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diff Objects/stringlib/string_format.h @ 80014:21fb1767e185 2.7

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Issue #14700: Fix buggy overflow checks for large precision and width in new-style and old-style formatting.
author Mark Dickinson <mdickinson@enthought.com>
date Sun, 28 Oct 2012 10:00:46 +0000
parents ee5350f1d0c2
children
line wrap: on
line diff
--- a/Objects/stringlib/string_format.h	Sun Oct 28 10:51:35 2012 +0100
+++ b/Objects/stringlib/string_format.h	Sun Oct 28 10:00:46 2012 +0000
@@ -197,7 +197,6 @@
 {
     Py_ssize_t accumulator = 0;
     Py_ssize_t digitval;
-    Py_ssize_t oldaccumulator;
     STRINGLIB_CHAR *p;
 
     /* empty string is an error */
@@ -209,19 +208,17 @@
         if (digitval < 0)
             return -1;
         /*
-           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;
     }
     return accumulator;
 }