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

[Alice Ryhl <aliceryhl@google.com>]

On Fri, May 02, 2025 at 01:53:57PM +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) };
> 
>       aref
>   }

I don't think it's entirely impossible to write an AlwaysRefCounted
value that can be on the stack. The type just needs a lifetime
parameter. For example, this API is not unsound:

struct MyDataStorage {
    // ...
}

impl MyDataStorage {
    fn as_aref(&self) -> ARef<MyData<'_>> {
        unsafe { ARef::from_raw(ptr::from_ref(self).cast()) }
    }
}

#[repr(transparent)]
struct MyData<'s> {
    storage: MyDataStorage,
    _lifetime: PhantomData<&'s MyDataStorage>,
}

unsafe impl AlwaysRefCounted for MyData<'_> {
    fn inc_ref(&self) {}
    unsafe fn dec_ref(_obj: NonNull<Self>) {}
}

impl Deref for MyData<'_> {
    type Target = MyDataStorage;
    fn deref(&self) -> &MyDataStorage {
        &self.storage
    }
}

Alice