[Libguestfs] [PATCH 09/11] python: do not try to malloc 0 elements in get_all_event_callbacks

Pino Toscano
Fri, 03 Mar 2017 14:33:03 -0000

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.  Since we assume a null pointer means failure,
then always add a null element at the end, so we do not rely on
implementation-defined behaviour of malloc.

The output parameter 'len_rtn' already keeps the number of valid items
in the returned array, so there are no behaviour changes for callers of
get_all_event_callbacks.
---
 python/handle.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/python/handle.c b/python/handle.c
index f4cc8ca..ef2fdd8 100644
--- a/python/handle.c
+++ b/python/handle.c
@@ -261,7 +261,7 @@ get_all_event_callbacks (guestfs_h *g, size_t *len_rtn)
   }

   /* Copy them into the return array. */
-  r = malloc (sizeof (PyObject *) * (*len_rtn));
+  r = malloc (sizeof (PyObject *) * (*len_rtn + 1));
   if (r == NULL) {
     PyErr_SetNone (PyExc_MemoryError);
     return NULL;
@@ -276,6 +276,7 @@ get_all_event_callbacks (guestfs_h *g, size_t *len_rtn)
     }
     cb = guestfs_next_private (g, &key);
   }
+  r[i] = NULL;

   return r;
 }
--
2.9.3