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

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Message-Id: <DBLUVHKRGVCO.19XQMAX6S1M7W@kernel.org>
Date: Sat, 26 Jul 2025 10:49:12 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alexandre Courbot" <acourbot@...dia.com>, "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>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>
Cc: "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 CEST, Alexandre Courbot 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>

Reviewed-by: Benno Lossin <lossin@...nel.org>

> ---
> 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) }
> +    }
> +}
>  
>  macro_rules! impl_asbytes {
>      ($($({$($generics:tt)*})? $t:ty, )*) => {
>
> ---
> base-commit: 14ae91a81ec8fa0bc23170d4aa16dd2a20d54105

Normally I'd expect this to be `rust-next` or an `-rc` tag... I did find
this in one of the trees that I fetch, so no worries, but maybe in the
future use one of those? Thanks!

---
Cheers,
Benno

> change-id: 20250725-as_bytes-6cbc11f2e8c3
>
> Best regards,