Re: [PATCH bpf-next 3/5] libbpf: add low level TC-BPF API

[Daniel Borkmann]

On 3/31/21 11:44 AM, Kumar Kartikeya Dwivedi wrote:
> On Wed, Mar 31, 2021 at 02:55:47AM IST, Daniel Borkmann wrote:
> > Do we even need the _block variant? I would rather prefer to take the chance
> > and make it as simple as possible, and only iff really needed extend with
> > other APIs, for example:
>
> The block variant can be dropped, I'll use the TC_BLOCK/TC_DEV alternative which
> sets parent_id/ifindex properly.
>
> >    bpf_tc_attach(prog_fd, ifindex, {INGRESS,EGRESS});
> >
> > Internally, this will create the sch_clsact qdisc & cls_bpf filter instance
> > iff not present yet, and attach to a default prio 1 handle 1, and _always_ in
> > direct-action mode. This is /as simple as it gets/ and we don't need to bother
> > users with more complex tc/cls_bpf internals unless desired. For example,
> > extended APIs could add prio/parent so that multi-prog can be attached to a
> > single cls_bpf instance, but even that could be a second step, imho.
>
> I am not opposed to clsact qdisc setup if INGRESS/EGRESS is supplied (not sure
> how others feel about it).

What speaks against it? It would be 100% clear from API side where the prog is
being attached. Same as with tc cmdline where you specify 'ingress'/'egress'.

> We could make direct_action mode default, and similarly choose prio

To be honest, I wouldn't even support a mode from the lib/API side where direct_action
is not set. It should always be forced to true. Everything else is rather broken
setup-wise, imho, since it won't scale. We added direct_action a bit later to the
kernel than original cls_bpf, but if I would do it again today, I'd make it the
only available option. I don't see a reasonable use-case where you have it to false.

> as 1 by default instead of letting the kernel do it. Then you can just pass in
> NULL for bpf_tc_cls_opts and be close to what you're proposing. For protocol we
> can choose ETH_P_ALL by default too if the user doesn't set it.

Same here with ETH_P_ALL, I'm not sure anyone uses anything other than ETH_P_ALL,
so yes, that should be default.

> With these modifications, the equivalent would look like
> 	bpf_tc_cls_attach(prog_fd, TC_DEV(ifindex, INGRESS), NULL, &id);

Few things compared to bpf_tc_attach(prog_fd, ifindex, {INGRESS,EGRESS}):

1) nit, but why even 'cls' in the name. I think we shouldn't expose such old-days
   tc semantics to a user. Just bpf_tc_attach() is cleaner/simpler to understand.
2) What's the 'TC_DEV(ifindex, INGRESS)' macro doing exactly? Looks unnecessary,
   why not regular args to the API?
3) Exposed bpf_tc_attach() API could internally call a bpf_tc_attach_opts() API
   with preset defaults, and the latter could have all the custom bits if the user
   needs to go beyond the simple API, so from your bpf_tc_cls_attach() I'd also
   drop the NULL.
4) For the simple API I'd likely also drop the id (you could have a query API if
   needed).

> So as long as the user doesn't care about other details, they can just pass opts
> as NULL.