inet: implement lockless IP_MINTTL

inet->min_ttl is already read with READ_ONCE().

Implementing IP_MINTTL socket option set/read
without holding the socket lock is easy.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_sockglue.c

@@ -1030,6 +1030,17 @@ int do_ip_setsockopt(struct sock *sk, int level, int optname,
 			return -EINVAL;
 		WRITE_ONCE(inet->uc_ttl, val);
 		return 0;
+	case IP_MINTTL:
+		if (optlen < 1)
+			return -EINVAL;
+		if (val < 0 || val > 255)
+			return -EINVAL;
+
+		if (val)
+			static_branch_enable(&ip4_min_ttl);
+
+		WRITE_ONCE(inet->min_ttl, val);
+		return 0;
 	}
 
 	err = 0;
@@ -1326,21 +1337,6 @@ int do_ip_setsockopt(struct sock *sk, int level, int optname,
 		err = xfrm_user_policy(sk, optname, optval, optlen);
 		break;
 
-	case IP_MINTTL:
-		if (optlen < 1)
-			goto e_inval;
-		if (val < 0 || val > 255)
-			goto e_inval;
-
-		if (val)
-			static_branch_enable(&ip4_min_ttl);
-
-		/* tcp_v4_err() and tcp_v4_rcv() might read min_ttl
-		 * while we are changint it.
-		 */
-		WRITE_ONCE(inet->min_ttl, val);
-		break;
-
 	case IP_LOCAL_PORT_RANGE:
 	{
 		const __u16 lo = val;
@@ -1595,6 +1591,9 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 		if (val < 0)
 			val = READ_ONCE(sock_net(sk)->ipv4.sysctl_ip_default_ttl);
 		goto copyval;
+	case IP_MINTTL:
+		val = READ_ONCE(inet->min_ttl);
+		goto copyval;
 	}
 
 	if (needs_rtnl)
@@ -1731,9 +1730,6 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 		len -= msg.msg_controllen;
 		return copy_to_sockptr(optlen, &len, sizeof(int));
 	}
-	case IP_MINTTL:
-		val = inet->min_ttl;
-		break;
 	case IP_LOCAL_PORT_RANGE:
 		val = inet->local_port_range.hi << 16 | inet->local_port_range.lo;
 		break;