[PATCH v5 0/2] rust: transmute: add `as_bytes(_mut)` methods for `AsBytes` trait

[PATCH v5 0/2] rust: transmute: add `as_bytes(_mut)` methods for `AsBytes` trait

[Alexandre Courbot]

This is the sister patch of [1], providing an `as_bytes` method for
`AsBytes`, and an `as_bytes_mut` accessor for types also implementing
`FromBytes`.

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
so far.

[1] https://lore.kernel.org/rust-for-linux/20250624042802.105623-1-christiansantoslima21@gmail.com/

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
Changes in v5:
- Add `CAST:` comments above cast operations. (thanks Benno!)
- Link to v4: https://lore.kernel.org/r/20250728-as_bytes-v4-0-b9156af37e33@nvidia.com

Changes in v4:
- Add the `as_bytes_mut` method for types also implementing `FromBytes`.
  (thanks Alice!)
- Link to v3: https://lore.kernel.org/r/20250726-as_bytes-v3-1-eb7514faab28@nvidia.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

---
Alexandre Courbot (2):
      rust: transmute: add `as_bytes` method for `AsBytes` trait
      rust: transmute: add `as_bytes_mut` method to `AsBytes` trait

 rust/kernel/transmute.rs | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)
---
base-commit: 14ae91a81ec8fa0bc23170d4aa16dd2a20d54105
change-id: 20250725-as_bytes-6cbc11f2e8c3

Best regards,
-- 
Alexandre Courbot <acourbot@nvidia.com>

[PATCH v5 1/2] rust: transmute: add `as_bytes` method for `AsBytes` trait

[Alexandre Courbot]

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>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/transmute.rs | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index 1c7d43771a37b90150de86699f114a2ffb84db91..08e6f0ff92a563e90895efa31f31f887aa05a0ae 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -47,7 +47,17 @@ 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] {
+        // CAST: `Self` implements `AsBytes` thus all bytes of `self` are initialized.
+        let data = core::ptr::from_ref(self).cast::<u8>();
+        let len = size_of_val(self);
+
+        // SAFETY: `data` is non-null and valid for reads of `len * sizeof::<u8>()` bytes.
+        unsafe { core::slice::from_raw_parts(data, len) }
+    }
+}
 
 macro_rules! impl_asbytes {
     ($($({$($generics:tt)*})? $t:ty, )*) => {

-- 
2.50.1

[PATCH v5 2/2] rust: transmute: add `as_bytes_mut` method to `AsBytes` trait

[Alexandre Courbot]

Types that implement both `AsBytes` and `FromBytes` can be safely
modified as a slice of bytes. Add a `as_bytes_mut` method for that
purpose.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/transmute.rs | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index 08e6f0ff92a563e90895efa31f31f887aa05a0ae..5b2a53aa5df041e56be0c23993f684aa803551ae 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -57,6 +57,21 @@ fn as_bytes(&self) -> &[u8] {
         // SAFETY: `data` is non-null and valid for reads of `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,
+    {
+        // CAST: `Self` implements both `AsBytes` and `FromBytes` thus making `Self`
+        // bi-directionally transmutable to `[u8; size_of_val(self)]`.
+        let data = core::ptr::from_mut(self).cast::<u8>();
+        let len = size_of_val(self);
+
+        // SAFETY: `data` is non-null and valid for read and writes of `len * sizeof::<u8>()`
+        // bytes.
+        unsafe { core::slice::from_raw_parts_mut(data, len) }
+    }
 }
 
 macro_rules! impl_asbytes {

-- 
2.50.1

Re: [PATCH v5 2/2] rust: transmute: add `as_bytes_mut` method to `AsBytes` trait

[Benno Lossin]

On Fri Aug 1, 2025 at 3:24 PM CEST, Alexandre Courbot wrote:
> Types that implement both `AsBytes` and `FromBytes` can be safely
> modified as a slice of bytes. Add a `as_bytes_mut` method for that
> purpose.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Danilo Krummrich <dakr@kernel.org>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

Reviewed-by: Benno Lossin <lossin@kernel.org>

---
Cheers,
Benno

> ---
>  rust/kernel/transmute.rs | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)

Re: [PATCH v5 0/2] rust: transmute: add `as_bytes(_mut)` methods for `AsBytes` trait

[Danilo Krummrich]

On 8/1/25 3:24 PM, Alexandre Courbot wrote:
> This is the sister patch of [1], providing an `as_bytes` method for
> `AsBytes`, and an `as_bytes_mut` accessor for types also implementing
> `FromBytes`.
> 
> 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
> so far.

Miguel, any concerns taking this through the nova tree?

Re: [PATCH v5 0/2] rust: transmute: add `as_bytes(_mut)` methods for `AsBytes` trait

[Miguel Ojeda]

On Tue, Aug 19, 2025 at 11:49 AM Danilo Krummrich <dakr@kernel.org> wrote:
>
> Miguel, any concerns taking this through the nova tree?

If it is needed for something this same cycle in Nova, then please go ahead:

Acked-by: Miguel Ojeda <ojeda@kernel.org>

Thanks!

Cheers,
Miguel

Re: [PATCH v5 0/2] rust: transmute: add `as_bytes(_mut)` methods for `AsBytes` trait

[Alexandre Courbot]

On Thu Aug 21, 2025 at 11:38 PM JST, Miguel Ojeda wrote:
> On Tue, Aug 19, 2025 at 11:49 AM Danilo Krummrich <dakr@kernel.org> wrote:
>>
>> Miguel, any concerns taking this through the nova tree?
>
> If it is needed for something this same cycle in Nova, then please go ahead:
>
> Acked-by: Miguel Ojeda <ojeda@kernel.org>

Thanks Miguel! Yes, we are planning use it this cycle in Nova.

I'll push this into nova-next later today.