bpo-36946: Fix possible signed integer overflow when handling slices. (GH-13375)

The final addition (cur += step) may overflow, so use size_t for "cur".
"cur" is always positive (even for negative steps), so it is safe to use
size_t here.
Co-Authored-By: Martin Panter <vadmium+py@gmail.com>

Lib/ctypes/test/test_arrays.py

@@ -69,6 +69,17 @@ class ArrayTestCase(unittest.TestCase):
         from operator import delitem
         self.assertRaises(TypeError, delitem, ca, 0)
 
+    def test_step_overflow(self):
+        a = (c_int * 5)()
+        a[3::sys.maxsize] = (1,)
+        self.assertListEqual(a[3::sys.maxsize], [1])
+        a = (c_char * 5)()
+        a[3::sys.maxsize] = b"A"
+        self.assertEqual(a[3::sys.maxsize], b"A")
+        a = (c_wchar * 5)()
+        a[3::sys.maxsize] = u"X"
+        self.assertEqual(a[3::sys.maxsize], u"X")
+
     def test_numeric_arrays(self):
 
         alen = 5

Lib/test/seq_tests.py

@@ -209,6 +209,7 @@ class CommonTest(unittest.TestCase):
         a = self.type2test([0,1,2,3,4])
         self.assertEqual(a[ -pow(2,128): 3 ], self.type2test([0,1,2]))
         self.assertEqual(a[ 3: pow(2,145) ], self.type2test([3,4]))
+        self.assertEqual(a[3::sys.maxsize], self.type2test([3]))
 
     def test_contains(self):
         u = self.type2test([0, 1, 2])

Lib/test/string_tests.py

@@ -1135,7 +1135,7 @@ class MixinStrUnicodeUserStringTest:
     def test_extended_getslice(self):
         # Test extended slicing by comparing with list slicing.
         s = string.ascii_letters + string.digits
-        indices = (0, None, 1, 3, 41, -1, -2, -37)
+        indices = (0, None, 1, 3, 41, sys.maxsize, -1, -2, -37)
         for start in indices:
             for stop in indices:
                 # Skip step 0 (invalid)

Lib/test/test_array.py

@@ -746,7 +746,7 @@ class BaseTest:
         # Test extended slicing by comparing with list slicing
         # (Assumes list conversion works correctly, too)
         a = array.array(self.typecode, self.example)
-        indices = (0, None, 1, 3, 19, 100, -1, -2, -31, -100)
+        indices = (0, None, 1, 3, 19, 100, sys.maxsize, -1, -2, -31, -100)
         for start in indices:
             for stop in indices:
                 # Everything except the initial 0 (invalid step)
@@ -844,7 +844,7 @@ class BaseTest:
         self.assertRaises(TypeError, a.__setitem__, slice(0, 1), b)
 
     def test_extended_set_del_slice(self):
-        indices = (0, None, 1, 3, 19, 100, -1, -2, -31, -100)
+        indices = (0, None, 1, 3, 19, 100, sys.maxsize, -1, -2, -31, -100)
         for start in indices:
             for stop in indices:
                 # Everything except the initial 0 (invalid step)

Lib/test/test_bytes.py

@@ -285,7 +285,7 @@ class BaseBytesTest:
         # Test extended slicing by comparing with list slicing.
         L = list(range(255))
         b = self.type2test(L)
-        indices = (0, None, 1, 3, 19, 100, -1, -2, -31, -100)
+        indices = (0, None, 1, 3, 19, 100, sys.maxsize, -1, -2, -31, -100)
         for start in indices:
             for stop in indices:
                 # Skip step 0 (invalid)
@@ -1242,7 +1242,8 @@ class ByteArrayTest(BaseBytesTest, unittest.TestCase):
         self.assertLessEqual(sys.getsizeof(b), size)
 
     def test_extended_set_del_slice(self):
-        indices = (0, None, 1, 3, 19, 300, 1<<333, -1, -2, -31, -300)
+        indices = (0, None, 1, 3, 19, 300, 1<<333, sys.maxsize,
+            -1, -2, -31, -300)
         for start in indices:
             for stop in indices:
                 # Skip invalid step 0

Lib/test/test_mmap.py

@@ -439,7 +439,7 @@ class MmapTests(unittest.TestCase):
         m = mmap.mmap(-1, len(s))
         m[:] = s
         self.assertEqual(m[:], s)
-        indices = (0, None, 1, 3, 19, 300, -1, -2, -31, -300)
+        indices = (0, None, 1, 3, 19, 300, sys.maxsize, -1, -2, -31, -300)
         for start in indices:
             for stop in indices:
                 # Skip step 0 (invalid)
@@ -451,7 +451,7 @@ class MmapTests(unittest.TestCase):
         # Test extended slicing by comparing with list slicing.
         s = bytes(reversed(range(256)))
         m = mmap.mmap(-1, len(s))
-        indices = (0, None, 1, 3, 19, 300, -1, -2, -31, -300)
+        indices = (0, None, 1, 3, 19, 300, sys.maxsize, -1, -2, -31, -300)
         for start in indices:
             for stop in indices:
                 # Skip invalid step 0

Misc/NEWS.d/next/Core and Builtins/2019-05-16-23-53-45.bpo-36946.qjxr0Y.rst

+Fix possible signed integer overflow when handling slices.

