linux-kernel - Re: [PATCH v3] rust: transmute: add `as

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Message-ID: <CAH5fLghNDDo0HKupPXe8G6z2TP4TJE881Bd76k0LDjSm75KcEQ@mail.gmail.com>
Date: Sun, 27 Jul 2025 08:52:00 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Abdiel Janulgue <abdiel.janulgue@...il.com>, Danilo Krummrich <dakr@...nel.org>, 
	Daniel Almeida <daniel.almeida@...labora.com>, Robin Murphy <robin.murphy@....com>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	"Christian S. Lima" <christiansantoslima21@...il.com>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait

On Sat, Jul 26, 2025 at 4:47 AM Alexandre Courbot <acourbot@...dia.com> wrote:
>
> Every type that implements `AsBytes` should be able to provide its byte
> representation. Introduce the `as_bytes` method that returns the
> implementer as a stream of bytes, and provide a default implementation
> that should be suitable for any type that satisfies `AsBytes`'s safety
> requirements.
>
> Reviewed-by: Danilo Krummrich <dakr@...nel.org>
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
> This is the sister patch of [1], providing an `as_bytes` method for
> `AsBytes`.
>
> It is going to be used in Nova, but should also be universally useful -
> if anything, it felt a bit strange that `AsBytes` did not provide this
> method so far.
>
> [1] https://lore.kernel.org/rust-for-linux/20250624042802.105623-1-christiansantoslima21@gmail.com/
> ---
> Changes in v3:
> - Use `ptr::from_ref` instead of `as *const T`.
> - Link to v2: https://lore.kernel.org/r/20250725-as_bytes-v2-1-c6584c211a6c@nvidia.com
>
> Changes in v2:
> - Use `size_of_val` to provide a default implementation for both `Sized`
>   and non-`Sized` types, and remove `AsBytesSized`. (thanks Alice!)
> - Link to v1: https://lore.kernel.org/r/20250725-as_bytes-v1-1-6f06a3744f69@nvidia.com
> ---
>  rust/kernel/transmute.rs | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index 1c7d43771a37b90150de86699f114a2ffb84db91..69c46c19a89191d8a2abc5801564cacda232218c 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
>  ///
>  /// Values of this type may not contain any uninitialized bytes. This type must not have interior
>  /// mutability.
> -pub unsafe trait AsBytes {}
> +pub unsafe trait AsBytes {
> +    /// Returns `self` as a slice of bytes.
> +    fn as_bytes(&self) -> &[u8] {
> +        let data = core::ptr::from_ref(self).cast::<u8>();
> +        let len = size_of_val(self);
> +
> +        // SAFETY: `data` is non-null and valid for `len * sizeof::<u8>()` bytes.
> +        unsafe { core::slice::from_raw_parts(data, len) }
> +    }
> +}

Let's also have an as_bytes_mut() method. I would require the type to
also implement FromBytes as it lets you replace the value with another
set of bytes.

Alice