bpf: Allow to retrieve sol_socket opts from sock_addr progs

The _bpf_setsockopt() is able to set some of the SOL_SOCKET level options,
however, _bpf_getsockopt() has little support to actually retrieve them.
This small patch adds few misc options such as SO_MARK, SO_PRIORITY and
SO_BINDTOIFINDEX. For the latter getter and setter are added. The mark and
priority in particular allow to retrieve the options from BPF cgroup hooks
to then implement custom behavior / settings on the syscall hooks compared
to other sockets that stick to the defaults, for example.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/cba44439b801e5ddc1170e5be787f4dc93a2d7f9.1610406333.git.daniel@iogearbox.net

net/core/filter.c

@@ -4770,6 +4770,10 @@ static int _bpf_setsockopt(struct sock *sk, int level, int optname,
 				ifindex = dev->ifindex;
 				dev_put(dev);
 			}
+			fallthrough;
+		case SO_BINDTOIFINDEX:
+			if (optname == SO_BINDTOIFINDEX)
+				ifindex = val;
 			ret = sock_bindtoindex(sk, ifindex, false);
 			break;
 		case SO_KEEPALIVE:
@@ -4932,8 +4936,25 @@ static int _bpf_getsockopt(struct sock *sk, int level, int optname,
 
 	sock_owned_by_me(sk);
 
+	if (level == SOL_SOCKET) {
+		if (optlen != sizeof(int))
+			goto err_clear;
+
+		switch (optname) {
+		case SO_MARK:
+			*((int *)optval) = sk->sk_mark;
+			break;
+		case SO_PRIORITY:
+			*((int *)optval) = sk->sk_priority;
+			break;
+		case SO_BINDTOIFINDEX:
+			*((int *)optval) = sk->sk_bound_dev_if;
+			break;
+		default:
+			goto err_clear;
+		}
 #ifdef CONFIG_INET
-	if (level == SOL_TCP && sk->sk_prot->getsockopt == tcp_getsockopt) {
+	} else if (level == SOL_TCP && sk->sk_prot->getsockopt == tcp_getsockopt) {
 		struct inet_connection_sock *icsk;
 		struct tcp_sock *tp;
 
@@ -4986,12 +5007,12 @@ static int _bpf_getsockopt(struct sock *sk, int level, int optname,
 		default:
 			goto err_clear;
 		}
+#endif
 #endif
 	} else {
 		goto err_clear;
 	}
 	return 0;
-#endif
 err_clear:
 	memset(optval, 0, optlen);
 	return -EINVAL;