Re: [PATCH v10 1/5] rust: types: Add Ownable/Owned types

[Alice Ryhl <aliceryhl@google.com>]

On Fri, May 02, 2025 at 09:02:29AM +0000, Oliver Mangold wrote:
> From: Asahi Lina <lina@asahilina.net>
> 
> By analogy to AlwaysRefCounted and ARef, an Ownable type is a (typically
> C FFI) type that *may* be owned by Rust, but need not be. Unlike
> AlwaysRefCounted, this mechanism expects the reference to be unique
> within Rust, and does not allow cloning.
> 
> Conceptually, this is similar to a KBox<T>, except that it delegates
> resource management to the T instead of using a generic allocator.
> 
> Link: https://lore.kernel.org/all/20250202-rust-page-v1-1-e3170d7fe55e@asahilina.net/
> Signed-off-by: Asahi Lina <lina@asahilina.net>
> [ om:
>   - split code into separate file and `pub use` it from types.rs
>   - make from_raw() and into_raw() public
>   - fixes to documentation
> ]
> Signed-off-by: Oliver Mangold <oliver.mangold@pm.me>
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> ---
>  rust/kernel/types.rs         |   3 ++
>  rust/kernel/types/ownable.rs | 117 +++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 120 insertions(+)
> 
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 9d0471afc9648f2973235488b441eb109069adb1..5d8a99dcba4bf733107635bf3f0c15840ec33e4c 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -11,6 +11,9 @@
>  };
>  use pin_init::{PinInit, Zeroable};
>  
> +pub mod ownable;
> +pub use ownable::{Ownable, OwnableMut, Owned};
> +
>  /// Used to transfer ownership to and from foreign (non-Rust) languages.
>  ///
>  /// Ownership is transferred from Rust to a foreign language by calling [`Self::into_foreign`] and
> diff --git a/rust/kernel/types/ownable.rs b/rust/kernel/types/ownable.rs
> new file mode 100644
> index 0000000000000000000000000000000000000000..52e7a69019f1e2bbbe3cf715651b67a5a5c7c13d
> --- /dev/null
> +++ b/rust/kernel/types/ownable.rs
> @@ -0,0 +1,117 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Owned reference types.
> +
> +use core::{
> +    marker::PhantomData,
> +    mem::ManuallyDrop,
> +    ops::{Deref, DerefMut},
> +    ptr::NonNull,
> +};
> +
> +/// Types that may be owned by Rust code or borrowed, but have a lifetime managed by C code.
> +///
> +/// It allows such types to define their own custom destructor function to be called when
> +/// a Rust-owned reference is dropped.
> +///
> +/// This is usually implemented by wrappers to existing structures on the C side of the code.
> +///
> +/// # Safety
> +///
> +/// Implementers must ensure that:
> +/// - Any objects owned by Rust as [`Owned<T>`] stay alive while that owned reference exists (i.e.
> +///   until the [`release()`](Ownable::release) trait method is called).
> +/// - That the C code follows the usual mutable reference requirements. That is, the kernel will
> +///   never mutate the [`Ownable`] (excluding internal mutability that follows the usual rules)
> +///   while Rust owns it.

This seems too strong? Or does the exception mean to say that this does
not apply to anything containing `Opaque`? By far most structs using
this will use Opaque, so maybe directly mention Opaque instead?

That C code follows the usual aliasing rules. That is, unless the value
is wrapped in `Opaque` (or `UnsafeCell`), then the value must not be
modified in any way while Rust owns it, unless that modification happens
inside a `&mut T` method on the value.

> +pub unsafe trait Ownable {
> +    /// Releases the object (frees it or returns it to foreign ownership).
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that the object is no longer referenced after this call.
> +    unsafe fn release(this: NonNull<Self>);
> +}
> +
> +/// A subtrait of Ownable that asserts that an [`Owned<T>`] or `&mut Owned<T>` Rust reference
> +/// may be dereferenced into a `&mut T`.
> +///
> +/// # Safety
> +///
> +/// Implementers must ensure that access to a `&mut T` is safe, implying that it is okay to call
> +/// [`core::mem::swap`] on the `Ownable`. This excludes pinned types (meaning: most kernel types).
> +pub unsafe trait OwnableMut: Ownable {}
> +
> +/// An owned reference to an ownable kernel object.
> +///
> +/// The object is automatically freed or released when an instance of [`Owned`] is
> +/// dropped.
> +///
> +/// # Invariants
> +///
> +/// The pointer stored in `ptr` is valid for the lifetime of the [`Owned`] instance.

This should probably talk about ownership.

> +pub struct Owned<T: Ownable> {
> +    ptr: NonNull<T>,
> +    _p: PhantomData<T>,
> +}

Alice