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

[Benno Lossin]

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.

-- 
Cheers,
Benno