[Libguestfs] [PATCH v2 2/4] python: do not try to malloc 0 elements in get_all_event_callbacks
In case there are no event handlers registered with the handle,
get_all_event_callbacks will count 0 elements, trying to malloc a buffer
of that size. POSIX says that this can result in either a null pointer,
or an unusable pointer.
Short-circuit get_all_event_callbacks to allocate nothing when there are
no events, making sure to use its results only when there were events.
---
python/handle.c | 22 +++++++++++++++++-----
1 file changed, 17 insertions(+), 5 deletions(-)
diff --git a/python/handle.c b/python/handle.c
index f4cc8ca..806408f 100644
--- a/python/handle.c
+++ b/python/handle.c
@@ -71,7 +71,7 @@ guestfs_int_py_close (PyObject *self, PyObject *args)
PyThreadState *py_save = NULL;
PyObject *py_g;
guestfs_h *g;
- size_t i, len;
+ size_t len;
PyObject **callbacks;
if (!PyArg_ParseTuple (args, (char *) "O:guestfs_close", &py_g))
@@ -81,9 +81,14 @@ guestfs_int_py_close (PyObject *self, PyObject *args)
/* As in the OCaml bindings, there is a hard to solve case where the
* caller can delete a callback from within the callback, resulting
* in a double-free here. XXX
+ *
+ * Take care of the result of get_all_event_callbacks: NULL can be
+ * both an error (and some PyErr_* was called), and no events.
+ * 'len' is specifically 0 only in the latter case, so filter that
+ * out.
*/
callbacks = get_all_event_callbacks (g, &len);
- if (callbacks == NULL)
+ if (len != 0 && callbacks == NULL)
return NULL;
if (PyEval_ThreadsInitialized ())
@@ -92,9 +97,12 @@ guestfs_int_py_close (PyObject *self, PyObject *args)
if (PyEval_ThreadsInitialized ())
PyEval_RestoreThread (py_save);
- for (i = 0; i < len; ++i)
- Py_XDECREF (callbacks[i]);
- free (callbacks);
+ if (len > 0) {
+ size_t i;
+ for (i = 0; i < len; ++i)
+ Py_XDECREF (callbacks[i]);
+ free (callbacks);
+ }
Py_INCREF (Py_None);
return Py_None;
@@ -260,6 +268,10 @@ get_all_event_callbacks (guestfs_h *g, size_t *len_rtn)
cb = guestfs_next_private (g, &key);
}
+ /* No events, so no need to allocate anything. */
+ if (*len_rtn == 0)
+ return NULL;
+
/* Copy them into the return array. */
r = malloc (sizeof (PyObject *) * (*len_rtn));
if (r == NULL) {
--
2.9.3