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

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Sep 26, 2023 at 05:15:52PM +0000, Benno Lossin wrote:
[...]
> > 
> > But here the difference it that we only derive a `*mut` from a `&`,
> > rather than transmute to a `&mut`, right? We only use `&` to get a
> > pointer value (a usize), so I don't think that rule applies here? Or in
> > other words, does the following implemenation look good to you?
> > 
> > 	impl<T: ?Sized> Arc<T> {
> > 	    pub fn as_with_ref(&self) -> &WithRef<T> {
> > 		// expose
> > 		let _ = self.ptr.as_ptr() as usize;
> > 		unsafe { self.ptr.as_ref() }
> > 	    }
> > 	}
> > 
> > 	impl<T: ?Sized> From<&WithRef<T>> for Arc<T> {
> > 	    fn from(b: &WithRef<T>) -> Self {
> > 		// from exposed
> > 		let ptr = unsafe { NonNull::new_unchecked(b as *const _ as usize as *mut _) };
> > 		// SAFETY: The existence of `b` guarantees that the refcount is non-zero. `ManuallyDrop`
> > 		// guarantees that `drop` isn't called, so it's ok that the temporary `Arc` doesn't own the
> > 		// increment.
> > 		ManuallyDrop::new(unsafe { Arc::from_inner(ptr) })
> > 		    .deref()
> > 		    .clone()
> > 	    }
> > 	}
> > 
> > 
> > An equivalent code snippet is as below (in case anyone wants to try it
> > in miri):
> > ```rust
> >      let raw = Box::into_raw(arc);
> > 
> >      // as_with_ref()
> >      let _ = raw as usize;
> >      let reference = unsafe { &*raw };
> > 
> >      // from()
> >      let raw: *mut T = reference as *const _ as usize as  *mut _ ;
> > 
> >      // drop()
> >      let arc = unsafe { Box::from_raw(raw) };
> > ```
> 
> I don't understand why we are trying to use ptr2int to fix this.
> Simply wrapping the `T` field inside `WithRef` with `UnsafeCell`
> should be enough.
> 

BTW, how do you fix this with only wrapping `T` field in `WithRef`?

Let say `WithRef` is defined as:

struct WithRef<T> {
    refcount: Opaque<bindings::refcount_t>,
    data: UnsafeCell<T>,
}


impl<T: ?Sized> From<&WithRef<T>> for Arc<T> {
    fn from(b: &WithRef<T>) -> Self {
        let data_ptr: *mut T = b.data.get();

	let ptr = ?; // how to get a pointer to `WithRef<T>` with the
                     // provenance to the whole data?

        ManuallyDrop::new(unsafe { Arc::from_inner(ptr) })
            .deref()
            .clone()
    }
}

The `data_ptr` above only has provenance to part of the struct for the
similar reason that my proposal of (ab)using `b.refcount.get()`. Am I
missing something here?

Regards,
Boqun

> -- 
> Cheers,
> Benno
> 
>