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

[Alice Ryhl <aliceryhl@google.com>]

On Tue, May 06, 2025 at 07:10:43AM -0700, Boqun Feng wrote:
> 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.

You can change the example to go through `impl From<&T> for ARef<T>`,
and then you have the same situation without this unsafe op.

Alice