Merge pull request #4580 from JoJoDeveloping/fix-4579-protector-0sized

Fix #4579 by checking if the strong protector is actually "active".

Author: RalfJung

src/tools/miri/src/borrow_tracker/tree_borrows/tree.rs

@@ -756,6 +756,8 @@ impl<'tcx> Tree {
                             // Don't check for protector if it is a Cell (see `unsafe_cell_deallocate` in `interior_mutability.rs`).
                             // Related to https://github.com/rust-lang/rust/issues/55005.
                             && !perm.permission().is_cell()
+                            // Only trigger UB if the accessed bit is set, i.e. if the protector is actually protecting this offset. See #4579.
+                            && perm.is_accessed()
                         {
                             Err(TransitionError::ProtectedDealloc)
                         } else {

src/tools/miri/tests/fail/both_borrows/zero-sized-protected.rs

@@ -1,6 +1,7 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
 #![feature(allocator_api)]
+use std::alloc::{Layout, alloc, dealloc};
 use std::cell::Cell;
 use std::ptr;
 
@@ -305,5 +306,14 @@ fn zst() {
         let ptr = &raw mut *b as *mut ();
         drop(b);
         let _ref = &mut *ptr;
+
+        // zero-sized protectors do not affect deallocation
+        fn with_protector(_x: &mut (), ptr: *mut u8, l: Layout) {
+            // `_x` here is strongly protected but covers zero bytes.
+            unsafe { dealloc(ptr, l) };
+        }
+        let l = Layout::from_size_align(1, 1).unwrap();
+        let ptr = alloc(l);
+        with_protector(&mut *ptr.cast::<()>(), ptr, l);
     }
 }