diff -r 46567fda0b29 Objects/bytearrayobject.c
--- a/Objects/bytearrayobject.c	Sat Jul 09 11:05:42 2016 +0200
+++ b/Objects/bytearrayobject.c	Sun Jul 10 21:55:33 2016 +0800
@@ -246,7 +246,6 @@
 PyObject *
 PyByteArray_Concat(PyObject *a, PyObject *b)
 {
-    Py_ssize_t size;
     Py_buffer va, vb;
     PyByteArrayObject *result = NULL;
 
@@ -259,13 +258,13 @@
             goto done;
     }
 
-    size = va.len + vb.len;
-    if (size < 0) {
-            PyErr_NoMemory();
-            goto done;
+    if (va.len > PY_SSIZE_T_MAX - vb.len) {
+        PyErr_NoMemory();
+        goto done;
     }
 
-    result = (PyByteArrayObject *) PyByteArray_FromStringAndSize(NULL, size);
+    result = (PyByteArrayObject *) \
+        PyByteArray_FromStringAndSize(NULL, va.len + vb.len);
     if (result != NULL) {
         memcpy(result->ob_bytes, va.buf, va.len);
         memcpy(result->ob_bytes + va.len, vb.buf, vb.len);
diff -r 46567fda0b29 Objects/bytesobject.c
--- a/Objects/bytesobject.c	Sat Jul 09 11:05:42 2016 +0200
+++ b/Objects/bytesobject.c	Sun Jul 10 21:55:33 2016 +0800
@@ -1388,7 +1388,6 @@
 static PyObject *
 bytes_concat(PyObject *a, PyObject *b)
 {
-    Py_ssize_t size;
     Py_buffer va, vb;
     PyObject *result = NULL;
 
@@ -1413,13 +1412,12 @@
         goto done;
     }
 
-    size = va.len + vb.len;
-    if (size < 0) {
+    if (va.len > PY_SSIZE_T_MAX - vb.len) {
         PyErr_NoMemory();
         goto done;
     }
 
-    result = PyBytes_FromStringAndSize(NULL, size);
+    result = PyBytes_FromStringAndSize(NULL, va.len + vb.len);
     if (result != NULL) {
         memcpy(PyBytes_AS_STRING(result), va.buf, va.len);
         memcpy(PyBytes_AS_STRING(result) + va.len, vb.buf, vb.len);