[PATCH net-next v2] netfilter: 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

This also aligns the behavior to "xtables-nft-multi ebtables" which uses
nft_limit instead of ebt_limit. In tests nft_limit did not suffer from
this issue and rate limited to 650 just fine.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>

---

Changelog v2:

- Adjusted commit message (adjusted title, added test results with
  nft_limit for comparison)
- Excluded rate limiting variables from zeroing when passed to userspace
  by increasing .usersize. This became necessary with 4.11 /
  commit ec2318904965 ("xtables: extend matches and targets with .usersize")
- Retested with 4.20-rc4 and current net-next/master (83af01ba1c2d)

v1 was:

"[net-next] bridge: ebtables: Avoid resetting limit rule state"
-> https://lore.kernel.org/patchwork/patch/854802/
---
 net/bridge/netfilter/ebt_limit.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/bridge/netfilter/ebt_limit.c b/net/bridge/netfilter/ebt_limit.c
index 165b9d678cf1..2cf9861c3bce 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)) {
@@ -105,7 +109,7 @@ static struct xt_match ebt_limit_mt_reg __read_mostly = {
 	.match		= ebt_limit_mt,
 	.checkentry	= ebt_limit_mt_check,
 	.matchsize	= sizeof(struct ebt_limit_info),
-	.usersize	= offsetof(struct ebt_limit_info, prev),
+	.usersize	= sizeof(struct ebt_limit_info),
 #ifdef CONFIG_COMPAT
 	.compatsize	= sizeof(struct ebt_compat_limit_info),
 #endif
-- 
2.11.0