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

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Message-Id: <DBNRVIEXN4ZR.1VG0GDN6BDXE8@kernel.org>
Date: Mon, 28 Jul 2025 16:53:31 +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 2/2] rust: transmute: add `as_bytes_mut` method to
 `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 d541c9960904d8c7f755351f22d06e4e8dbd546a..000fda2a78f8e6e8378bbe93cddd8a5008db20cc 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -56,6 +56,19 @@ fn as_bytes(&self) -> &[u8] {
>          // SAFETY: `data` is non-null and valid for reads over `len * sizeof::<u8>()` bytes.
>          unsafe { core::slice::from_raw_parts(data, len) }
>      }
> +
> +    /// Returns `self` as a mutable slice of bytes.
> +    fn as_bytes_mut(&mut self) -> &mut [u8]
> +    where
> +        Self: FromBytes,
> +    {
> +        let data = core::ptr::from_mut(self).cast::<u8>();

Also add a `CAST` comment above this line:

    // CAST: `Self` implements both `AsBytes` and `FromBytes` thus making `Self` bi-directionally
    // transmutable to `[u8; size_of_val(self)]`.

On that note, `FromBytes` probably entails `Sized`, or does someone have
a use-case in mind? (because then we could replace the
`size_of_val(self)` above with `size_of::<Self>()` which I think makes
it simpler to understand)

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

s/over/of/

> +        // bytes. Since `Self` implements `FromBytes` it can be represented by any value.

Don't need the second sentence due to the `CAST` comment above.

---
Cheers,
Benno

> +        unsafe { core::slice::from_raw_parts_mut(data, len) }
> +    }
>  }
>  
>  macro_rules! impl_asbytes {