bpf: Fix [::] -> [::1] rewrite in sys_sendmsg

sys_sendmsg has supported unspecified destination IPv6 (wildcard) for
unconnected UDP sockets since 876c7f41. When [::] is passed by user as
destination, sys_sendmsg rewrites it with [::1] to be consistent with
BSD (see "BSD'ism" comment in the code).

This didn't work when cgroup-bpf was enabled though since the rewrite
[::] -> [::1] happened before passing control to cgroup-bpf block where
fl6.daddr was updated with passed by user sockaddr_in6.sin6_addr (that
might or might not be changed by BPF program). That way if user passed
[::] as dst IPv6 it was first rewritten with [::1] by original code from
876c7f41, but then rewritten back with [::] by cgroup-bpf block.

It happened even when BPF_CGROUP_UDP6_SENDMSG program was not present
(CONFIG_CGROUP_BPF=y was enough).

The fix is to apply BSD'ism after cgroup-bpf block so that [::] is
replaced with [::1] no matter where it came from: passed by user to
sys_sendmsg or set by BPF_CGROUP_UDP6_SENDMSG program.

Fixes: 1cedee13 ("bpf: Hooks for sys_sendmsg")
Reported-by: Nitin Rawat <nitin.rawat@intel.com>
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/ipv6/udp.c

@@ -1390,10 +1390,7 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	ipc6.opt = opt;
 
 	fl6.flowi6_proto = sk->sk_protocol;
-	if (!ipv6_addr_any(daddr))
-		fl6.daddr = *daddr;
-	else
-		fl6.daddr.s6_addr[15] = 0x1; /* :: means loopback (BSD'ism) */
+	fl6.daddr = *daddr;
 	if (ipv6_addr_any(&fl6.saddr) && !ipv6_addr_any(&np->saddr))
 		fl6.saddr = np->saddr;
 	fl6.fl6_sport = inet->inet_sport;
@@ -1421,6 +1418,9 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		}
 	}
 
+	if (ipv6_addr_any(&fl6.daddr))
+		fl6.daddr.s6_addr[15] = 0x1; /* :: means loopback (BSD'ism) */
+
 	final_p = fl6_update_dst(&fl6, opt, &final);
 	if (final_p)
 		connected = false;