bpf: Allow sk_lookup with IPv6 module

This is a more complete fix than d71019b5 ("net: core: Fix build
with CONFIG_IPV6=m"), so that IPv6 sockets may be looked up if the IPv6
module is loaded (not just if it's compiled in).
Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/net/addrconf.h

@@ -265,6 +265,11 @@ extern const struct ipv6_stub *ipv6_stub __read_mostly;
 struct ipv6_bpf_stub {
 	int (*inet6_bind)(struct sock *sk, struct sockaddr *uaddr, int addr_len,
 			  bool force_bind_address_no_port, bool with_lock);
+	struct sock *(*udp6_lib_lookup)(struct net *net,
+					const struct in6_addr *saddr, __be16 sport,
+					const struct in6_addr *daddr, __be16 dport,
+					int dif, int sdif, struct udp_table *tbl,
+					struct sk_buff *skb);
 };
 extern const struct ipv6_bpf_stub *ipv6_bpf_stub __read_mostly;
 

net/core/filter.c

@@ -4717,7 +4717,7 @@ static struct sock *sk_lookup(struct net *net, struct bpf_sock_tuple *tuple,
 			sk = __udp4_lib_lookup(net, src4, tuple->ipv4.sport,
 					       dst4, tuple->ipv4.dport,
 					       dif, sdif, &udp_table, skb);
-#if IS_REACHABLE(CONFIG_IPV6)
+#if IS_ENABLED(CONFIG_IPV6)
 	} else {
 		struct in6_addr *src6 = (struct in6_addr *)&tuple->ipv6.saddr;
 		struct in6_addr *dst6 = (struct in6_addr *)&tuple->ipv6.daddr;
@@ -4728,10 +4728,12 @@ static struct sock *sk_lookup(struct net *net, struct bpf_sock_tuple *tuple,
 					    src6, tuple->ipv6.sport,
 					    dst6, tuple->ipv6.dport,
 					    dif, sdif, &refcounted);
-		else
-			sk = __udp6_lib_lookup(net, src6, tuple->ipv6.sport,
-					       dst6, tuple->ipv6.dport,
-					       dif, sdif, &udp_table, skb);
+		else if (likely(ipv6_bpf_stub))
+			sk = ipv6_bpf_stub->udp6_lib_lookup(net,
+							    src6, tuple->ipv6.sport,
+							    dst6, tuple->ipv6.dport,
+							    dif, sdif,
+							    &udp_table, skb);
 #endif
 	}
 

net/ipv6/af_inet6.c

@@ -901,6 +901,7 @@ static const struct ipv6_stub ipv6_stub_impl = {
 
 static const struct ipv6_bpf_stub ipv6_bpf_stub_impl = {
 	.inet6_bind = __inet6_bind,
+	.udp6_lib_lookup = __udp6_lib_lookup,
 };
 
 static int __init inet6_init(void)