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# HG changeset patch
# User Benjamin Peterson <benjamin@python.org>
# Date 1397441438 14400
# Node ID 50c07ed1743da9cd4540d83de0c30bd17aeb41b0
# Parent  218e28a935ab4494d05215c243e2129625a71893
in scan_once, prevent the reading of arbitrary memory when passed a negative index

Bug reported by Guido Vranken.

diff --git a/Lib/json/tests/test_decode.py b/Lib/json/tests/test_decode.py
--- a/Lib/json/tests/test_decode.py
+++ b/Lib/json/tests/test_decode.py
@@ -60,5 +60,10 @@ class TestDecode(object):
         msg = 'escape'
         self.assertRaisesRegexp(ValueError, msg, self.loads, s)
 
+    def test_negative_index(self):
+        d = self.json.JSONDecoder()
+        self.assertRaises(ValueError, d.raw_decode, 'a'*42, -50000)
+        self.assertRaises(ValueError, d.raw_decode, u'a'*42, -50000)
+
 class TestPyDecode(TestDecode, PyTest): pass
 class TestCDecode(TestDecode, CTest): pass
diff --git a/Modules/_json.c b/Modules/_json.c
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -1468,7 +1468,10 @@ scan_once_str(PyScannerObject *s, PyObje
     PyObject *res;
     char *str = PyString_AS_STRING(pystr);
     Py_ssize_t length = PyString_GET_SIZE(pystr);
-    if (idx >= length) {
+    if (idx < 0)
+        /* Compatibility with the Python version. */
+        idx += length;
+    if (idx < 0 || idx >= length) {
         PyErr_SetNone(PyExc_StopIteration);
         return NULL;
     }
@@ -1555,7 +1558,10 @@ scan_once_unicode(PyScannerObject *s, Py
     PyObject *res;
     Py_UNICODE *str = PyUnicode_AS_UNICODE(pystr);
     Py_ssize_t length = PyUnicode_GET_SIZE(pystr);
-    if (idx >= length) {
+    if (idx < 0)
+        /* Compatibility with Python version. */
+        idx += length;
+    if (idx < 0 || idx >= length) {
         PyErr_SetNone(PyExc_StopIteration);
         return NULL;
     }