Re: [PATCH v2] rust: elaborate safety requirements for `AlwaysReferenceCounted`

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, May 06, 2025 at 10:29:02AM +0200, Andreas Hindborg wrote:
> Clarify that implementers of `AlwaysReferenceCounted` must prevent the
> implementer from being directly initialized by users.
> 
> It is a violation of the safety requirements of `AlwaysReferenceCounted` if
> its implementers can be initialized on the stack by users. Although this
> follows from the safety requirements, it is not immediately obvious.
> 
> The following example demonstrates the issue. Note that the safety
> requirements for implementing `AlwaysRefCounted` and for calling
> `ARef::from_raw` are satisfied.
> 
>   struct Empty {}
> 
>   unsafe impl AlwaysRefCounted for Empty {
>       fn inc_ref(&self) {}
>       unsafe fn dec_ref(_obj: NonNull<Self>) {}
>   }
> 
>   fn unsound() -> ARef<Empty> {
>       use core::ptr::NonNull;
>       use kernel::types::{ARef, RefCounted};
> 
>       let mut data = Empty {};
>       let ptr = NonNull::<Empty>::new(&mut data).unwrap();
>       let aref: ARef<Empty> = unsafe { ARef::from_raw(ptr) };
> 

Hmm.. I would say in this case, what gets violated is the safe
requirement of ARef::from_raw(), because callers are supposed to
guarantee that an refcount increment was passed to `ARef` and in this
case, and unsound() cannot guarantee that here because it's going to
clean up `data` when the it returns.

Regards,
Boqun

>       aref
>   }
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> ---
> Changes in v2:
> - Express safety requirement in terms of ownership rather than
>   initialization.
> - Link to v1: https://lore.kernel.org/r/20250502-aref-from-raw-v1-1-eb0630626bba@kernel.org
> ---
>  rust/kernel/types.rs | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 9d0471afc964..52683d686c8a 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -409,6 +409,10 @@ pub const fn raw_get(this: *const Self) -> *mut T {
>  /// Implementers must also ensure that all instances are reference-counted. (Otherwise they
>  /// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
>  /// alive.)
> +///
> +/// Implementers of this trait must ensure that values of types implementing this trait can never be
> +/// owned by value. Instead, values must be owned and used through a pointer type. That is, a type
> +/// that implements [`Deref`].
>  pub unsafe trait AlwaysRefCounted {
>      /// Increments the reference count on the object.
>      fn inc_ref(&self);
> 
> ---
> base-commit: b4432656b36e5cc1d50a1f2dc15357543add530e
> change-id: 20250502-aref-from-raw-e110b3e6dbf5
> 
> Best regards,
> -- 
> Andreas Hindborg <a.hindborg@kernel.org>
> 
>