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

[Alexandre Courbot]

On Wed Aug 27, 2025 at 9:51 AM JST, John Hubbard wrote:
> 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.

That's much simpler and better. I'll just add "... at the cost of a
copy." to not lose this information.

>
>> +    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`.

Taken as-is, thanks!