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

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

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

> On Fri, May 2, 2025 at 2:32 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>> "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?
>
> Hmm. Perhaps we should say that you can never own it "by value". There
> must always be pointer indirection.

Yes, that could work. I'll send a new version.


Best regards,
Andreas Hindborg