Some error messages were:

 - too generic: "missing input", "invalid request"

 - not precise enough: "limit greater than maximum" but what's the max?

 - missing: subflow not found, or connect error.

This can be easily improved by being more precise, or adding new error
messages.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/pm_netlink.c   |  6 ++++--
 net/mptcp/pm_userspace.c | 10 +++++++++-
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
index 98ac73938bd8196e196d5ee8c264784ba8d37645..a60217faf95debf870dd87ecf1afc1cde7c69bcf 100644
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -1875,7 +1875,9 @@ static int parse_limit(struct genl_info *info, int id, unsigned int *limit)
 
 	*limit = nla_get_u32(attr);
 	if (*limit > MPTCP_PM_ADDR_MAX) {
-		GENL_SET_ERR_MSG(info, "limit greater than maximum");
+		NL_SET_ERR_MSG_ATTR_FMT(info->extack, attr,
+					"limit greater than maximum (%u)",
+					MPTCP_PM_ADDR_MAX);
 		return -EINVAL;
 	}
 	return 0;
@@ -2003,7 +2005,7 @@ int mptcp_pm_nl_set_flags(struct sk_buff *skb, struct genl_info *info)
 	if (addr.addr.family == AF_UNSPEC) {
 		lookup_by_id = 1;
 		if (!addr.addr.id) {
-			GENL_SET_ERR_MSG(info, "missing required inputs");
+			GENL_SET_ERR_MSG(info, "missing address ID");
 			return -EOPNOTSUPP;
 		}
 	}
diff --git a/net/mptcp/pm_userspace.c b/net/mptcp/pm_userspace.c
index cdc83fabb7c2c45bc3d7c954a824c8f27bb85718..e350d6cc23bf2e23c5f255ede51570d8596b4585 100644
--- a/net/mptcp/pm_userspace.c
+++ b/net/mptcp/pm_userspace.c
@@ -190,7 +190,7 @@ static struct mptcp_sock *mptcp_userspace_pm_get_sock(const struct genl_info *in
 	}
 
 	if (!mptcp_pm_is_userspace(msk)) {
-		GENL_SET_ERR_MSG(info, "invalid request; userspace PM not selected");
+		GENL_SET_ERR_MSG(info, "userspace PM not selected");
 		sock_put((struct sock *)msk);
 		return NULL;
 	}
@@ -428,6 +428,9 @@ int mptcp_pm_nl_subflow_create_doit(struct sk_buff *skb, struct genl_info *info)
 	err = __mptcp_subflow_connect(sk, &local, &addr_r);
 	release_sock(sk);
 
+	if (err)
+		GENL_SET_ERR_MSG_FMT(info, "connect error: %d", err);
+
 	spin_lock_bh(&msk->pm.lock);
 	if (err)
 		mptcp_userspace_pm_delete_local_addr(msk, &entry);
@@ -552,6 +555,7 @@ int mptcp_pm_nl_subflow_destroy_doit(struct sk_buff *skb, struct genl_info *info
 	lock_sock(sk);
 	ssk = mptcp_nl_find_ssk(msk, &addr_l.addr, &addr_r);
 	if (!ssk) {
+		GENL_SET_ERR_MSG(info, "subflow not found");
 		err = -ESRCH;
 		goto release_sock;
 	}
@@ -625,6 +629,10 @@ int mptcp_userspace_pm_set_flags(struct sk_buff *skb, struct genl_info *info)
 	ret = mptcp_pm_nl_mp_prio_send_ack(msk, &loc.addr, &rem.addr, bkup);
 	release_sock(sk);
 
+	/* mptcp_pm_nl_mp_prio_send_ack() only fails in one case */
+	if (ret < 0)
+		GENL_SET_ERR_MSG(info, "subflow not found");
+
 set_flags_err:
 	sock_put(sk);
 	return ret;

-- 
2.47.1