netdev: Reject empty names in netdev_open().

The empty string is not a valid name for a network device.  I would have
expected that each of the netdev provider implementations would reject an
empty string, but there was a special case for Linux tap devices where they
instead caused unexpected behavior.  This commit should fix the problem for
those devices and every other kind.

Reported-by: Gabor Locsei <[email protected]>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-discuss/2017-February/043613.html
Signed-off-by: Ben Pfaff <[email protected]>
Acked-by: Girish Moodalbail <[email protected]>
Acked-by: Andy Zhou <[email protected]>

lib/netdev.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2016 Nicira, Inc.
+ * Copyright (c) 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2016, 2017 Nicira, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -343,6 +343,14 @@ netdev_open(const char *name, const char *type, struct netdev **netdevp)
     struct netdev *netdev;
     int error;
 
+    if (!name[0]) {
+        /* Reject empty names.  This saves the providers having to do this.  At
+         * least one screwed this up: the netdev-linux "tap" implementation
+         * passed the name directly to the Linux TUNSETIFF call, which treats
+         * an empty string as a request to generate a unique name. */
+        return EINVAL;
+    }
+
     netdev_initialize();
 
     ovs_mutex_lock(&netdev_mutex);