inet: implement lockless getsockopt(IP_MULTICAST_IF)

Add missing annotations to inet->mc_index and inet->mc_addr
to fix data-races.

getsockopt(IP_MULTICAST_IF) can be lockless.

setsockopt() side is left for later.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/datagram.c

@@ -39,9 +39,9 @@ int __ip4_datagram_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len
 	saddr = inet->inet_saddr;
 	if (ipv4_is_multicast(usin->sin_addr.s_addr)) {
 		if (!oif || netif_index_is_l3_master(sock_net(sk), oif))
-			oif = inet->mc_index;
+			oif = READ_ONCE(inet->mc_index);
 		if (!saddr)
-			saddr = inet->mc_addr;
+			saddr = READ_ONCE(inet->mc_addr);
 	} else if (!oif) {
 		oif = READ_ONCE(inet->uc_index);
 	}

net/ipv4/ip_sockglue.c

@@ -1168,8 +1168,8 @@ int do_ip_setsockopt(struct sock *sk, int level, int optname,
 
 		if (!mreq.imr_ifindex) {
 			if (mreq.imr_address.s_addr == htonl(INADDR_ANY)) {
-				inet->mc_index = 0;
-				inet->mc_addr  = 0;
+				WRITE_ONCE(inet->mc_index, 0);
+				WRITE_ONCE(inet->mc_addr, 0);
 				err = 0;
 				break;
 			}
@@ -1194,8 +1194,8 @@ int do_ip_setsockopt(struct sock *sk, int level, int optname,
 		    midx != sk->sk_bound_dev_if)
 			break;
 
-		inet->mc_index = mreq.imr_ifindex;
-		inet->mc_addr  = mreq.imr_address.s_addr;
+		WRITE_ONCE(inet->mc_index, mreq.imr_ifindex);
+		WRITE_ONCE(inet->mc_addr, mreq.imr_address.s_addr);
 		err = 0;
 		break;
 	}
@@ -1673,19 +1673,11 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 	case IP_UNICAST_IF:
 		val = (__force int)htonl((__u32) READ_ONCE(inet->uc_index));
 		goto copyval;
-	}
-
-	if (needs_rtnl)
-		rtnl_lock();
-	sockopt_lock_sock(sk);
-
-	switch (optname) {
 	case IP_MULTICAST_IF:
 	{
 		struct in_addr addr;
 		len = min_t(unsigned int, len, sizeof(struct in_addr));
-		addr.s_addr = inet->mc_addr;
-		sockopt_release_sock(sk);
+		addr.s_addr = READ_ONCE(inet->mc_addr);
 
 		if (copy_to_sockptr(optlen, &len, sizeof(int)))
 			return -EFAULT;
@@ -1693,6 +1685,13 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 			return -EFAULT;
 		return 0;
 	}
+	}
+
+	if (needs_rtnl)
+		rtnl_lock();
+	sockopt_lock_sock(sk);
+
+	switch (optname) {
 	case IP_MSFILTER:
 	{
 		struct ip_msfilter msf;

net/ipv4/ping.c

@@ -773,9 +773,9 @@ static int ping_v4_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 
 	if (ipv4_is_multicast(daddr)) {
 		if (!ipc.oif || netif_index_is_l3_master(sock_net(sk), ipc.oif))
-			ipc.oif = inet->mc_index;
+			ipc.oif = READ_ONCE(inet->mc_index);
 		if (!saddr)
-			saddr = inet->mc_addr;
+			saddr = READ_ONCE(inet->mc_addr);
 	} else if (!ipc.oif)
 		ipc.oif = READ_ONCE(inet->uc_index);
 

net/ipv4/raw.c

@@ -579,9 +579,9 @@ static int raw_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	uc_index = READ_ONCE(inet->uc_index);
 	if (ipv4_is_multicast(daddr)) {
 		if (!ipc.oif || netif_index_is_l3_master(sock_net(sk), ipc.oif))
-			ipc.oif = inet->mc_index;
+			ipc.oif = READ_ONCE(inet->mc_index);
 		if (!saddr)
-			saddr = inet->mc_addr;
+			saddr = READ_ONCE(inet->mc_addr);
 	} else if (!ipc.oif) {
 		ipc.oif = uc_index;
 	} else if (ipv4_is_lbcast(daddr) && uc_index) {

net/ipv4/udp.c

@@ -1177,9 +1177,9 @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	uc_index = READ_ONCE(inet->uc_index);
 	if (ipv4_is_multicast(daddr)) {
 		if (!ipc.oif || netif_index_is_l3_master(sock_net(sk), ipc.oif))
-			ipc.oif = inet->mc_index;
+			ipc.oif = READ_ONCE(inet->mc_index);
 		if (!saddr)
-			saddr = inet->mc_addr;
+			saddr = READ_ONCE(inet->mc_addr);
 		connected = 0;
 	} else if (!ipc.oif) {
 		ipc.oif = uc_index;