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>
(cherry picked from commit 14514d9084a40f599c57da853a305aa264562a43)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>

4 files changed, 8 insertions, 4 deletions

diff --git a/Objects/bytearrayobject.c b/Objects/bytearrayobject.c
index 3f3e6bcf0c..8a0994ffa4 100644
--- a/Objects/bytearrayobject.c
+++ b/Objects/bytearrayobject.c
@@ -420,7 +420,8 @@ 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;
         }
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index a5319da48d..b4ba1a01ab 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -1673,7 +1673,8 @@ 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;
diff --git a/Objects/tupleobject.c b/Objects/tupleobject.c
index 9bb91a5e65..7ee06e20e4 100644
--- a/Objects/tupleobject.c
+++ b/Objects/tupleobject.c
@@ -730,7 +730,8 @@ 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;
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index e18937981b..ed1e4a4dd5 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -13989,7 +13989,8 @@ 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;