Exercise setsockopt/getsockopt of IP_RECVERR and IPV6_RECVERR on the
MPTCP parent socket, including the empty-errqueue EAGAIN contract on
MSG_ERRQUEUE|MSG_DONTWAIT.

End-to-end errqueue delivery (ICMP, TX timestamps, zerocopy) depends on
subflow-side producers that are out of scope for this series and will be
covered by follow-up work.

Assisted-by: Codex:gpt-5
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 .../selftests/net/mptcp/mptcp_sockopt.c       | 55 +++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
index b6e58d936ebe..95bb2cc8e2ff 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
@@ -769,6 +769,60 @@ static void test_ip_tos_sockopt(int fd)
 		xerror("expect socklen_t == -1");
 }
 
+static void test_ip_recverr_sockopt(int fd)
+{
+	struct iovec iov = {
+		.iov_base = &(char){ 0 },
+		.iov_len = 1,
+	};
+	struct msghdr msg = {
+		.msg_iov = &iov,
+		.msg_iovlen = 1,
+	};
+	int one = 1, zero = 0, val = -1;
+	socklen_t s = sizeof(val);
+	int level, optname, r;
+
+	switch (pf) {
+	case AF_INET:
+		level = SOL_IP;
+		optname = IP_RECVERR;
+		break;
+	case AF_INET6:
+		level = SOL_IPV6;
+		optname = IPV6_RECVERR;
+		break;
+	default:
+		xerror("Unknown pf %d\n", pf);
+	}
+
+	r = setsockopt(fd, level, optname, &one, sizeof(one));
+	if (r)
+		die_perror("setsockopt recverr on");
+
+	r = getsockopt(fd, level, optname, &val, &s);
+	if (r)
+		die_perror("getsockopt recverr on");
+	if (s != sizeof(val) || val != one)
+		xerror("recverr on mismatch val=%d len=%u", val, s);
+
+	r = recvmsg(fd, &msg, MSG_ERRQUEUE | MSG_DONTWAIT);
+	if (r != -1 || errno != EAGAIN)
+		xerror("expected empty errqueue to return EAGAIN, ret=%d errno=%d", r, errno);
+
+	r = setsockopt(fd, level, optname, &zero, sizeof(zero));
+	if (r)
+		die_perror("setsockopt recverr off");
+
+	val = -1;
+	s = sizeof(val);
+	r = getsockopt(fd, level, optname, &val, &s);
+	if (r)
+		die_perror("getsockopt recverr off");
+	if (s != sizeof(val) || val != zero)
+		xerror("recverr off mismatch val=%d len=%u", val, s);
+}
+
 static int client(int pipefd)
 {
 	int fd = -1;
@@ -787,6 +841,7 @@ static int client(int pipefd)
 	}
 
 	test_ip_tos_sockopt(fd);
+	test_ip_recverr_sockopt(fd);
 
 	connect_one_server(fd, pipefd);
 
-- 
2.53.0