[PATCH net 03/13] net: sched: cls_u32: disallow linking to root hnode

[Al Viro]

From: Al Viro <v...@zeniv.linux.org.uk>

Operation makes no sense.  Nothing will actually break if we do so
(depth limit in u32_classify() will prevent infinite loops), but
according to maintainers it's best prohibited outright.

NOTE: doing so guarantees that u32_destroy() will trigger the call
of u32_destroy_hnode(); we might want to make that unconditional.

Test:
tc qdisc add dev eth0 ingress
tc filter add dev eth0 parent ffff: protocol ip prio 100 u32 \
        link 800: offset at 0 mask 0f00 shift 6 plus 0 eat match ip protocol 6 
ff
should fail with
Error: cls_u32: Not linking to root node

Signed-off-by: Al Viro <v...@zeniv.linux.org.uk>
---
 net/sched/cls_u32.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index c4782aa808c7..72459b09d910 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -797,6 +797,10 @@ static int u32_set_parms(struct net *net, struct tcf_proto 
*tp,
                                NL_SET_ERR_MSG_MOD(extack, "Link hash table not 
found");
                                return -EINVAL;
                        }
+                       if (ht_down->is_root) {
+                               NL_SET_ERR_MSG_MOD(extack, "Not linking to root 
node");
+                               return -EINVAL;
+                       }
                        ht_down->refcnt++;
                }
 
-- 
2.11.0