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Message-ID: <aBoYYzj7sGEbsQzw@Mac.home>
Date: Tue, 6 May 2025 07:10:43 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...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>,
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 06, 2025 at 10:29:02AM +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) };
>
Hmm.. I would say in this case, what gets violated is the safe
requirement of ARef::from_raw(), because callers are supposed to
guarantee that an refcount increment was passed to `ARef` and in this
case, and unsound() cannot guarantee that here because it's going to
clean up `data` when the it returns.
Regards,
Boqun
> aref
> }
>
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
> Changes in v2:
> - Express safety requirement in terms of ownership rather than
> initialization.
> - Link to v1: https://lore.kernel.org/r/20250502-aref-from-raw-v1-1-eb0630626bba@kernel.org
> ---
> rust/kernel/types.rs | 4 ++++
> 1 file changed, 4 insertions(+)
>
> 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`].
> 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,
> --
> Andreas Hindborg <a.hindborg@...nel.org>
>
>
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