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

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Message-ID: <CAH5fLggpqHAXekdLTxLu4XqvDDae6ctOu4b1FHQsUF+WVW30MQ@mail.gmail.com>
Date: Sun, 27 Jul 2025 17:21:49 +0200
From: Alice Ryhl <aliceryhl@...gle.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>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.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 v3] rust: transmute: add `as_bytes` method for `AsBytes` trait

On Sun, Jul 27, 2025 at 2:39 PM Benno Lossin <lossin@...nel.org> wrote:
>
> On Sun Jul 27, 2025 at 12:23 PM CEST, Alice Ryhl wrote:
> > On Sun, Jul 27, 2025 at 08:52:00AM +0200, Alice Ryhl wrote:
> >> On Sat, Jul 26, 2025 at 4:47 AM Alexandre Courbot <acourbot@...dia.com> wrote:
> >> > 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.
> >
> > s/I would/It would/
> >
> > FromBytes is needed only for as_bytes_mut(), not for the existing
> > method.
>
> I agree with your suggestion, but it can be an independent patch and
> doesn't need to go in via this one, right?

Sure, that's fine.

Alice