net sched actions: fix dumping which requires several messages to user space

Fixes a bug in the tcf_dump_walker function that can cause some actions
to not be reported when dumping a large number of actions. This issue
became more aggrevated when cookies feature was added. In particular
this issue is manifest when large cookie values are assigned to the
actions and when enough actions are created that the resulting table
must be dumped in multiple batches.

The number of actions returned in each batch is limited by the total
number of actions and the memory buffer size.  With small cookies
the numeric limit is reached before the buffer size limit, which avoids
the code path triggering this bug. When large cookies are used buffer
fills before the numeric limit, and the erroneous code path is hit.

For example after creating 32 csum actions with the cookie
aaaabbbbccccdddd

$ tc actions ls action csum
total acts 26

    action order 0: csum (tcp) action continue
    index 1 ref 1 bind 0
    cookie aaaabbbbccccdddd

    .....

    action order 25: csum (tcp) action continue
    index 26 ref 1 bind 0
    cookie aaaabbbbccccdddd
total acts 6

    action order 0: csum (tcp) action continue
    index 28 ref 1 bind 0
    cookie aaaabbbbccccdddd

    ......

    action order 5: csum (tcp) action continue
    index 32 ref 1 bind 0
    cookie aaaabbbbccccdddd

Note that the action with index 27 is omitted from the report.

Fixes: 4b3550ef ("[NET_SCHED]: Use nla_nest_start/nla_nest_end")"
Signed-off-by: Craig Dillabaugh <cdillaba@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/act_api.c

@@ -135,8 +135,10 @@ static int tcf_dump_walker(struct tcf_idrinfo *idrinfo, struct sk_buff *skb,
 			continue;
 
 		nest = nla_nest_start(skb, n_i);
-		if (!nest)
+		if (!nest) {
+			index--;
 			goto nla_put_failure;
+		}
 		err = tcf_action_dump_1(skb, p, 0, 0);
 		if (err < 0) {
 			index--;