---

yaml
---
r: 229059
b: refs/heads/try
c: 22e2100
h: refs/heads/master
i:
  229057: eb17999
  229055: 845969a
v: v3

Author: Gankra

[refs]

@@ -1,7 +1,7 @@
 ---
 refs/heads/master: aca2057ed5fb7af3f8905b2bc01f72fa001c35c8
 refs/heads/snap-stage3: 1af31d4974e33027a68126fa5a5a3c2c6491824f
-refs/heads/try: 1ffe3453cb2bd4cc031b4f8a4bdb88279e01e094
+refs/heads/try: 22e21004582902cc1b7d1bef89d09728cbe64ca2
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/tags/release-0.2: c870d2dffb391e14efb05aa27898f1f6333a9596
 refs/tags/release-0.3: b5f0d0f648d9a6153664837026ba1be43d3e2503

branches/try/src/liballoc/arc.rs

@@ -78,16 +78,18 @@ use core::atomic::Ordering::{Relaxed, Release, Acquire, SeqCst};
 use core::fmt;
 use core::cmp::Ordering;
 use core::mem::{align_of_val, size_of_val};
-use core::intrinsics::drop_in_place;
+use core::intrinsics::{drop_in_place, abort};
 use core::mem;
 use core::nonzero::NonZero;
 use core::ops::{Deref, CoerceUnsized};
 use core::ptr;
 use core::marker::Unsize;
 use core::hash::{Hash, Hasher};
-use core::usize;
+use core::{usize, isize};
 use heap::deallocate;
 
+const MAX_REFCOUNT: usize = (isize::MAX) as usize;
+
 /// An atomically reference counted wrapper for shared state.
 ///
 /// # Examples
@@ -311,7 +313,21 @@ impl<T: ?Sized> Clone for Arc<T> {
         // another must already provide any required synchronization.
         //
         // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
-        self.inner().strong.fetch_add(1, Relaxed);
+        let old_size = self.inner().strong.fetch_add(1, Relaxed);
+
+        // However we need to guard against massive refcounts in case someone
+        // is `mem::forget`ing Arcs. If we don't do this the count can overflow
+        // and users will use-after free. We racily saturate to `isize::MAX` on
+        // the assumption that there aren't ~2 billion threads incrementing
+        // the reference count at once. This branch will never be taken in
+        // any realistic program.
+        //
+        // We abort because such a program is incredibly degenerate, and we
+        // don't care to support it.
+        if old_size > MAX_REFCOUNT {
+            unsafe { abort(); }
+        }
+
         Arc { _ptr: self._ptr }
     }
 }
@@ -612,7 +628,13 @@ impl<T: ?Sized> Clone for Weak<T> {
         // fetch_add (ignoring the lock) because the weak count is only locked
         // where are *no other* weak pointers in existence. (So we can't be
         // running this code in that case).
-        self.inner().weak.fetch_add(1, Relaxed);
+        let old_size = self.inner().weak.fetch_add(1, Relaxed);
+
+        // See comments in Arc::clone() for why we do this (for mem::forget).
+        if old_size > MAX_REFCOUNT {
+            unsafe { abort(); }
+        }
+
         return Weak { _ptr: self._ptr }
     }
 }