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

[Alice Ryhl <aliceryhl@google.com>]

On Wed, May 07, 2025 at 10:35:16AM +0200, Benno Lossin wrote:
> On Tue May 6, 2025 at 10:29 AM CEST, Andreas Hindborg wrote:
> > 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`].
> 
> I don't think this covers every case, if I modify your example above
> with Alice's suggestion and using `Box` instead of the stack, I get the
> same problem:
> 
>     struct Empty {}
>  
>     unsafe impl AlwaysRefCounted for Empty {
>         fn inc_ref(&self) {}
>         unsafe fn dec_ref(_obj: NonNull<Self>) {}
>     }
>  
>     fn unsound() -> ARef<Empty> {
>         use kernel::types::{ARef, RefCounted};
>  
>         let data = Box::new(Empty {});
>         let aref = ARef::from(&data);
>  
>         aref
>     }
> 
> The same should be true if one uses `Arc` instead of `Box`. So, even
> though we store it in a "pointer type that implements `Deref`", it is
> unsound.
> 
> I think that types that implement `AlwaysRefCounted` must only be store
> inside of `ARef<T>`. So something like "Values of this trait must only
> be exposed as `ARef<Self>` or `&Self`." I'm not satisfied with the
> wording 'exposed', maybe you have a better word or can expand the
> sentence.

I mean, in some sense the problem is that Empty violates the existing
requirement:

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.)

Alice