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 09:03:52PM +0000, Benno Lossin wrote:
> On 25.09.23 20:51, Boqun Feng wrote:
> > 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>`.
> 
> No, the code above only obtains a pointer that has provenance valid
> for a `bindings::refcount_t` (or type with the same layout, such as
> `Opaque<bindings::refcount_t>`). But not the whole `WithRef<T>`, so accessing
> it by reading/writing will still be UB.
> 

Hmm... but we do the similar thing in `Arc::from_raw()`, right?

    	pub unsafe fn from_raw(ptr: *const T) -> Self {
	    ..
	}

, what we have is a pointer to T, and we construct a pointer to
`ArcInner<T>/WithRef<T>`, in that function. Because the `sub` on pointer
gets away from provenance? If so, we can also do a sub(0) in the above
code.

Regards,
Boqun

> -- 
> Cheers,
> Benno
> 
>