Issue #13848: open() and the FileIO constructor now check for NUL characters in the file name.

Patch by Hynek Schlawack.

Include/unicodeobject.h

@@ -1914,6 +1914,12 @@ PyAPI_FUNC(int) PyUnicode_Contains(
     PyObject *element           /* Element string */
     );
 
+/* Checks whether the string contains any NUL characters. */
+
+#ifndef Py_LIMITED_API
+PyAPI_FUNC(int) _PyUnicode_HasNULChars(PyObject *);
+#endif
+
 /* Checks whether argument is a valid identifier. */
 
 PyAPI_FUNC(int) PyUnicode_IsIdentifier(PyObject *s);

Lib/test/test_fileio.py

@@ -306,6 +306,11 @@ class OtherFileTests(unittest.TestCase):
         finally:
             os.unlink(TESTFN)
 
+    def testConstructorHandlesNULChars(self):
+        fn_with_NUL = 'foo\0bar'
+        self.assertRaises(TypeError, _FileIO, fn_with_NUL, 'w')
+        self.assertRaises(TypeError, _FileIO, bytes(fn_with_NUL, 'ascii'), 'w')
+
     def testInvalidFd(self):
         self.assertRaises(ValueError, _FileIO, -10)
         self.assertRaises(OSError, _FileIO, make_bad_fd())

Lib/test/test_io.py

@@ -363,6 +363,11 @@ class IOTest(unittest.TestCase):
             self.assertRaises(exc, fp.seek, 1, self.SEEK_CUR)
             self.assertRaises(exc, fp.seek, -1, self.SEEK_END)
 
+    def test_open_handles_NUL_chars(self):
+        fn_with_NUL = 'foo\0bar'
+        self.assertRaises(TypeError, self.open, fn_with_NUL, 'w')
+        self.assertRaises(TypeError, self.open, bytes(fn_with_NUL, 'ascii'), 'w')
+
     def test_raw_file_io(self):
         with self.open(support.TESTFN, "wb", buffering=0) as f:
             self.assertEqual(f.readable(), False)

Misc/NEWS

@@ -461,6 +461,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #13848: open() and the FileIO constructor now check for NUL
+  characters in the file name.  Patch by Hynek Schlawack.
+
 - Issue #13806: The size check in audioop decompression functions was too
   strict and could reject valid compressed data.  Patch by Oleg Plakhotnyuk.
 

Modules/_io/fileio.c

@@ -258,6 +258,12 @@ fileio_init(PyObject *oself, PyObject *args, PyObject *kwds)
 
 #ifdef MS_WINDOWS
     if (PyUnicode_Check(nameobj)) {
+        int rv = _PyUnicode_HasNULChars(nameobj);
+        if (rv) {
+            if (rv != -1)
+                PyErr_SetString(PyExc_TypeError, "embedded NUL character");
+            return -1;
+        }
         widename = PyUnicode_AsUnicode(nameobj);
         if (widename == NULL)
             return -1;
@@ -265,28 +271,10 @@ fileio_init(PyObject *oself, PyObject *args, PyObject *kwds)
 #endif
     if (fd < 0)
     {
-        if (PyBytes_Check(nameobj) || PyByteArray_Check(nameobj)) {
-            Py_ssize_t namelen;
-            if (PyObject_AsCharBuffer(nameobj, &name, &namelen) < 0)
-                return -1;
-        }
-        else {
-            PyObject *u = PyUnicode_FromObject(nameobj);
-
-            if (u == NULL)
-                return -1;
-
-            stringobj = PyUnicode_EncodeFSDefault(u);
-            Py_DECREF(u);
-            if (stringobj == NULL)
-                return -1;
-            if (!PyBytes_Check(stringobj)) {
-                PyErr_SetString(PyExc_TypeError,
-                                "encoder failed to return bytes");
-                goto error;
-            }
-            name = PyBytes_AS_STRING(stringobj);
+        if (!PyUnicode_FSConverter(nameobj, &stringobj)) {
+            return -1;
         }
+        name = PyBytes_AS_STRING(stringobj);
     }
 
     s = mode;

Objects/unicodeobject.c

@@ -3574,6 +3574,19 @@ PyUnicode_DecodeFSDefaultAndSize(const char *s, Py_ssize_t size)
 }
 
 
+int
+_PyUnicode_HasNULChars(PyObject* s)
+{
+    static PyObject *nul = NULL;
+
+    if (nul == NULL)
+        nul = PyUnicode_FromStringAndSize("\0", 1);
+    if (nul == NULL)
+        return -1;
+    return PyUnicode_Contains(s, nul);
+}
+
+
 int
 PyUnicode_FSConverter(PyObject* arg, void* addr)
 {