[PATCH net-next] bridge: ebtables: Avoid resetting limit rule state

["Linus Lüssing" <linus.luessing@c0d3.blue>]

So far any changes with ebtables will reset the state of limit rules,
leading to spikes in traffic. This is especially noticeable if changes
are done frequently, for instance via a daemon.

This patch fixes this by bailing out from (re)setting if the limit
rule was initialized before.

When sending packets every 250ms for 600s, with a
"--limit 1/sec --limit-burst 50" rule and a command like this
in the background:

$ ebtables -N VOIDCHAIN
$ while true; do ebtables -F VOIDCHAIN; sleep 30; done

The results are:

Before: ~1600 packets
After: 650 packets

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
---
 net/bridge/netfilter/ebt_limit.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/bridge/netfilter/ebt_limit.c b/net/bridge/netfilter/ebt_limit.c
index 61a9f1be1263..f74b48633feb 100644
--- a/net/bridge/netfilter/ebt_limit.c
+++ b/net/bridge/netfilter/ebt_limit.c
@@ -69,6 +69,10 @@ static int ebt_limit_mt_check(const struct xt_mtchk_param *par)
 {
 	struct ebt_limit_info *info = par->matchinfo;
 
+	/* Do not reset state on unrelated table changes */
+	if (info->prev)
+		return 0;
+
 	/* Check for overflow. */
 	if (info->burst == 0 ||
 	    user2credits(info->avg * info->burst) < user2credits(info->avg)) {
-- 
2.11.0