inet: implement lockless IP_MULTICAST_TTL

inet->mc_ttl can be read locklessly.

Implement proper lockless reads and writes to inet->mc_ttl
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/ip_output.c

@@ -1430,7 +1430,7 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
 	if (cork->ttl != 0)
 		ttl = cork->ttl;
 	else if (rt->rt_type == RTN_MULTICAST)
-		ttl = inet->mc_ttl;
+		ttl = READ_ONCE(inet->mc_ttl);
 	else
 		ttl = ip_select_ttl(inet, &rt->dst);
 

net/ipv4/ip_sockglue.c

@@ -1039,6 +1039,17 @@ int do_ip_setsockopt(struct sock *sk, int level, int optname,
 
 		WRITE_ONCE(inet->min_ttl, val);
 		return 0;
+	case IP_MULTICAST_TTL:
+		if (sk->sk_type == SOCK_STREAM)
+			return -EINVAL;
+		if (optlen < 1)
+			return -EINVAL;
+		if (val == -1)
+			val = 1;
+		if (val < 0 || val > 255)
+			return -EINVAL;
+		WRITE_ONCE(inet->mc_ttl, val);
+		return 0;
 	}
 
 	err = 0;
@@ -1101,17 +1112,6 @@ int do_ip_setsockopt(struct sock *sk, int level, int optname,
 			goto e_inval;
 		inet->pmtudisc = val;
 		break;
-	case IP_MULTICAST_TTL:
-		if (sk->sk_type == SOCK_STREAM)
-			goto e_inval;
-		if (optlen < 1)
-			goto e_inval;
-		if (val == -1)
-			val = 1;
-		if (val < 0 || val > 255)
-			goto e_inval;
-		inet->mc_ttl = val;
-		break;
 	case IP_UNICAST_IF:
 	{
 		struct net_device *dev = NULL;
@@ -1592,6 +1592,9 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 	case IP_MINTTL:
 		val = READ_ONCE(inet->min_ttl);
 		goto copyval;
+	case IP_MULTICAST_TTL:
+		val = READ_ONCE(inet->mc_ttl);
+		goto copyval;
 	}
 
 	if (needs_rtnl)
@@ -1649,9 +1652,6 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 		}
 		break;
 	}
-	case IP_MULTICAST_TTL:
-		val = inet->mc_ttl;
-		break;
 	case IP_UNICAST_IF:
 		val = (__force int)htonl((__u32) inet->uc_index);
 		break;
@@ -1718,7 +1718,8 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 			put_cmsg(&msg, SOL_IP, IP_PKTINFO, sizeof(info), &info);
 		}
 		if (inet_test_bit(TTL, sk)) {
-			int hlim = inet->mc_ttl;
+			int hlim = READ_ONCE(inet->mc_ttl);
+
 			put_cmsg(&msg, SOL_IP, IP_TTL, sizeof(hlim), &hlim);
 		}
 		if (inet_test_bit(TOS, sk)) {

net/netfilter/ipvs/ip_vs_sync.c

@@ -1316,7 +1316,7 @@ static void set_mcast_ttl(struct sock *sk, u_char ttl)
 
 	/* setsockopt(sock, SOL_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl)); */
 	lock_sock(sk);
-	inet->mc_ttl = ttl;
+	WRITE_ONCE(inet->mc_ttl, ttl);
 #ifdef CONFIG_IP_VS_IPV6
 	if (sk->sk_family == AF_INET6) {
 		struct ipv6_pinfo *np = inet6_sk(sk);