net: sched: cls_u32: mark root hnode explicitly

... and produce consistent error on attempt to delete such.
Existing check in u32_delete() is inconsistent - after

tc qdisc add dev eth0 ingress
tc filter add dev eth0 parent ffff: protocol ip prio 100 handle 1: u32 \
divisor 1
tc filter add dev eth0 parent ffff: protocol ip prio 200 handle 2: u32 \
divisor 1

both

tc filter delete dev eth0 parent ffff: protocol ip prio 100 handle 801: u32

and

tc filter delete dev eth0 parent ffff: protocol ip prio 100 handle 800: u32

will fail (at least with refcounting fixes), but the former will complain
about an attempt to remove a busy table, while the latter will recognize
it as root and yield "Not allowed to delete root node" instead.

The problem with the existing check is that several tcf_proto instances
might share the same tp->data and handle-to-hnode lookup will be the same
for all of them. So comparing an hnode to be deleted with tp->root won't
catch the case when one tp is used to try deleting the root of another.
Solution is trivial - mark the root hnodes explicitly upon allocation and
check for that.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_u32.c

@@ -84,6 +84,7 @@ struct tc_u_hnode {
 	int			refcnt;
 	unsigned int		divisor;
 	struct idr		handle_idr;
+	bool			is_root;
 	struct rcu_head		rcu;
 	u32			flags;
 	/* The 'ht' field MUST be the last field in structure to allow for
@@ -377,6 +378,7 @@ static int u32_init(struct tcf_proto *tp)
 	root_ht->refcnt++;
 	root_ht->handle = tp_c ? gen_new_htid(tp_c, root_ht) : 0x80000000;
 	root_ht->prio = tp->prio;
+	root_ht->is_root = true;
 	idr_init(&root_ht->handle_idr);
 
 	if (tp_c == NULL) {
@@ -692,7 +694,7 @@ static int u32_delete(struct tcf_proto *tp, void *arg, bool *last,
 		goto out;
 	}
 
-	if (root_ht == ht) {
+	if (ht->is_root) {
 		NL_SET_ERR_MSG_MOD(extack, "Not allowed to delete root node");
 		return -EINVAL;
 	}