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

[Alice Ryhl <alice@ryhl.io>]

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