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

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Message-ID: <aIeQPrGSa2rwckiH@tardis-2.local>
Date: Mon, 28 Jul 2025 07:59:10 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <lossin@...nel.org>
Cc: 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>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Alice Ryhl <aliceryhl@...gle.com>, 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 v4 2/2] rust: transmute: add `as_bytes_mut` method to
 `AsBytes` trait

On Mon, Jul 28, 2025 at 04:53:31PM +0200, Benno Lossin wrote:
> 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

Because if `T` is `FromBytes` then we want `[T]` is `FromBytes` too?
Also it would be weird if `[u8]` itself doesn't implement `FromBytes`.

Regards,
Boqun

> `size_of_val(self)` above with `size_of::<Self>()` which I think makes
> it simpler to understand)
> 
[...]