[Python-checkins] cpython: Issue #1621: Fix undefined behaviour from signed overflow in get_integer

Sat Sep 24 20:11:59 CEST 2011

http://hg.python.org/cpython/rev/5e456e1a9e8c
changeset: 72466:5e456e1a9e8c
user: Mark Dickinson <mdickinson at enthought.com>
date: Sat Sep 24 19:11:53 2011 +0100
summary:
 Issue #1621: Fix undefined behaviour from signed overflow in get_integer (stringlib/formatter.h)
files:
 Objects/stringlib/formatter.h | 16 +++++++---------
 1 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/Objects/stringlib/formatter.h b/Objects/stringlib/formatter.h
--- a/Objects/stringlib/formatter.h
+++ b/Objects/stringlib/formatter.h
@@ -73,7 +73,7 @@
 get_integer(STRINGLIB_CHAR **ptr, STRINGLIB_CHAR *end,
 Py_ssize_t *result)
 {
- Py_ssize_t accumulator, digitval, oldaccumulator;
+ Py_ssize_t accumulator, digitval;
 int numdigits;
 accumulator = numdigits = 0;
 for (;;(*ptr)++, numdigits++) {
@@ -83,19 +83,17 @@
 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