Modules/_ctypes/_ctypes.c

@@ -4487,7 +4487,8 @@ Array_subscript(PyObject *myself, PyObject *item)
         StgDictObject *stgdict, *itemdict;
         PyObject *proto;
         PyObject *np;
-        Py_ssize_t start, stop, step, slicelen, cur, i;
+        Py_ssize_t start, stop, step, slicelen, i;
+        size_t cur;
 
         if (PySlice_Unpack(item, &start, &stop, &step) < 0) {
             return NULL;
@@ -4627,7 +4628,8 @@ Array_ass_subscript(PyObject *myself, PyObject *item, PyObject *value)
         return Array_ass_item(myself, i, value);
     }
     else if (PySlice_Check(item)) {
-        Py_ssize_t start, stop, step, slicelen, otherlen, i, cur;
+        Py_ssize_t start, stop, step, slicelen, otherlen, i;
+        size_t cur;
 
         if (PySlice_Unpack(item, &start, &stop, &step) < 0) {
             return -1;

Modules/_elementtree.c

@@ -1809,7 +1809,8 @@ element_subscr(PyObject* self_, PyObject* item)
         return element_getitem(self_, i);
     }
     else if (PySlice_Check(item)) {
-        Py_ssize_t start, stop, step, slicelen, cur, i;
+        Py_ssize_t start, stop, step, slicelen, i;
+        size_t cur;
         PyObject* list;
 
         if (!self->extra)
@@ -1861,7 +1862,8 @@ element_ass_subscr(PyObject* self_, PyObject* item, PyObject* value)
         return element_setitem(self_, i, value);
     }
     else if (PySlice_Check(item)) {
-        Py_ssize_t start, stop, step, slicelen, newlen, cur, i;
+        Py_ssize_t start, stop, step, slicelen, newlen, i;
+        size_t cur;
 
         PyObject* recycle = NULL;
         PyObject* seq;

Modules/arraymodule.c

@@ -2343,7 +2343,8 @@ array_subscr(arrayobject* self, PyObject* item)
         return array_item(self, i);
     }
     else if (PySlice_Check(item)) {
-        Py_ssize_t start, stop, step, slicelength, cur, i;
+        Py_ssize_t start, stop, step, slicelength, i;
+        size_t cur;
         PyObject* result;
         arrayobject* ar;
         int itemsize = self->ob_descr->itemsize;
@@ -2527,7 +2528,8 @@ array_ass_subscr(arrayobject* self, PyObject* item, PyObject* value)
         return 0;
     }
     else {
-        Py_ssize_t cur, i;
+        size_t cur;
+        Py_ssize_t i;
 
         if (needed != slicelength) {
             PyErr_Format(PyExc_ValueError,

Modules/mmapmodule.c

@@ -806,7 +806,8 @@ mmap_subscript(mmap_object *self, PyObject *item)
                                               slicelen);
         else {
             char *result_buf = (char *)PyMem_Malloc(slicelen);
-            Py_ssize_t cur, i;
+            size_t cur;
+            Py_ssize_t i;
             PyObject *result;
 
             if (result_buf == NULL)
@@ -926,7 +927,8 @@ mmap_ass_subscript(mmap_object *self, PyObject *item, PyObject *value)
             memcpy(self->data + start, vbuf.buf, slicelen);
         }
         else {
-            Py_ssize_t cur, i;
+            size_t cur;
+            Py_ssize_t i;
 
             for (cur = start, i = 0;
                  i < slicelen;

Objects/bytearrayobject.c

@@ -410,7 +410,8 @@ bytearray_subscript(PyByteArrayObject *self, PyObject *index)
         return PyLong_FromLong((unsigned char)(PyByteArray_AS_STRING(self)[i]));
     }
     else if (PySlice_Check(index)) {
-        Py_ssize_t start, stop, step, slicelength, cur, i;
+        Py_ssize_t start, stop, step, slicelength, i;
+        size_t cur;
         if (PySlice_Unpack(index, &start, &stop, &step) < 0) {
             return NULL;
         }

Objects/bytesobject.c

@@ -1677,7 +1677,8 @@ bytes_subscript(PyBytesObject* self, PyObject* item)
         return PyLong_FromLong((unsigned char)self->ob_sval[i]);
     }
     else if (PySlice_Check(item)) {
-        Py_ssize_t start, stop, step, slicelength, cur, i;
+        Py_ssize_t start, stop, step, slicelength, i;
+        size_t cur;
         char* source_buf;
         char* result_buf;
         PyObject* result;

Objects/tupleobject.c

@@ -753,7 +753,8 @@ tuplesubscript(PyTupleObject* self, PyObject* item)
         return tupleitem(self, i);
     }
     else if (PySlice_Check(item)) {
-        Py_ssize_t start, stop, step, slicelength, cur, i;
+        Py_ssize_t start, stop, step, slicelength, i;
+        size_t cur;
         PyObject* result;
         PyObject* it;
         PyObject **src, **dest;

Objects/unicodeobject.c

@@ -13991,7 +13991,8 @@ unicode_subscript(PyObject* self, PyObject* item)
             i += PyUnicode_GET_LENGTH(self);
         return unicode_getitem(self, i);
     } else if (PySlice_Check(item)) {
-        Py_ssize_t start, stop, step, slicelength, cur, i;
+        Py_ssize_t start, stop, step, slicelength, i;
+        size_t cur;
         PyObject *result;
         void *src_data, *dest_data;
         int src_kind, dest_kind;