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

[Alice Ryhl]

On 9/25/23 23:55, Boqun Feng wrote:
> 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.

Not sure what you mean. Operations on raw pointers leave provenance 
unchanged.

Alice