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

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Message-ID: <10506c57-1629-4ca2-9c3f-3e97e68caa7a@kernel.org>
Date: Mon, 28 Jul 2025 12:01:29 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Benno Lossin <lossin@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Abdiel Janulgue <abdiel.janulgue@...il.com>,
 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 7/28/25 5:39 AM, Alexandre Courbot wrote:
> 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.

I'd make it a series and add as_bytes_mut() as a separate patch.