Subject: Re: [PATCH nf-next] netfilter: nf_tables: add ebpf expression

From: Florian Westphal <[email-masked]>
Date: Fri, 2 Sep 2022 19:52:16 +0200

Alexei Starovoitov <[email-masked]> wrote:
> > See my reply to Alexey, immediate goal was to get rid of the indirect
> > calls by providing a tailored/jitted equivalent of nf_hook_slow().
> >
> > The next step could be to allow implementation of netfilter hooks
> > (i.e., kernel modules that call nf_register_net_hook()) in bpf
> > but AFAIU it requires addition of BPF_PROG_TYPE_NETFILTER etc.
> 
> We were adding new prog and maps types in the past.
> Now new features are being added differently.
> All of the networking either works with sk_buff-s or xdp frames.
> We try hard not to add any new uapi helpers.
> Everything is moving to kfuncs.
> Other sub-systems should be able to use bpf without touching
> the bpf core. See hid-bpf as an example.
> It needs several verifier improvements, but doesn't need
> new prog types, helpers, etc.

I don't see how it can be done without a new prog type, the bpf progs
would need access to "nf_hook_state" struct, passed as argument
to nf_hook_slow() (and down to the individual xt_foo modules...).

We can't change the existing netfilter hook prototype to go by
sk_buff * as that doesn't have all information, most prominent are
the input and output net_device, but also okfn is needed for async
reinject (nf_queue), the hook location and so on.

> > After that, yes, one could think about how to jit nft_do_chain() and
> > all the rest of the nft machinery.
> 
> Sounds like a ton of work. All that just to accelerate nft a bit?
> I think there are more impactful projects to work on.
> For example, accelerating classic iptables with bpf would immediately
> help a bunch of users.

Maybe, but from the problem points and the required effort it doesn't matter
if the chosen target is iptables or nftables; as far as the time/effort
needed I'd say they are identical.

The hard issues that need to be solved first are the same; they reside
in the netfilter core and not in the specific interpreter (nft_do_chain
vs. ipt_do_table and friends).

nf_tables might be *slightly* easier once that point would be reached
because the core functionality is more integrated with nf_tables whereas
in iptables there is more copypastry (ipt_do_table, ip6t_do_table,
ebt_do_table, ...).