vmaps and default action

[Florian Westphal <fw@strlen.de>]

Hi,

currently a ruleset like this loads fine:

table inet filter {
        chain input {
                type filter hook input priority filter; policy accept;
                meta l4proto vmap { tcp : jump tcp_chain, udp : jump udp_chain } counter packets 0 bytes 0 jump other_chain
        }

        chain forward {
                type filter hook forward priority filter; policy accept;
        }

        chain output {
                type filter hook output priority filter; policy accept;
        }

        chain tcp_chain {
                counter packets 18 bytes 1017
        }

        chain udp_chain {
                counter packets 0 bytes 0
        }

        chain other_chain {
                counter packets 0 bytes 0
        }
}

but it doesn't do what would be expected, tcp or udp traffic will
jump to the appropriate chain, but 'other' protocols will never
make it to other_chain.

Instead, users need a workaround like this:

table inet filter {
        chain input {
                type filter hook input priority filter; policy accept;
                jump test_proto_chain
        }

        chain test_proto_chain {
                meta l4proto vmap { tcp : goto tcp_chain, udp : goto udp_chain }
                counter packets 2 bytes 168 goto other_chain
        }

        chain forward {
                type filter hook forward priority filter; policy accept;
        }

        chain output {
                type filter hook output priority filter; policy accept;
        }

        chain tcp_chain {
                counter packets 29 bytes 1966
        }

        chain udp_chain {
                counter packets 4 bytes 774
        }

        chain other_chain {
                counter packets 2 bytes 168
        }
}

The intermediate chain (test_proto_chain) allows to then call the real chains
via goto, so the remaining rules in test_proto_chain get omitted in case the
goto label is found, and if not the next rule does the catchall/default handling.

This isn't really nice, there should be a better way to do this.

It would be possible to make ruleset A just work by resurrecting my old patch
to not set NFT_BREAK in case no vmap entry is found, then the rule would
continue evaluation and hit the '... jump other' expression.

If thats unwanted, nft should at least complain/warn that the 'counter jump other_chain'
part will never be run, i.e. similar to how nft handles constructs like this:

inet-filter:7:25-31: Error: Statement after terminal statement has no effect
jump test_proto_chain counter accept
~~~~~~~~~~~~~~~~~~~~~ ^^^^^^^

I can make a patch that adds this warning, are there any suggestions on how to handle/add
default/catchall support?

I still think making 'A' "just work" is the most sane option, it needs very little
kernel changes, needs no extra keywords and it looks "natural" to me to make vmap a no-op
if no jump/goto was executed.