On 12 February 2014 15:16, Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx> wrote: > On Wed, Feb 12, 2014 at 09:58:28AM -0700, Mathieu Poirier wrote: > [...] >> > I can see that packet and byte counters are not in sync, but I don't >> > see why that should be a problem for your quota extension. Your >> > accounting is based on packet *OR* byte counters. >> >> If we have a filter reporting the amount of packets and bytes that >> have gone through a chain the least we can do is make them coherent. >> Otherwise the numbers simply can't be trusted. Fixing that problem is >> what I've done in the 4th version of my patchset sent on Monday. >> >> > Moreover, if you inspect the result of the counter update via >> > atomic_inc_return / atomic_add_return, you can ensure that packets >> > running out of the quota limit will always match. >> >> Thanks for Florian Westphal's hint on 'cmpxchg' I came up with a match >> algorithm that requires no spinlock and doesn't mandate that packet >> count be synchronised with bytes. Before sending it out I'd like us >> to reach a consensus on the above - I think the current accounting >> method is broken and ought to be fixed. > > I really think we should skip that spinlock with atomic64. That > doesn't make sense to me. > >> If the current situation is fine with you I'll simply send another >> version that works without counts being synchronised - but I really >> think we can do better. > > Given the interferences that you described, I think you can use this > approach to ensure that only one event is delivered: > > if (old < info->quota && new >= info->quota && > test_and_set_bit(0, info->quota_reached)) > deliver_event(...); > > On top of that, there's another problem that needs to be solved now > that we have a quota_reached field in the info area. This needs a > "database of quota objects" that are identified by a name, otherwise > iptables rule updates will trigger a new (unnecessary event) event > every time *any* new rule is added. > > If you want to test what I mean, try this: > > iptables -I INPUT -m quota --quota 1000 -j LOG --log-prefix "quota: " > > then keep an eye on /var/log/messages, ping somewhere to generate > traffic. You should see log messages for each packet until the quota > is reached. After a couple of ICMP packets, you will see no more logs > since the quota has been reached. > > Now, keep monitoring /var/log/messages and add any rule, eg. iptables > -I INPUT. The quota matches again and you'll start seeing more log > messages since every rule updates "resets" the internal state of the > match. This is a known problem. To solve this, you have to use a > specific quota object with a refcnt that is identified by a name. See > xt_hashlimit.c for instance. Note that the quota_reached can't be > added to the existing nfacct object since it's a valid configuration > to have two rules using with different quota values that refer to the > same nfacct object (think of a scenario in which you want to throttle > to N Kbytes/s after X bytes has been consumed, then throttle to a > lower M Kbytes/s after X+Y bytes has been consumed). > > Thus, the iptables command for this should look like: > > iptables -I INPUT -p icmp \ > -m nfacct --mode packets --quota 100 --quota-name icmp-quota-100 > > In _mt_check you have to look up for "icmp-quota-100" in the list of > quota objects to attach it to the rule. If it doesn't exist you create > it. In the _destroy path, you have to put the refcnt, if it reaches > zero, it will be released. It may sound tricky, but this will work > fine in practise. > > Please, take the time to digest this and let us know if you have any > question. Thank you. I traced things a little and I understand exactly what is happening - I will implement the same mechanism as in hashlimit. Since accounting objects can be delete from nfnetlink_acct I suggest we add a notifier to it. That way objects in the database can be deleted when accounting objects are deleted from userspace with nfacct. What do you think? -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
