Re: [PATCH v9 2/5] rust: types: implement ForeignOwnable for ARef<T>

["Gary Guo" <gary@garyguo.net>]

On Wed Aug 5, 2026 at 3:59 PM BST, Philipp Stanner wrote:
> From: Danilo Krummrich <dakr@kernel.org>
>
> Implement ForeignOwnable for ARef<T>, making it possible for C code to
> own an ARef<T>.
>
> Since ARef represents shared ownership, BorrowedMut is &T rather than
> &mut T, matching the semantics of the underlying reference-counted type.
>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
> ---
>  rust/kernel/sync/aref.rs | 40 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
>
> diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
> index b721b2e00b98..540766613659 100644
> --- a/rust/kernel/sync/aref.rs
> +++ b/rust/kernel/sync/aref.rs
> @@ -24,6 +24,11 @@
>      ptr::NonNull, //
>  };
>  
> +use crate::{
> +    prelude::*,
> +    types::ForeignOwnable, //
> +};
> +
>  /// Types that are _always_ reference counted.
>  ///
>  /// It allows such types to define their own custom ref increment and decrement functions.
> @@ -188,6 +193,41 @@ fn eq(&self, other: &ARef<U>) -> bool {
>  }
>  impl<T: AlwaysRefCounted + Eq> Eq for ARef<T> {}
>  
> +// SAFETY: `into_foreign` returns a pointer from `NonNull::as_ptr`, so it's non-null. The
> +// `ARef` invariant guarantees that `ptr` points to a valid `T`, so it's aligned to `T`.
> +unsafe impl<T: AlwaysRefCounted + 'static> ForeignOwnable for ARef<T> {

This doesn't need to be static, if you can add `where Self: 'a` on `Borrowed`
and `BorrowedMut` instead.

Best,
Gary

> +    const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
> +
> +    type Borrowed<'a> = &'a T;
> +    type BorrowedMut<'a> = &'a T;
> +
> +    fn into_foreign(self) -> *mut c_void {
> +        ARef::into_raw(self).as_ptr().cast()
> +    }
> +
> +    unsafe fn from_foreign(ptr: *mut c_void) -> Self {
> +        // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
> +        // call to `Self::into_foreign`.
> +        let ptr = unsafe { NonNull::new_unchecked(ptr.cast()) };
> +
> +        // SAFETY: `ptr` came from `into_foreign`, which consumed an `ARef` without decrementing
> +        // the refcount, so we can transfer the ownership to the new `ARef`.
> +        unsafe { ARef::from_raw(ptr) }
> +    }
> +
> +    unsafe fn borrow<'a>(ptr: *mut c_void) -> &'a T {
> +        // SAFETY: The safety requirements of this method ensure that the object remains alive and
> +        // immutable for the duration of 'a.
> +        unsafe { &*ptr.cast() }
> +    }
> +
> +    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> &'a T {
> +        // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety
> +        // requirements for `borrow`.
> +        unsafe { <Self as ForeignOwnable>::borrow(ptr) }
> +    }
> +}
> +
>  impl<T, U> PartialEq<&'_ U> for ARef<T>
>  where
>      T: AlwaysRefCounted + PartialEq<U>,