Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Sep 25, 2023 at 05:00:45PM +0000, Benno Lossin wrote:
> On 25.09.23 18:16, Boqun Feng wrote:
> > On Mon, Sep 25, 2023 at 03:07:44PM +0000, Benno Lossin wrote:
> >> ```rust
> >> struct MutatingDrop {
> >>       value: i32,
> >> }
> >>
> >> impl Drop for MutatingDrop {
> >>       fn drop(&mut self) {
> >>           self.value = 0;
> >>       }
> >> }
> >>
> >> let arc = Arc::new(MutatingDrop { value: 42 });
> >> let wr = arc.as_with_ref(); // this creates a shared `&` reference to the MutatingDrop
> >> let arc2: Arc<MutatingDrop> = wr.into(); // increments the reference count to 2
> > 
> > More precisely, here we did a
> > 
> > 	&WithRef<_> -> NonNull<WithRef<_>>
> > 
> > conversion, and later on, we may use the `NonNull<WithRef<_>>` in
> > `drop` to get a `Box<WithRef<_>>`.
> 
> Indeed.
> 

Can we workaround this issue by (ab)using the `UnsafeCell` inside
`WithRef<T>`?

impl<T: ?Sized> From<&WithRef<T>> for Arc<T> {
    fn from(b: &WithRef<T>) -> Self {
        // SAFETY: The existence of the references proves that
	// `b.refcount.get()` is a valid pointer to `WithRef<T>`.
	let ptr = unsafe { NonNull::new_unchecked(b.refcount.get().cast::<WithRef<T>>()) };

	// SAFETY: see the SAFETY above `let ptr = ..` line.
        ManuallyDrop::new(unsafe { Arc::from_inner(ptr) })
            .deref()
            .clone()
    }
}

This way, the raw pointer in the new Arc no longer derives from the
reference of `WithRef<T>`.

Regards,
Boqun

> > 
> >> drop(arc); // this decrements the reference count to 1
> >> drop(arc2); // this decrements the reference count to 0, so it will drop it
> >> ```
> >> When dropping `arc2` it will run the destructor for `MutatingDrop`,
> >> which mutates `value`. This is a problem, because the mutable reference
> >> supplied was derived from a `&`, that is not allowed in Rust.
> >>
> > 
> > Is this an UB? I kinda wonder what's the real damage we can get, because
> > in this case, we just use a reference to carry a value of a pointer,
> > i.e.
> > 
> > 	ptr -> reference -> ptr
> > 
> > I cannot think of any real damage compiler can make, but I'm happy to be
> > surprised ;-)
> 
> This is UB, so anything can happen :)
> 
> -- 
> Cheers,
> Benno
> 
>