Re: [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait

[Alice Ryhl <aliceryhl@google.com>]

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@nvidia.com> wrote:
> >
> > Every type that implements `AsBytes` should be able to provide its byte
> > representation. Introduce the `as_bytes` method that returns the
> > implementer as a stream of bytes, and provide a default implementation
> > that should be suitable for any type that satisfies `AsBytes`'s safety
> > requirements.
> >
> > Reviewed-by: Danilo Krummrich <dakr@kernel.org>
> > Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> > ---
> > This is the sister patch of [1], providing an `as_bytes` method for
> > `AsBytes`.
> >
> > It is going to be used in Nova, but should also be universally useful -
> > if anything, it felt a bit strange that `AsBytes` did not provide this
> > method so far.
> >
> > [1] https://lore.kernel.org/rust-for-linux/20250624042802.105623-1-christiansantoslima21@gmail.com/
> > ---
> > Changes in v3:
> > - Use `ptr::from_ref` instead of `as *const T`.
> > - Link to v2: https://lore.kernel.org/r/20250725-as_bytes-v2-1-c6584c211a6c@nvidia.com
> >
> > Changes in v2:
> > - Use `size_of_val` to provide a default implementation for both `Sized`
> >   and non-`Sized` types, and remove `AsBytesSized`. (thanks Alice!)
> > - Link to v1: https://lore.kernel.org/r/20250725-as_bytes-v1-1-6f06a3744f69@nvidia.com
> > ---
> >  rust/kernel/transmute.rs | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > 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.

Alice