bpo-36389: Change PyMem_SetupDebugHooks() constants (GH-12782)

Modify CLEANBYTE, DEADDYTE and FORBIDDENBYTE constants: use 0xCD,
0xDD and 0xFD, rather than 0xCB, 0xBB and 0xFB, to use the same byte
patterns than Windows CRT debug malloc() and free().

Doc/c-api/memory.rst

@@ -440,8 +440,9 @@ Customize Memory Allocators
 
    Setup hooks to detect bugs in the Python memory allocator functions.
 
-   Newly allocated memory is filled with the byte ``0xCB``, freed memory is
-   filled with the byte ``0xDB``.
+   Newly allocated memory is filled with the byte ``0xCD`` (``CLEANBYTE``),
+   freed memory is filled with the byte ``0xDD`` (``DEADBYTE``). Memory blocks
+   are surrounded by "forbidden bytes" (``FORBIDDENBYTE``: byte ``0xFD``).
 
    Runtime checks:
 
@@ -471,6 +472,12 @@ Customize Memory Allocators
       if the GIL is held when functions of :c:data:`PYMEM_DOMAIN_OBJ` and
       :c:data:`PYMEM_DOMAIN_MEM` domains are called.
 
+   .. versionchanged:: 3.8.0
+      Byte patterns ``0xCB`` (``CLEANBYTE``), ``0xDB`` (``DEADBYTE``) and
+      ``0xFB`` (``FORBIDDENBYTE``) have been replaced with ``0xCD``, ``0xDD``
+      and ``0xFD`` to use the same values than Windows CRT debug ``malloc()``
+      and ``free()``.
+
 
 .. _pymalloc:
 

Include/internal/pycore_pymem.h

@@ -160,21 +160,20 @@ PyAPI_FUNC(int) _PyMem_SetDefaultAllocator(
    pointer value is checked.
 
    The heuristic relies on the debug hooks on Python memory allocators which
-   fills newly allocated memory with CLEANBYTE (0xCB) and newly freed memory
-   with DEADBYTE (0xDB). Detect also "untouchable bytes" marked
-   with FORBIDDENBYTE (0xFB). */
+   fills newly allocated memory with CLEANBYTE (0xCD) and newly freed memory
+   with DEADBYTE (0xDD). Detect also "untouchable bytes" marked
+   with FORBIDDENBYTE (0xFD). */
 static inline int _PyMem_IsPtrFreed(void *ptr)
 {
     uintptr_t value = (uintptr_t)ptr;
 #if SIZEOF_VOID_P == 8
-    return (value == (uintptr_t)0xCBCBCBCBCBCBCBCB
-            || value == (uintptr_t)0xDBDBDBDBDBDBDBDB
-            || value == (uintptr_t)0xFBFBFBFBFBFBFBFB
-            );
+    return (value == (uintptr_t)0xCDCDCDCDCDCDCDCD
+            || value == (uintptr_t)0xDDDDDDDDDDDDDDDD
+            || value == (uintptr_t)0xFDFDFDFDFDFDFDFD);
 #elif SIZEOF_VOID_P == 4
-    return (value == (uintptr_t)0xCBCBCBCB
-            || value == (uintptr_t)0xDBDBDBDB
-            || value == (uintptr_t)0xFBFBFBFB);
+    return (value == (uintptr_t)0xCDCDCDCD
+            || value == (uintptr_t)0xDDDDDDDD
+            || value == (uintptr_t)0xFDFDFDFD);
 #else
 #  error "unknown pointer size"
 #endif

Lib/test/test_capi.py

@@ -480,11 +480,11 @@ class PyMemDebugTests(unittest.TestCase):
                  r"    The [0-9] pad bytes at p-[0-9] are FORBIDDENBYTE, as expected.\n"
                  r"    The [0-9] pad bytes at tail={ptr} are not all FORBIDDENBYTE \(0x[0-9a-f]{{2}}\):\n"
                  r"        at tail\+0: 0x78 \*\*\* OUCH\n"
-                 r"        at tail\+1: 0xfb\n"
-                 r"        at tail\+2: 0xfb\n"
+                 r"        at tail\+1: 0xfd\n"
+                 r"        at tail\+2: 0xfd\n"
                  r"        .*\n"
                  r"    The block was made by call #[0-9]+ to debug malloc/realloc.\n"
-                 r"    Data at p: cb cb cb .*\n"
+                 r"    Data at p: cd cd cd .*\n"
                  r"\n"
                  r"Enable tracemalloc to get the memory block allocation traceback\n"
                  r"\n"
@@ -500,7 +500,7 @@ class PyMemDebugTests(unittest.TestCase):
                  r"    The [0-9] pad bytes at p-[0-9] are FORBIDDENBYTE, as expected.\n"
                  r"    The [0-9] pad bytes at tail={ptr} are FORBIDDENBYTE, as expected.\n"
                  r"    The block was made by call #[0-9]+ to debug malloc/realloc.\n"
-                 r"    Data at p: cb cb cb .*\n"
+                 r"    Data at p: cd cd cd .*\n"
                  r"\n"
                  r"Enable tracemalloc to get the memory block allocation traceback\n"
                  r"\n"

Misc/NEWS.d/next/C API/2019-04-11-12-20-35.bpo-36389.P9QFoP.rst

+Change the value of ``CLEANBYTE``, ``DEADDYTE`` and ``FORBIDDENBYTE`` internal
+constants used by debug hooks on Python memory allocators
+(:c:func:`PyMem_SetupDebugHooks` function). Byte patterns ``0xCB``, ``0xDB``
+and ``0xFB`` have been replaced with ``0xCD``, ``0xDD`` and ``0xFD`` to use the
+same values than Windows CRT debug ``malloc()`` and ``free()``.

Objects/object.c

@@ -415,13 +415,12 @@ _Py_BreakPoint(void)
 }
 
 
-/* Heuristic checking if the object memory has been deallocated.
-   Rely on the debug hooks on Python memory allocators which fills the memory
-   with DEADBYTE (0xDB) when memory is deallocated.
+/* Heuristic checking if the object memory is uninitialized or deallocated.
+   Rely on the debug hooks on Python memory allocators:
+   see _PyMem_IsPtrFreed().
 
    The function can be used to prevent segmentation fault on dereferencing
-   pointers like 0xdbdbdbdbdbdbdbdb. Such pointer is very unlikely to be mapped
-   in memory. */
+   pointers like 0xDDDDDDDDDDDDDDDD. */
 int
 _PyObject_IsFreed(PyObject *op)
 {

Objects/obmalloc.c

@@ -1915,13 +1915,16 @@ _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. If modified, _PyMem_IsPtrFreed() should be updated as well.
+ *
+ * Byte patterns 0xCB, 0xBB and 0xFB have been replaced with 0xCD, 0xDD and
+ * 0xFD to use the same values than Windows CRT debug malloc() and free().
  */
 #undef CLEANBYTE
 #undef DEADBYTE
 #undef FORBIDDENBYTE
-#define CLEANBYTE      0xCB    /* clean (newly allocated) memory */
-#define DEADBYTE       0xDB    /* dead (newly freed) memory */
-#define FORBIDDENBYTE  0xFB    /* untouchable bytes at each end of a block */
+#define CLEANBYTE      0xCD    /* clean (newly allocated) memory */
+#define DEADBYTE       0xDD    /* dead (newly freed) memory */
+#define FORBIDDENBYTE  0xFD    /* untouchable bytes at each end of a block */
 
 static size_t serialno = 0;     /* incremented on each debug {m,re}alloc */