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

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

On Wed, Oct 21, 2020 at 12:43:21PM +0200, Pablo Neira Ayuso wrote:
> 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.

For clarity here, I mean: iptables does not use the generic match
infrastructure for matching on these fields, instead it is using
ip_packet_match() which is called from ipt_do_table() which is the
core function that evaluates the packet.

> 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.