Prevent expandtabs() on string and unicode objects from causing a segfault when

a large width is passed on 32-bit platforms.  Found by Google.

It would be good for people to review this especially carefully and verify
I don't have an off by one error and there is no other way to cause overflow.

Lib/test/test_str.py

@@ -1,4 +1,6 @@
+
 import unittest
+import sys
 from test import test_support, string_tests
 
 
@@ -82,6 +84,15 @@ def __unicode__(self):
         self.assertEqual(str(Foo9("foo")), "string")
         self.assertEqual(unicode(Foo9("foo")), u"not unicode")
 
+    def test_expandtabs_overflows_gracefully(self):
+        # This test only affects 32-bit platforms because expandtabs can only take
+        # an int as the max value, not a 64-bit C long.  If expandtabs is changed
+        # to take a 64-bit long, this test should apply to all platforms.
+        if sys.maxint > (1 << 32):
+            return
+        self.assertRaises(OverflowError, 't\tt\t'.expandtabs, sys.maxint)
+
+
 def test_main():
     test_support.run_unittest(StrTest)
 

Lib/test/test_unicode.py

@@ -817,8 +817,13 @@ def __repr__(self):
         self.assertEqual(repr(s1()), '\\n')
         self.assertEqual(repr(s2()), '\\n')
 
-
-
+    def test_expandtabs_overflows_gracefully(self):
+        # This test only affects 32-bit platforms because expandtabs can only take
+        # an int as the max value, not a 64-bit C long.  If expandtabs is changed
+        # to take a 64-bit long, this test should apply to all platforms.
+        if sys.maxint > (1 << 32):
+            return
+        self.assertRaises(OverflowError, u't\tt\t'.expandtabs, sys.maxint)
 
 
 def test_main():

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 2.6 alpha 1?
 Core and builtins
 -----------------
 
+- Prevent expandtabs() on string and unicode objects from causing a segfault when
+  a large width is passed on 32-bit platforms.
+
 - Bug #1733488: Fix compilation of bufferobject.c on AIX.
 
 - Bug #1722485: remove docstrings again when running with -OO.

Objects/stringobject.c

@@ -3307,7 +3307,7 @@ string_expandtabs(PyStringObject *self, PyObject *args)
 {
     const char *e, *p;
     char *q;
-    Py_ssize_t i, j;
+    Py_ssize_t i, j, old_j;
     PyObject *u;
     int tabsize = 8;
 
@@ -3315,12 +3315,18 @@ string_expandtabs(PyStringObject *self, PyObject *args)
 	return NULL;
 
     /* First pass: determine size of output string */
-    i = j = 0;
+    i = j = old_j = 0;
     e = PyString_AS_STRING(self) + PyString_GET_SIZE(self);
     for (p = PyString_AS_STRING(self); p < e; p++)
         if (*p == '\t') {
-	    if (tabsize > 0)
+	    if (tabsize > 0) {
 		j += tabsize - (j % tabsize);
+		if (old_j > j) {
+		    PyErr_SetString(PyExc_OverflowError, "new string is too long");
+		    return NULL;
+		}
+		old_j = j;
+            }
 	}
         else {
             j++;
@@ -3330,6 +3336,11 @@ string_expandtabs(PyStringObject *self, PyObject *args)
             }
         }
 
+    if ((i + j) < 0) {
+        PyErr_SetString(PyExc_OverflowError, "new string is too long");
+        return NULL;
+    }
+
     /* Second pass: create output string and fill it */
     u = PyString_FromStringAndSize(NULL, i + j);
     if (!u)

Objects/unicodeobject.c

@@ -5690,7 +5690,7 @@ unicode_expandtabs(PyUnicodeObject *self, PyObject *args)
     Py_UNICODE *e;
     Py_UNICODE *p;
     Py_UNICODE *q;
-    Py_ssize_t i, j;
+    Py_ssize_t i, j, old_j;
     PyUnicodeObject *u;
     int tabsize = 8;
 
@@ -5698,12 +5698,18 @@ unicode_expandtabs(PyUnicodeObject *self, PyObject *args)
 	return NULL;
 
     /* First pass: determine size of output string */
-    i = j = 0;
+    i = j = old_j = 0;
     e = self->str + self->length;
     for (p = self->str; p < e; p++)
         if (*p == '\t') {
-	    if (tabsize > 0)
+	    if (tabsize > 0) {
 		j += tabsize - (j % tabsize);
+		if (old_j > j) {
+		    PyErr_SetString(PyExc_OverflowError, "new string is too long");
+		    return NULL;
+		}
+		old_j = j;
+	    }
 	}
         else {
             j++;
@@ -5713,6 +5719,11 @@ unicode_expandtabs(PyUnicodeObject *self, PyObject *args)
             }
         }
 
+    if ((i + j) < 0) {
+        PyErr_SetString(PyExc_OverflowError, "new string is too long");
+        return NULL;
+    }
+
     /* Second pass: create output string and fill it */
     u = _PyUnicode_New(i + j);
     if (!u)