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

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Message-Id: <DBNDJGIUQC1L.3EYGJ1ROIGMBY@nvidia.com>
Date: Mon, 28 Jul 2025 12:39:31 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Benno Lossin" <lossin@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.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 9:39 PM JST, Benno Lossin 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?

Given where we are in the merge cycle, it seems like we have a couple of
months until that code gets merged anyway, so I don't see any reason to
not send a v4 with Alice's suggestion? The only drawback I see is that I
would have to reset the Reviewed-by tags.