netfilter: Factor out the hook list selection from nf_register_hook

- Add a new function find_nf_hook_list to select the nf_hook_list

- Fail nf_register_hook when asked for a per netdevice hook list when
  support for per netdevice hook lists is not built into the kernel.

- Move the hook list head selection outside of nf_hook_mutex as
  nothing in the selection requires the hook list, and error handling
  is simpler if a mutex is not held.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/core.c

@@ -62,27 +62,31 @@ EXPORT_SYMBOL(nf_hooks_needed);
 
 static DEFINE_MUTEX(nf_hook_mutex);
 
-int nf_register_hook(struct nf_hook_ops *reg)
+static struct list_head *find_nf_hook_list(const struct nf_hook_ops *reg)
 {
-	struct list_head *nf_hook_list;
-	struct nf_hook_ops *elem;
+	struct list_head *nf_hook_list = NULL;
 
-	mutex_lock(&nf_hook_mutex);
-	switch (reg->pf) {
-	case NFPROTO_NETDEV:
+	if (reg->pf != NFPROTO_NETDEV)
+		nf_hook_list = &nf_hooks[reg->pf][reg->hooknum];
+	else if (reg->hooknum == NF_NETDEV_INGRESS) {
 #ifdef CONFIG_NETFILTER_INGRESS
-		if (reg->hooknum == NF_NETDEV_INGRESS) {
-			BUG_ON(reg->dev == NULL);
+		if (reg->dev)
 			nf_hook_list = &reg->dev->nf_hooks_ingress;
-			break;
-		}
 #endif
-		/* Fall through. */
-	default:
-		nf_hook_list = &nf_hooks[reg->pf][reg->hooknum];
-		break;
 	}
+	return nf_hook_list;
+}
+
+int nf_register_hook(struct nf_hook_ops *reg)
+{
+	struct list_head *nf_hook_list;
+	struct nf_hook_ops *elem;
 
+	nf_hook_list = find_nf_hook_list(reg);
+	if (!nf_hook_list)
+		return -ENOENT;
+
+	mutex_lock(&nf_hook_mutex);
 	list_for_each_entry(elem, nf_hook_list, list) {
 		if (reg->priority < elem->priority)
 			break;