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

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Message-Id: <DBMUEH5MYR2L.CXM12OIUH7TZ@kernel.org>
Date: Sun, 27 Jul 2025 14:39:39 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>, "Alexandre Courbot"
 <acourbot@...dia.com>
Cc: "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 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?

---
Cheers,
Benno