mptcp: implement IPV6_TCLASS socket option support

This patch adds full support for the IPV6_TCLASS socket option in MPTCP,
ensuring consistent traffic class handling across all subflows.

Key changes:
1. setsockopt(IPV6_TCLASS):
   - First sets the traffic class value on the main MPTCP socket
   - Propagates the new value to all existing subflows
   - Uses lock synchronization to ensure atomic updates

2. getsockopt(IPV6_TCLASS):
   - Returns the current traffic class value stored in the main socket's
     inet6_sk structure

This implementation mirrors the existing IPv4 TOS handling (IP_TOS) and
ensures QoS settings are consistently applied to all underlying TCP
connections in the MPTCP session.

Closes: multipath-tcp/mptcp_net-next#568
Signed-off-by: Geliang Tang <[email protected]>

Author: Geliang Tang

net/mptcp/sockopt.c

@@ -384,6 +384,33 @@ static int mptcp_setsockopt_sol_socket(struct mptcp_sock *msk, int optname,
 	return -EOPNOTSUPP;
 }
 
+static int mptcp_setsockopt_v6_set_tclass(struct mptcp_sock *msk, int optname,
+					  sockptr_t optval, unsigned int optlen)
+{
+	struct mptcp_subflow_context *subflow;
+	struct sock *sk = (struct sock *)msk;
+	int err, val;
+
+	err = ipv6_setsockopt(sk, SOL_IPV6, optname, optval, optlen);
+	if (err)
+		return err;
+
+	lock_sock(sk);
+	sockopt_seq_inc(msk);
+	val = READ_ONCE(inet6_sk(sk)->tclass);
+	mptcp_for_each_subflow(msk, subflow) {
+		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+		bool slow;
+
+		slow = lock_sock_fast(ssk);
+		inet6_sk(ssk)->tclass = val;
+		unlock_sock_fast(ssk, slow);
+	}
+	release_sock(sk);
+
+	return 0;
+}
+
 static int mptcp_setsockopt_v6(struct mptcp_sock *msk, int optname,
 			       sockptr_t optval, unsigned int optlen)
 {
@@ -426,6 +453,8 @@ static int mptcp_setsockopt_v6(struct mptcp_sock *msk, int optname,
 
 		release_sock(sk);
 		break;
+	case IPV6_TCLASS:
+		return mptcp_setsockopt_v6_set_tclass(msk, optname, optval, optlen);
 	}
 
 	return ret;
@@ -1475,6 +1504,9 @@ static int mptcp_getsockopt_v6(struct mptcp_sock *msk, int optname,
 	case IPV6_FREEBIND:
 		return mptcp_put_int_option(msk, optval, optlen,
 					    inet_test_bit(FREEBIND, sk));
+	case IPV6_TCLASS:
+		return mptcp_put_int_option(msk, optval, optlen,
+					    inet6_sk(sk)->tclass);
 	}
 
 	return -EOPNOTSUPP;