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.