[PATCH v2] rust: transmute: add `as_bytes` method for `AsBytes` trait

[PATCH v2] 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.

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 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..d4036986a3d7fb97e5da3e121e9590ad23b784e9 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 = (self as *const 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) }
+    }
+}
 
 macro_rules! impl_asbytes {
     ($($({$($generics:tt)*})? $t:ty, )*) => {

---
base-commit: 14ae91a81ec8fa0bc23170d4aa16dd2a20d54105
change-id: 20250725-as_bytes-6cbc11f2e8c3

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

Re: [PATCH v2] rust: transmute: add `as_bytes` method for `AsBytes` trait

[Danilo Krummrich]

On 7/25/25 3:08 PM, Alexandre Courbot wrote:
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index 1c7d43771a37b90150de86699f114a2ffb84db91..d4036986a3d7fb97e5da3e121e9590ad23b784e9 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 = (self as *const Self).cast::<u8>();

I'd probably use from_ref() instead. With that,

	Reviewed-by: Danilo Krummrich <dakr@kernel.org>

> +        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) }
> +    }
> +}

Re: [PATCH v2] rust: transmute: add `as_bytes` method for `AsBytes` trait

[Alexandre Courbot]

On Sat Jul 26, 2025 at 12:07 AM JST, Danilo Krummrich wrote:
> On 7/25/25 3:08 PM, Alexandre Courbot wrote:
>> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
>> index 1c7d43771a37b90150de86699f114a2ffb84db91..d4036986a3d7fb97e5da3e121e9590ad23b784e9 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 = (self as *const Self).cast::<u8>();
>
> I'd probably use from_ref() instead. With that,

Oh yes I always forget about this one...

>
> 	Reviewed-by: Danilo Krummrich <dakr@kernel.org>

Thanks!