linux-kernel - Re: [PATCH v4 1/2] rust: transmute: add `as

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Message-Id: <DBNRDKQBN1TK.6Y8U7XN2G15H@kernel.org>
Date: Mon, 28 Jul 2025 16:30:05 +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 v4 1/2] rust: transmute: add `as_bytes` method for
 `AsBytes` trait

On Mon Jul 28, 2025 at 2:47 PM CEST, Alexandre Courbot wrote:
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index 1c7d43771a37b90150de86699f114a2ffb84db91..d541c9960904d8c7f755351f22d06e4e8dbd546a 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's add a `CAST` comment above this that says:

    // CAST: `Self` implements `AsBytes` thus all bytes of `self` are initialized.

> +        let len = size_of_val(self);
> +
> +        // SAFETY: `data` is non-null and valid for reads over `len * sizeof::<u8>()` bytes.

s/over/of/

---
Cheers,
Benno

> +        unsafe { core::slice::from_raw_parts(data, len) }
> +    }
> +}
>  
>  macro_rules! impl_asbytes {
>      ($($({$($generics:tt)*})? $t:ty, )*) => {