net: tc: improve qdisc error messages

The existing error message ("Invalid qdisc name") is confusing
because it suggests that there is no qdisc with the given name. In
fact, the name does refer to a valid qdisc, but it doesn't match
the kind of an existing qdisc being modified or replaced. The
new error message provides more detail to eliminate confusion.

Signed-off-by: John Ousterhout <[email protected]>
Acked-by: Jamal Hadi Salim <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: johnousterhout

net/sched/sch_api.c

@@ -1560,7 +1560,7 @@ static int tc_get_qdisc(struct sk_buff *skb, struct nlmsghdr *n,
 	}
 
 	if (tca[TCA_KIND] && nla_strcmp(tca[TCA_KIND], q->ops->id)) {
-		NL_SET_ERR_MSG(extack, "Invalid qdisc name");
+		NL_SET_ERR_MSG(extack, "Invalid qdisc name: must match existing qdisc");
 		return -EINVAL;
 	}
 
@@ -1670,7 +1670,7 @@ static int tc_modify_qdisc(struct sk_buff *skb, struct nlmsghdr *n,
 				}
 				if (tca[TCA_KIND] &&
 				    nla_strcmp(tca[TCA_KIND], q->ops->id)) {
-					NL_SET_ERR_MSG(extack, "Invalid qdisc name");
+					NL_SET_ERR_MSG(extack, "Invalid qdisc name: must match existing qdisc");
 					return -EINVAL;
 				}
 				if (q->flags & TCQ_F_INGRESS) {
@@ -1746,7 +1746,7 @@ static int tc_modify_qdisc(struct sk_buff *skb, struct nlmsghdr *n,
 		return -EEXIST;
 	}
 	if (tca[TCA_KIND] && nla_strcmp(tca[TCA_KIND], q->ops->id)) {
-		NL_SET_ERR_MSG(extack, "Invalid qdisc name");
+		NL_SET_ERR_MSG(extack, "Invalid qdisc name: must match existing qdisc");
 		return -EINVAL;
 	}
 	err = qdisc_change(q, tca, extack);