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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>
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        Daniel Borkmann <daniel@iogearbox.net>,
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        Alexei Starovoitov <ast@kernel.org>,
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Subject: Re: [PATCH bpf-next 3/5] libbpf: add low level TC-BPF API
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Date:   Tue, 06 Apr 2021 12:06:24 +0200
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Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Sat, Apr 3, 2021 at 10:47 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> 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@gmail.com> 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.

-Toke