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

[Andreas Hindborg <a.hindborg@kernel.org>]

"Alice Ryhl" <aliceryhl@google.com> writes:

> 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
>     }
> }

Right. I would rephrase then:

It is a violation of the safety requirements of `AlwaysReferenceCounted`
if its implementers can be initialized on the stack by users and an
`ARef` referencing the object can outlive the object. Although this follows from
the safety requirements, it is not immediately obvious.

and

+/// Note: This means that implementers must prevent users from directly
+/// initializing the implementer when the implementer is `'static`. Otherwise users could
+/// initialize the implementer on
+/// the stack, which would violate the safety requirements.

What do you think?


Best regards,
Andreas Hindborg