[Python-checkins] cpython: Issue #9530: Fix undefined behaviour due to signed overflow in

Thu Dec 1 16:27:09 CET 2011

http://hg.python.org/cpython/rev/7e37598a25a6
changeset: 73805:7e37598a25a6
user: Mark Dickinson <mdickinson at enthought.com>
date: Thu Dec 01 15:27:04 2011 +0000
summary:
 Issue #9530: Fix undefined behaviour due to signed overflow in Python/formatter_unicode.c.
files:
 Python/formatter_unicode.c | 16 +++++++---------
 1 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -51,7 +51,7 @@
 get_integer(PyObject *str, Py_ssize_t *pos, Py_ssize_t end,
 Py_ssize_t *result)
 {
- Py_ssize_t accumulator, digitval, oldaccumulator;
+ Py_ssize_t accumulator, digitval;
 int numdigits;
 accumulator = numdigits = 0;
 for (;;(*pos)++, numdigits++) {
@@ -61,19 +61,17 @@
 if (digitval < 0)
 break;
 /*
- This trick was copied from old Unicode format code. It's cute,
- but would really suck on an old machine with a slow divide
- implementation. Fortunately, in the normal case we do not
- expect too many digits.
+ Detect possible overflow before it happens:
+
+ accumulator * 10 + digitval > PY_SSIZE_T_MAX if and only if
+ accumulator > (PY_SSIZE_T_MAX - digitval) / 10.
 */
- oldaccumulator = accumulator;
- accumulator *= 10;
- if ((accumulator+10)/10 != oldaccumulator+1) {
+ if (accumulator > (PY_SSIZE_T_MAX - digitval) / 10) {
 PyErr_Format(PyExc_ValueError,
 "Too many decimal digits in format string");
 return -1;
 }
- accumulator += digitval;
+ accumulator = accumulator * 10 + digitval;
 }
 *result = accumulator;
 return numdigits;
-- 
Repository URL: http://hg.python.org/cpython
