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

[Andrii Nakryiko]

On Tue, Apr 6, 2021 at 3:06 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:
>
> > On Sat, Apr 3, 2021 at 10:47 AM Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> >>
> >> On Sat, Apr 03, 2021 at 12:38:06AM +0530, Kumar Kartikeya Dwivedi wrote:
> >> > On Sat, Apr 03, 2021 at 12:02:14AM IST, Alexei Starovoitov wrote:
> >> > > On Fri, Apr 2, 2021 at 8:27 AM Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote:
> >> > > > [...]
> >> > >
> >> > > All of these things are messy because of tc legacy. bpf tried to follow tc style
> >> > > with cls and act distinction and it didn't quite work. cls with
> >> > > direct-action is the only
> >> > > thing that became mainstream while tc style attach wasn't really addressed.
> >> > > There were several incidents where tc had tens of thousands of progs attached
> >> > > because of this attach/query/index weirdness described above.
> >> > > I think the only way to address this properly is to introduce bpf_link style of
> >> > > attaching to tc. Such bpf_link would support ingress/egress only.
> >> > > direction-action will be implied. There won't be any index and query
> >> > > will be obvious.
> >> >
> >> > Note that we already have bpf_link support working (without support for pinning
> >> > ofcourse) in a limited way. The ifindex, protocol, parent_id, priority, handle,
> >> > chain_index tuple uniquely identifies a filter, so we stash this in the bpf_link
> >> > and are able to operate on the exact filter during release.
> >>
> >> Except they're not unique. The library can stash them, but something else
> >> doing detach via iproute2 or their own netlink calls will detach the prog.
> >> This other app can attach to the same spot a different prog and now
> >> bpf_link__destroy will be detaching somebody else prog.
> >>
> >> > > So I would like to propose to take this patch set a step further from
> >> > > what Daniel said:
> >> > > int bpf_tc_attach(prog_fd, ifindex, {INGRESS,EGRESS}):
> >> > > and make this proposed api to return FD.
> >> > > To detach from tc ingress/egress just close(fd).
> >> >
> >> > You mean adding an fd-based TC API to the kernel?
> >>
> >> yes.
> >
> > I'm totally for bpf_link-based TC attachment.
> >
> > But I think *also* having "legacy" netlink-based APIs will allow
> > applications to handle older kernels in a much nicer way without extra
> > dependency on iproute2. We have a similar situation with kprobe, where
> > currently libbpf only supports "modern" fd-based attachment, but users
> > periodically ask questions and struggle to figure out issues on older
> > kernels that don't support new APIs.
>
> +1; I am OK with adding a new bpf_link-based way to attach TC programs,
> but we still need to support the netlink API in libbpf.
>
> > So I think we'd have to support legacy TC APIs, but I agree with
> > Alexei and Daniel that we should keep it to the simplest and most
> > straightforward API of supporting direction-action attachments and
> > setting up qdisc transparently (if I'm getting all the terminology
> > right, after reading Quentin's blog post). That coincidentally should
> > probably match how bpf_link-based TC API will look like, so all that
> > can be abstracted behind a single bpf_link__attach_tc() API as well,
> > right? That's the plan for dealing with kprobe right now, btw. Libbpf
> > will detect the best available API and transparently fall back (maybe
> > with some warning for awareness, due to inherent downsides of legacy
> > APIs: no auto-cleanup being the most prominent one).
>
> Yup, SGTM: Expose both in the low-level API (in bpf.c), and make the
> high-level API auto-detect. That way users can also still use the
> netlink attach function if they don't want the fd-based auto-close
> behaviour of bpf_link.

So I thought a bit more about this, and it feels like the right move
would be to expose only higher-level TC BPF API behind bpf_link. It
will keep the API complexity and amount of APIs that libbpf will have
to support to the minimum, and will keep the API itself simple:
direct-attach with the minimum amount of input arguments. By not
exposing low-level APIs we also table the whole bpf_tc_cls_attach_id
design discussion, as we now can keep as much info as needed inside
bpf_link_tc (which will embed bpf_link internally as well) to support
detachment and possibly some additional querying, if needed.

I think that's the best and least controversial step forward for
getting this API into libbpf.

>
> -Toke
>