bpo-36389: _PyObject_IsFreed() now also detects uninitialized memory (GH-12770)

Replace _PyMem_IsFreed() function with _PyMem_IsPtrFreed() inline
function. The function is now way more efficient, it became a simple
comparison on integers, rather than a short loop. It detects also
uninitialized bytes and "forbidden bytes" filled by debug hooks
on memory allocators.

Add unit tests on _PyObject_IsFreed().

2 files changed, 8 insertions, 25 deletions

diff --git a/Objects/object.c b/Objects/object.c
index bd44acacb61..c9aa479abdc 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -425,18 +425,17 @@ _Py_BreakPoint(void)
 int
 _PyObject_IsFreed(PyObject *op)
 {
-    uintptr_t ptr = (uintptr_t)op;
-    if (_PyMem_IsFreed(&ptr, sizeof(ptr))) {
+    if (_PyMem_IsPtrFreed(op) || _PyMem_IsPtrFreed(op->ob_type)) {
         return 1;
     }
-    int freed = _PyMem_IsFreed(&op->ob_type, sizeof(op->ob_type));
-    /* ignore op->ob_ref: the value can have be modified
+    /* ignore op->ob_ref: its value can have be modified
        by Py_INCREF() and Py_DECREF(). */
 #ifdef Py_TRACE_REFS
-    freed &= _PyMem_IsFreed(&op->_ob_next, sizeof(op->_ob_next));
-    freed &= _PyMem_IsFreed(&op->_ob_prev, sizeof(op->_ob_prev));
+    if (_PyMem_IsPtrFreed(op->_ob_next) || _PyMem_IsPtrFreed(op->_ob_prev)) {
+        return 1;
+    }
 #endif
-    return freed;
+    return 0;
 }
 
 
@@ -453,7 +452,7 @@ _PyObject_Dump(PyObject* op)
     if (_PyObject_IsFreed(op)) {
         /* It seems like the object memory has been freed:
            don't access it to prevent a segmentation fault. */
-        fprintf(stderr, "<freed object>\n");
+        fprintf(stderr, "<Freed object>\n");
         return;
     }
 
diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index 1c2a32050f9..e919fad595b 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -1914,7 +1914,7 @@ _Py_GetAllocatedBlocks(void)
 
 /* Special bytes broadcast into debug memory blocks at appropriate times.
  * Strings of these are unlikely to be valid addresses, floats, ints or
- * 7-bit ASCII.
+ * 7-bit ASCII. If modified, _PyMem_IsPtrFreed() should be updated as well.
  */
 #undef CLEANBYTE
 #undef DEADBYTE
@@ -2059,22 +2059,6 @@ _PyMem_DebugRawCalloc(void *ctx, size_t nelem, size_t elsize)
 }
 
 
-/* Heuristic checking if the memory has been freed. Rely on the debug hooks on
-   Python memory allocators which fills the memory with DEADBYTE (0xDB) when
-   memory is deallocated. */
-int
-_PyMem_IsFreed(void *ptr, size_t size)
-{
-    unsigned char *bytes = ptr;
-    for (size_t i=0; i < size; i++) {
-        if (bytes[i] != DEADBYTE) {
-            return 0;
-        }
-    }
-    return 1;
-}
-
-
 /* The debug free first checks the 2*SST bytes on each end for sanity (in
    particular, that the FORBIDDENBYTEs with the api ID are still intact).
    Then fills the original bytes with DEADBYTE.