netlink: change return-value logic of netlink_broadcast()

Currently, netlink_broadcast() reports errors to the caller if no
messages at all were delivered:

1) If, at least, one message has been delivered correctly, returns 0.
2) Otherwise, if no messages at all were delivered due to skb_clone()
   failure, return -ENOBUFS.
3) Otherwise, if there are no listeners, return -ESRCH.

With this patch, the caller knows if the delivery of any of the
messages to the listeners have failed:

1) If it fails to deliver any message (for whatever reason), return
   -ENOBUFS.
2) Otherwise, if all messages were delivered OK, returns 0.
3) Otherwise, if no listeners, return -ESRCH.

In the current ctnetlink code and in Netfilter in general, we can add
reliable logging and connection tracking event delivery by dropping the
packets whose events were not successfully delivered over Netlink. Of
course, this option would be settable via /proc as this approach reduces
performance (in terms of filtered connections per seconds by a stateful
firewall) but providing reliable logging and event delivery (for
conntrackd) in return.

This patch also changes some clients of netlink_broadcast() that
may report ENOBUFS errors via printk. This error handling is not
of any help. Instead, the userspace daemons that are listening to
those netlink messages should resync themselves with the kernel-side
if they hit ENOBUFS.

BTW, netlink_broadcast() clients include those that call
cn_netlink_send(), nlmsg_multicast() and genlmsg_multicast() since they
internally call netlink_broadcast() and return its error value.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 10 deletions

diff --git a/drivers/scsi/scsi_transport_iscsi.c b/drivers/scsi/scsi_transport_iscsi.c
index 75c9297694cb..2adfab8c11c1 100644
--- a/drivers/scsi/scsi_transport_iscsi.c
+++ b/drivers/scsi/scsi_transport_iscsi.c
@@ -966,15 +966,7 @@ iscsi_if_transport_lookup(struct iscsi_transport *tt)
 static int
 iscsi_broadcast_skb(struct sk_buff *skb, gfp_t gfp)
 {
-	int rc;
-
-	rc = netlink_broadcast(nls, skb, 0, 1, gfp);
-	if (rc < 0) {
-		printk(KERN_ERR "iscsi: can not broadcast skb (%d)\n", rc);
-		return rc;
-	}
-
-	return 0;
+	return netlink_broadcast(nls, skb, 0, 1, gfp);
 }
 
 static int
@@ -1207,7 +1199,7 @@ int iscsi_session_event(struct iscsi_cls_session *session,
 	 * the user and when the daemon is restarted it will handle it
 	 */
 	rc = iscsi_broadcast_skb(skb, GFP_KERNEL);
-	if (rc < 0)
+	if (rc == -ESRCH)
 		iscsi_cls_session_printk(KERN_ERR, session,
 					 "Cannot notify userspace of session "
 					 "event %u. Check iscsi daemon\n",