[PATCH net-next v2] netfilter: ebtables: avoid resetting limit rule state

From: Linus LÃssing
Date: Sun Dec 09 2018 - 01:21:25 EST

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 LÃssing <linus.luessing@xxxxxxxxx>


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),
.compatsize = sizeof(struct ebt_compat_limit_info),