Subject: Re: [PATCH bpf-next 0/7] Support defragmenting IPv(4|6) packets in BPF

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 29 Jun 2023 14:16:08 +0200

Florian Westphal <[email-masked]> writes:

> Toke Høiland-Jørgensen <[email-masked]> wrote:
>> > The basic idea is we bump a refcnt on the netfilter defrag module and
>> > then run the bpf prog after the defrag module runs. This allows bpf
>> > progs to transparently see full, reassembled packets. The nice thing
>> > about this is that progs don't have to carry around logic to detect
>> > fragments.
>> 
>> One high-level comment after glancing through the series: Instead of
>> allocating a flag specifically for the defrag module, why not support
>> loading (and holding) arbitrary netfilter modules in the UAPI?
>
> How would that work/look like?
>
> defrag (and conntrack) need special handling because loading these
> modules has no effect on the datapath.
>
> Traditionally, yes, loading was enough, but now with netns being
> ubiquitous we don't want these to get enabled unless needed.
>
> Ignoring bpf, this happens when user adds nftables/iptables rules
> that check for conntrack state, use some form of NAT or use e.g. tproxy.
>
> For bpf a flag during link attachment seemed like the best way
> to go.

Right, I wasn't disputing that having a flag to load a module was a good
idea. On the contrary, I was thinking we'd need many more of these
if/when BPF wants to take advantage of more netfilter code. Say, if a
BPF module wants to call into TPROXY, that module would also need go be
loaded and kept around, no?

I was thinking something along the lines of just having a field
'netfilter_modules[]' where userspace could put an arbitrary number of
module names into, and we'd load all of them and put a ref into the
bpf_link. In principle, we could just have that be a string array of
module names, but that's probably a bit cumbersome (and, well, building
a generic module loader interface into the bpf_like API is not
desirable either). But maybe with an explicit ENUM?

> At the moment I only see two flags for this, namely
> "need defrag" and "need conntrack".
>
> For conntrack, we MIGHT be able to not need a flag but
> maybe verifier could "guess" based on kfuncs used.

If the verifier can just identify the modules from the kfuncs and do the
whole thing automatically, that would of course be even better from an
ease-of-use PoV. Not sure what that would take, though? I seem to recall
having discussions around these lines before that fell down on various
points.

> But for defrag, I don't think its good to add a dummy do-nothing
> kfunc just for expressing the dependency on bpf prog side.

Agreed.

-Toke