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 03:34:56PM -0700, Boqun Feng wrote:
[...]
> > >>>
> > >>> 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.
> > > 
> > > Let's look at the function from_raw(), the input is a pointer to T,
> > > right? So you only have the provenance to T, but in that function, the
> > > pointer is casted to a pointer to WithRef<T>/ArcInner<T>, that means you
> > > have the provenance to the whole WithRef<T>/ArcInner<T>, right? My
> > > question is: why isn't that a UB?
> > 
> > The pointer was originally derived by a call to `into_raw`:
> > ```
> >      pub fn into_raw(self) -> *const T {
> >          let ptr = self.ptr.as_ptr();
> >          core::mem::forget(self);
> >          // SAFETY: The pointer is valid.
> >          unsafe { core::ptr::addr_of!((*ptr).data) }
> >      }
> > ```
> > So in this function the origin (also the origin of the provenance)
> > of the pointer is `ptr` which is of type `NonNull<WithRef<T>>`.
> > Raw pointers do not lose this provenance information when you cast
> > it and when using `addr_of`/`addr_of_mut`. So provenance is something
> > that is not really represented in the type system for raw pointers.
> 
> Ah, I see, that's the thing I was missing. Now it makes much sense to
> me, thank you both!
> 
> > 
> > When doing a round trip through a reference though, the provenance is
> > newly assigned and thus would only be valid for a `T`:
> > ```
> > let raw = arc.into_raw();
> > let reference = unsafe { &*raw };
> > let raw: *const T = reference;
> > let arc = unsafe { Arc::from_raw(raw) };
> > ```
> 
> Agreed. This example demonstrates the key point: the provenances of raw
> pointers are decided at derive time.
> 

So the original problem the Alice brought up is also because of the
provenance, right? To get a `&WithRef<T>`, we reborrow the pointer to
get a `&`, and any pointer derived from that reference will have a
different (and read-only) provenance, which causes the problem. Like:

```rust
let raw = Box::into_raw(arc);
let reference = unsafe { &*raw }; // as_with_ref()
let raw: *mut T = reference as *const _ as  *mut _ ;
let arc = unsafe { Box::from_raw(raw) };
```

Regards,
Boqun

> Regards,
> Boqun
> 
> 
> > Miri would complain about the above code.
> > 
> > -- 
> > Cheers,
> > Benno
> > 
> >