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

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Message-Id: <DBNT4B4LANGR.29PBM3597ZQBR@kernel.org>
Date: Mon, 28 Jul 2025 17:52:01 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>
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 4:59 PM CEST, Boqun Feng wrote:
> 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`.

Oh yeah that's true.

---
Cheers,
Benno