Bytes objects pickled in 3.x with protocol <=2 are unpickled incorrectly in 2.x

Modules/_pickle.c

 #include "Python.h"
 #include "structmember.h"
 
 PyDoc_STRVAR(pickle_module_doc,
 "Optimized C implementation for the Python pickle module.");
 
 /* Bump this when new opcodes are added to the pickle protocol. */
 enum {
     HIGHEST_PROTOCOL = 3,
     DEFAULT_PROTOCOL = 3
@@ skipping 1692 matching lines @@
 }
 
 static int
 save_bytes(PicklerObject *self, PyObject *obj)
 {
     if (self->proto < 3) {
         /* Older pickle protocols do not have an opcode for pickling bytes
            objects. Therefore, we need to fake the copy protocol (i.e.,
            the __reduce__ method) to permit bytes object unpickling. */
         PyObject *reduce_value = NULL;
-        PyObject *bytelist = NULL;
+        PyObject *bytearray = NULL;
         int status;
 
-        bytelist = PySequence_List(obj);
-        if (bytelist == NULL)
+        bytearray = PyByteArray_FromObject(obj);
+        if (bytearray == NULL)
             return -1;
 
         reduce_value = Py_BuildValue("(O(O))", (PyObject *)&PyBytes_Type,
-                                     bytelist);
+                                     bytearray);
         if (reduce_value == NULL) {
-            Py_DECREF(bytelist);
+            Py_DECREF(bytearray);
             return -1;
         }
 
         /* save_reduce() will memoize the object automatically. */
         status = save_reduce(self, reduce_value, obj);
         Py_DECREF(reduce_value);
-        Py_DECREF(bytelist);
+        Py_DECREF(bytearray);
         return status;
     }
     else {
         Py_ssize_t size;
         char header[5];
         Py_ssize_t len;
 
         size = PyBytes_Size(obj);
         if (size < 0)
             return -1;
@@ skipping 4652 matching lines @@
     if (PyModule_AddObject(m, "PicklingError", PicklingError) < 0)
         return NULL;
     if (PyModule_AddObject(m, "UnpicklingError", UnpicklingError) < 0)
         return NULL;
 
     if (initmodule() < 0)
         return NULL;
 
     return m;
 }