Re: [net-next PATCH 0/2] netfilter: Improve inverted IP prefix matches

[Pablo Neira Ayuso <pablo@netfilter.org>]

Hi Phil,

On Fri, Oct 02, 2020 at 11:00:33AM +0200, Phil Sutter wrote:
> Hi Florian,
> 
> On Fri, Oct 02, 2020 at 12:25:36AM +0200, Florian Westphal wrote:
> > Phil Sutter <phil@nwl.cc> wrote:
> > > The following two patches improve packet throughput in a test setup
> > > sending UDP packets (using iperf3) between two netns. The ruleset used
> > > on receiver side is like this:
> > > 
> > > | *filter
> > > | :test - [0:0]
> > > | -A INPUT -j test
> > > | -A INPUT -j ACCEPT
> > > | -A test ! -s 10.0.0.0/10 -j DROP # this line repeats 10000 times
> > > | COMMIT
> > > 
> > > These are the generated VM instructions for each rule:
> > > 
> > > | [ payload load 4b @ network header + 12 => reg 1 ]
> > > | [ bitwise reg 1 = (reg=1 & 0x0000c0ff ) ^ 0x00000000 ]
> > 
> > Not related to this patch, but we should avoid the bitop if the
> > netmask is divisble by 8 (can adjust the cmp -- adjusting the
> > payload expr is probably not worth it).
> 
> See the patch I just sent to this list. I adjusted both - it simply
> didn't appear to me that I could get by with reducing the cmp expression
> size only. The upside though is that detecting the prefix match based on
> payload expression length is quick and easy.
> 
> Someone will have to adjust nft tool, though. ;)
> 
> > > | [ cmp eq reg 1 0x0000000a ]
> > > | [ counter pkts 0 bytes 0 ]
> > 
> > Out of curiosity, does omitting 'counter' help?
> > 
> > nft counter is rather expensive due to bh disable,
> > iptables does it once at the evaluation loop only.
> 
> I changed the test to create the base ruleset using iptables-nft-restore
> just as before, but create the rules in 'test' chain like so:
> 
> | nft add rule filter test ip saddr != 10.0.0.0/10 drop
> 
> The VM code is as expected:
> 
> | [ payload load 4b @ network header + 12 => reg 1 ]
> | [ bitwise reg 1 = (reg=1 & 0x0000c0ff ) ^ 0x00000000 ]
> | [ cmp eq reg 1 0x0000000a ]
> | [ immediate reg 0 drop ]
> 
> Performance is ~7000pkt/s. So while it's faster than iptables-nft, it's
> still quite a bit slower than legacy iptables despite the skipped
> counters.

iptables is optimized for matching on input/output device name and
IPv4 address + mask (see ip_packet_match()) for historical reasons,
iptables does not use a match for this since the beginning.

One possibility (in the short-term) is to add an internal kernel
expression to achieve the same behaviour. The kernel needs to detects
for:

payload (nh, offset to ip saddr or ip daddr or ip protocol) + cmp
payload (nh, offset to ip saddr or ip daddr) + bitwise + cmp
meta (iifname or oifname) + bitwise + cmp
meta (iifname or oifname) + cmp

at the very beginning of the rule.

and squash these expressions into the "built-in" iptables match
expression which emulates ip_packet_match().

Not nice, but if microbenchmarks using thousand of rules really matter
(this is worst case O(n) linear list evaluation...) then it might make
sense to explore this.