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

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Message-Id: <20250506-aref-from-raw-v2-1-5a35e47f4ec2@kernel.org>
Date: Tue, 06 May 2025 10:29:02 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: 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, Andreas Hindborg <a.hindborg@...nel.org>
Subject: [PATCH v2] rust: elaborate safety requirements for
 `AlwaysReferenceCounted`

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
  }

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>