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

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

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

> 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.)

The example holds, even if you implement `inc_ref`/`dec_ref` properly,
right?


Best regards,
Andreas Hindborg