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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAH5fLgj3j2BEyOmVw+T_988e_h1TYRmYVuEDYaL-baRu_mQq4g@mail.gmail.com>
Date: Fri, 2 May 2025 14:37:19 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: 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>, 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] rust: elaborate safety requirements for `AlwaysReferenceCounted`

On Fri, May 2, 2025 at 2:32 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> "Alice Ryhl" <aliceryhl@...gle.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.

Alice