tunnel4: add cb_handler to struct xfrm_tunnel

This patch is to register a callback function tunnel4_rcv_cb with
is_ipip set in a xfrm_input_afinfo object for tunnel4 and tunnel64.

It will be called by xfrm_rcv_cb() from xfrm_input() when family
is AF_INET and proto is IPPROTO_IPIP or IPPROTO_IPV6.

v1->v2:
  - Fix a sparse warning caused by the missing "__rcu", as Jakub
    noticed.
  - Handle the err returned by xfrm_input_register_afinfo() in
    tunnel4_init/fini(), as Sabrina noticed.
v2->v3:
  - Add "#if IS_ENABLED(CONFIG_INET_XFRM_TUNNEL)" to fix the build error
    when xfrm is disabled, reported by kbuild test robot.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/net/xfrm.h

@@ -1416,6 +1416,7 @@ struct xfrm6_protocol {
 /* XFRM tunnel handlers.  */
 struct xfrm_tunnel {
 	int (*handler)(struct sk_buff *skb);
+	int (*cb_handler)(struct sk_buff *skb, int err);
 	int (*err_handler)(struct sk_buff *skb, u32 info);
 
 	struct xfrm_tunnel __rcu *next;

net/ipv4/tunnel4.c

@@ -110,6 +110,33 @@ static int tunnel4_rcv(struct sk_buff *skb)
 	return 0;
 }
 
+#if IS_ENABLED(CONFIG_INET_XFRM_TUNNEL)
+static int tunnel4_rcv_cb(struct sk_buff *skb, u8 proto, int err)
+{
+	struct xfrm_tunnel __rcu *head;
+	struct xfrm_tunnel *handler;
+	int ret;
+
+	head = (proto == IPPROTO_IPIP) ? tunnel4_handlers : tunnel64_handlers;
+
+	for_each_tunnel_rcu(head, handler) {
+		if (handler->cb_handler) {
+			ret = handler->cb_handler(skb, err);
+			if (ret <= 0)
+				return ret;
+		}
+	}
+
+	return 0;
+}
+
+static const struct xfrm_input_afinfo tunnel4_input_afinfo = {
+	.family		=	AF_INET,
+	.is_ipip	=	true,
+	.callback	=	tunnel4_rcv_cb,
+};
+#endif
+
 #if IS_ENABLED(CONFIG_IPV6)
 static int tunnel64_rcv(struct sk_buff *skb)
 {
@@ -230,6 +257,18 @@ static int __init tunnel4_init(void)
 #endif
 		goto err;
 	}
+#endif
+#if IS_ENABLED(CONFIG_INET_XFRM_TUNNEL)
+	if (xfrm_input_register_afinfo(&tunnel4_input_afinfo)) {
+		inet_del_protocol(&tunnel4_protocol, IPPROTO_IPIP);
+#if IS_ENABLED(CONFIG_IPV6)
+		inet_del_protocol(&tunnel64_protocol, IPPROTO_IPV6);
+#endif
+#if IS_ENABLED(CONFIG_MPLS)
+		inet_del_protocol(&tunnelmpls4_protocol, IPPROTO_MPLS);
+#endif
+		goto err;
+	}
 #endif
 	return 0;
 
@@ -240,6 +279,10 @@ static int __init tunnel4_init(void)
 
 static void __exit tunnel4_fini(void)
 {
+#if IS_ENABLED(CONFIG_INET_XFRM_TUNNEL)
+	if (xfrm_input_unregister_afinfo(&tunnel4_input_afinfo))
+		pr_err("tunnel4 close: can't remove input afinfo\n");
+#endif
 #if IS_ENABLED(CONFIG_MPLS)
 	if (inet_del_protocol(&tunnelmpls4_protocol, IPPROTO_MPLS))
 		pr_err("tunnelmpls4 close: can't remove protocol\n");