Nouveau - Aug 2025 - [PATCH v2 1/8] rust: transmute: add `from_bytes_copy` method to `FromBytes` trait

`FromBytes::from_bytes` comes with a few practical limitations:

- It requires the bytes slice to have the same alignment as the returned
  type, which might not be guaranteed in the case of a byte stream,
- It returns a reference, requiring the returned type to implement
  `Clone` if one wants to keep the value for longer than the lifetime of
  the slice.

To overcome these when needed, add a `from_bytes_copy` with a default
implementation in the trait. `from_bytes_copy` returns an owned value
that is populated using an unaligned read, removing the lifetime
constraint and making it usable even on non-aligned byte slices.

Reviewed-by: Alice Ryhl <aliceryhl at google.com>
Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
---
 rust/kernel/transmute.rs | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index
494bb3b1d059337520efef694fc8952972d44fbf..721dd8254dcedd71ed7c1fc0ee9292950c16c89e
100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -78,6 +78,23 @@ fn from_bytes_mut(bytes: &mut [u8]) ->
Option<&mut Self>
             None
         }
     }
+
+    /// Creates an owned instance of `Self` by copying `bytes`.
+    ///
+    /// As the data is copied into a properly-aligned location, this method can
be used even if
+    /// [`FromBytes::from_bytes`] would return `None` due to incompatible
alignment.
+    fn from_bytes_copy(bytes: &[u8]) -> Option<Self>
+    where
+        Self: Sized,
+    {
+        if bytes.len() == size_of::<Self>() {
+            // SAFETY: `bytes` has the same size as `Self`, and per the
invariants of `FromBytes`,
+            // any byte sequence is a valid value for `Self`.
+            Some(unsafe {
core::ptr::read_unaligned(bytes.as_ptr().cast::<Self>()) })
+        } else {
+            None
+        }
+    }
 }
 
 macro_rules! impl_frombytes {

-- 
2.50.1

On Tue Aug 26, 2025 at 6:07 AM CEST, Alexandre Courbot
wrote:
> `FromBytes::from_bytes` comes with a few practical limitations:
>
> - It requires the bytes slice to have the same alignment as the returned
>   type, which might not be guaranteed in the case of a byte stream,
> - It returns a reference, requiring the returned type to implement
>   `Clone` if one wants to keep the value for longer than the lifetime of
>   the slice.
I think that any `Sized` type that can implement `FromBytes` also can
implement `Copy`, so I don't feel like the last point is a strong one.
But the unaligned byte stream is motivation enough :)
> To overcome these when needed, add a `from_bytes_copy` with a default
> implementation in the trait. `from_bytes_copy` returns an owned value
> that is populated using an unaligned read, removing the lifetime
> constraint and making it usable even on non-aligned byte slices.
>
> Reviewed-by: Alice Ryhl <aliceryhl at google.com>
> Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
Reviewed-by: Benno Lossin <lossin at kernel.org>

---
Cheers,
Benno
> ---
>  rust/kernel/transmute.rs | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)

On 8/25/25 9:07 PM, Alexandre Courbot wrote:
> `FromBytes::from_bytes` comes with a few practical limitations:
> 
> - It requires the bytes slice to have the same alignment as the returned
>   type, which might not be guaranteed in the case of a byte stream,
> - It returns a reference, requiring the returned type to implement
>   `Clone` if one wants to keep the value for longer than the lifetime of
>   the slice.
> 
> To overcome these when needed, add a `from_bytes_copy` with a default
> implementation in the trait. `from_bytes_copy` returns an owned value
> that is populated using an unaligned read, removing the lifetime
> constraint and making it usable even on non-aligned byte slices.
> 
> Reviewed-by: Alice Ryhl <aliceryhl at google.com>
> Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
> ---
>  rust/kernel/transmute.rs | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index
494bb3b1d059337520efef694fc8952972d44fbf..721dd8254dcedd71ed7c1fc0ee9292950c16c89e
100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -78,6 +78,23 @@ fn from_bytes_mut(bytes: &mut [u8]) ->
Option<&mut Self>
>              None
>          }
>      }
> +
> +    /// Creates an owned instance of `Self` by copying `bytes`.
> +    ///
> +    /// As the data is copied into a properly-aligned location, this
method can be used even if
> +    /// [`FromBytes::from_bytes`] would return `None` due to incompatible
alignment.
Some very minor suggestions:

This wording less precise than it could be: "as the data is copied"
can mean
either "while it is being copied", or "because it is
copied". Also, there
should not be a hyphen in "properly aligned".

I'd suggest something like this instead:
 
    /// Unlike [`FromBytes::from_bytes`], which requires aligned input, this
    /// method can be used on non-aligned data.
> +    fn from_bytes_copy(bytes: &[u8]) -> Option<Self>
> +    where
> +        Self: Sized,
> +    {
> +        if bytes.len() == size_of::<Self>() {
> +            // SAFETY: `bytes` has the same size as `Self`, and per the
invariants of `FromBytes`,
> +            // any byte sequence is a valid value for `Self`.
More wording suggestions. How about this:

            // SAFETY: we just verified that `bytes` has the same size as
`Self`, and per the
            // invariants of `FromBytes`, any byte sequence of the correct
length is a valid value
            // for `Self`.
> +            Some(unsafe {
core::ptr::read_unaligned(bytes.as_ptr().cast::<Self>()) })
> +        } else {
> +            None
> +        }
> +    }
>  }
>  
>  macro_rules! impl_frombytes {
> 
I'm unable to find anything wrong with the code itself, and the above are
just
tiny nits, so whether or not you apply either or both of the above suggestions,
please feel free to add:


Reviewed-by: John Hubbard <jhubbard at nvidia.com>


thanks,
-- 
John Hubbard

On Tue Aug 26, 2025 at 1:07 PM JST, Alexandre Courbot
wrote:
> `FromBytes::from_bytes` comes with a few practical limitations:
>
> - It requires the bytes slice to have the same alignment as the returned
>   type, which might not be guaranteed in the case of a byte stream,
> - It returns a reference, requiring the returned type to implement
>   `Clone` if one wants to keep the value for longer than the lifetime of
>   the slice.
>
> To overcome these when needed, add a `from_bytes_copy` with a default
> implementation in the trait. `from_bytes_copy` returns an owned value
> that is populated using an unaligned read, removing the lifetime
> constraint and making it usable even on non-aligned byte slices.
>
> Reviewed-by: Alice Ryhl <aliceryhl at google.com>
> Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
We got 3 Reviewed-by on this patch - Miguel, are you ok if I merge it
together with Christian's `from_bytes` patch, since they are closely
related?