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.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/transmute.rs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

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>();
+        let len = size_of_val(self);
+
+        // SAFETY: `data` is non-null and valid for read and writes over `len * sizeof::<u8>()`
+        // bytes. Since `Self` implements `FromBytes` it can be represented by any value.
+        unsafe { core::slice::from_raw_parts_mut(data, len) }
+    }
 }
 
 macro_rules! impl_asbytes {

-- 
2.50.1

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
`size_of_val(self)` above with `size_of::<Self>()` which I think makes
it simpler to understand)

> +        let len = size_of_val(self);
> +
> +        // SAFETY: `data` is non-null and valid for read and writes over `len * sizeof::<u8>()`

s/over/of/

> +        // bytes. Since `Self` implements `FromBytes` it can be represented by any value.

Don't need the second sentence due to the `CAST` comment above.

---
Cheers,
Benno

> +        unsafe { core::slice::from_raw_parts_mut(data, len) }
> +    }
>  }
>  
>  macro_rules! impl_asbytes {

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`.

Regards,
Boqun

> `size_of_val(self)` above with `size_of::<Self>()` which I think makes
> it simpler to understand)
> 
[...]

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

On 7/28/25 2:47 PM, 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.
> 
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

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