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

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Message-Id: <D9PSH8MJ48JO.3OOA3Z3NSBGC9@kernel.org>
Date: Wed, 07 May 2025 10:35:16 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Andreas Hindborg" <a.hindborg@...nel.org>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun Feng"
 <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <benno.lossin@...ton.me>, "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor
 Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>
Cc: "Oliver Mangold" <oliver.mangold@...me>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] rust: elaborate safety requirements for
 `AlwaysReferenceCounted`

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.

---
Cheers,
Benno

>  pub unsafe trait AlwaysRefCounted {
>      /// Increments the reference count on the object.
>      fn inc_ref(&self);
>
> ---
> base-commit: b4432656b36e5cc1d50a1f2dc15357543add530e
> change-id: 20250502-aref-from-raw-e110b3e6dbf5
>
> Best regards,