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.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/pm_netlink.c   | 7 ++++---
 net/mptcp/pm_userspace.c | 9 ++++++++-
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
index 49c34494e156ff6c3277b107c3840ee29dd3afbb..3bb213e2a967e87d4244dc0f1ba58d3ba21fb371 100644
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -1878,8 +1878,9 @@ static int parse_limit(struct genl_info *info, int id, unsigned int *limit)
 
 	*limit = nla_get_u32(attr);
 	if (*limit > MPTCP_PM_ADDR_MAX) {
-		NL_SET_ERR_MSG_ATTR(info->extack, attr,
-				    "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;
@@ -2008,7 +2009,7 @@ int mptcp_pm_nl_set_flags(struct sk_buff *skb, struct genl_info *info)
 		lookup_by_id = 1;
 		if (!addr.addr.id) {
 			NL_SET_ERR_MSG_ATTR(info->extack, attr,
-					    "missing required inputs");
+					    "missing address ID");
 			return -EOPNOTSUPP;
 		}
 	}
diff --git a/net/mptcp/pm_userspace.c b/net/mptcp/pm_userspace.c
index 9c0bec588c498e0aa78acf4018db509abe90cad2..a8f73b082460ba00f7fa998cf7646368f8201e2e 100644
--- a/net/mptcp/pm_userspace.c
+++ b/net/mptcp/pm_userspace.c
@@ -191,7 +191,7 @@ static struct mptcp_sock *mptcp_userspace_pm_get_sock(const struct genl_info *in
 
 	if (!mptcp_pm_is_userspace(msk)) {
 		NL_SET_ERR_MSG_ATTR(info->extack, token,
-				    "invalid request; userspace PM not selected");
+				    "userspace PM not selected");
 		sock_put((struct sock *)msk);
 		return NULL;
 	}
@@ -433,6 +433,9 @@ 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);
@@ -557,6 +560,7 @@ 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;
 	}
@@ -634,6 +638,9 @@ 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);
 
+	if (ret)
+		GENL_SET_ERR_MSG(info, "subflow not found");
+
 set_flags_err:
 	sock_put(sk);
 	return ret;

-- 
2.47.1