is bound to the input `bytes`.
> +            unsafe { Some(core::slice::from_raw_parts(ptr, len)) }
> +        } else {
> +            None
> +        }

Nit: I’d rewrite this as 

if !ptr.is_aligned() {
 return None;
}

unsafe { Some(core::slice::from_raw_parts(ptr, len)) }

This saves one indentation level.

> +    }
> +
> +    /// Converts a mutable slice of bytes to a mutable slice of `Self`.
> +    ///
> +    /// Succeeds if the reference is properly aligned, and the size of `bytes` is a multiple of that
> +    /// of `Self`.
> +    ///
> +    /// Otherwise, returns [`None`].
> +    fn from_bytes_to_mut_slice(bytes: &mut [u8]) -> Option<&mut [Self]>
> +    where
> +        Self: AsBytes + Sized,
> +    {
> +        let size = size_of::<Self>();
> +        if size == 0 {
> +            return None;
> +        }
> +        if bytes.len() % size != 0 {
> +            return None;
> +        }
> +        let len = bytes.len() / size;
> +        let ptr = bytes.as_mut_ptr().cast::<Self>();
> +
> +        #[allow(clippy::incompatible_msrv)]
> +        if ptr.is_aligned() {
> +            // SAFETY:
> +            // - `ptr` is valid for reads and writes of `bytes.len()` bytes, which is
> +            //   `len * size_of::<Self>()`.
> +            // - `ptr` is aligned for `Self`.
> +            // - `ptr` points to `len` consecutive properly initialized values of type `Self`
> +            //   because `Self` implements `FromBytes`.
> +            // - `AsBytes` guarantees that writing a new `Self` to the resulting slice can safely
> +            //   be reflected as bytes in the original slice.
> +            // - The lifetime of the returned slice is bound to the input `bytes`.
> +            unsafe { Some(core::slice::from_raw_parts_mut(ptr, len)) }
> +        } else {
> +            None
> +        }

Same here.

> +    }
> +
>     /// Creates an owned instance of `Self` by copying `bytes`.
>     ///
>     /// Unlike [`FromBytes::from_bytes`], which requires aligned input, this method can be used on
> 
> -- 
> 2.52.0.305.g3fc767764a-goog
> 
> 

Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>