From cfbf1a33c17f5ea499a0419d94162d80c52ec782 Mon Sep 17 00:00:00 2001
From: Guido van Rossum <guido@python.org>
Date: Thu, 28 Mar 2002 20:17:52 +0000
Subject: [PATCH] Fix an issue that was reported in but unrelated to the main
problem of SF bug 535905 (Evil Trashcan and GC interaction).
The SETLOCAL() macro should not DECREF the local variable in-place and
then store the new value; it should copy the old value to a temporary
value, then store the new value, and then DECREF the temporary value.
This is because it is possible that during the DECREF the frame is
accessed by other code (e.g. a __del__ method or gc.collect()) and the
variable would be pointing to already-freed memory.
BUGFIX CANDIDATE!
---
Python/ceval.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/Python/ceval.c b/Python/ceval.c
index 97a734cf94..a93ceea128 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -554,8 +554,16 @@ eval_frame(PyFrameObject *f)
/* Local variable macros */
#define GETLOCAL(i) (fastlocals[i])
-#define SETLOCAL(i, value) do { Py_XDECREF(GETLOCAL(i)); \
- GETLOCAL(i) = value; } while (0)
+
+/* The SETLOCAL() macro must not DECREF the local variable in-place and
+ then store the new value; it must copy the old value to a temporary
+ value, then store the new value, and then DECREF the temporary value.
+ This is because it is possible that during the DECREF the frame is
+ accessed by other code (e.g. a __del__ method or gc.collect()) and the
+ variable would be pointing to already-freed memory. */
+#define SETLOCAL(i, value) do { PyObject *tmp = GETLOCAL(i); \
+ GETLOCAL(i) = value; \
+ Py_XDECREF(tmp); } while (0)
/* Start of code */
--
2.40.0