mptcp: set error messages for set_flags

In addition to returning the error value, this patch also sets an error
messages with GENL_SET_ERR_MSG or NL_SET_ERR_MSG_ATTR both for pm_netlink.c
and pm_userspace.c. It will help the userspace to identify the issue.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240305-upstream-net-next-20240304-mptcp-misc-cleanup-v1-3-c436ba5e569b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/pm_netlink.c

@@ -1908,8 +1908,10 @@ 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)
+		if (!addr.addr.id) {
+			GENL_SET_ERR_MSG(info, "missing required inputs");
 			return -EOPNOTSUPP;
+		}
 	}
 
 	if (addr.flags & MPTCP_PM_ADDR_FLAG_BACKUP)
@@ -1919,11 +1921,13 @@ int mptcp_pm_nl_set_flags(struct sk_buff *skb, struct genl_info *info)
 	entry = __lookup_addr(pernet, &addr.addr, lookup_by_id);
 	if (!entry) {
 		spin_unlock_bh(&pernet->lock);
+		GENL_SET_ERR_MSG(info, "address not found");
 		return -EINVAL;
 	}
 	if ((addr.flags & MPTCP_PM_ADDR_FLAG_FULLMESH) &&
 	    (entry->flags & MPTCP_PM_ADDR_FLAG_SIGNAL)) {
 		spin_unlock_bh(&pernet->lock);
+		GENL_SET_ERR_MSG(info, "invalid addr flags");
 		return -EINVAL;
 	}
 

net/mptcp/pm_userspace.c

@@ -563,13 +563,17 @@ int mptcp_userspace_pm_set_flags(struct sk_buff *skb, struct genl_info *info)
 	token_val = nla_get_u32(token);
 
 	msk = mptcp_token_get_sock(net, token_val);
-	if (!msk)
+	if (!msk) {
+		NL_SET_ERR_MSG_ATTR(info->extack, token, "invalid token");
 		return ret;
+	}
 
 	sk = (struct sock *)msk;
 
-	if (!mptcp_pm_is_userspace(msk))
+	if (!mptcp_pm_is_userspace(msk)) {
+		GENL_SET_ERR_MSG(info, "userspace PM not selected");
 		goto set_flags_err;
+	}
 
 	ret = mptcp_pm_parse_entry(attr, info, false, &loc);
 	if (ret < 0)
@@ -583,6 +587,7 @@ int mptcp_userspace_pm_set_flags(struct sk_buff *skb, struct genl_info *info)
 
 	if (loc.addr.family == AF_UNSPEC ||
 	    rem.addr.family == AF_UNSPEC) {
+		GENL_SET_ERR_MSG(info, "invalid address families");
 		ret = -EINVAL;
 		goto set_flags_err;
 	}