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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Apr 14, 2021 at 3:51 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
>
> > On Wed, Apr 14, 2021 at 3:58 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >>
> >> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
> >>
> >> > On Tue, Apr 6, 2021 at 3:06 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >> >>
> >> >> 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.
> >> >
> >> > 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.
> >>
> >> But then there would be no way for the caller to explicitly select a
> >> mechanism? I.e., if I write a BPF program using this mechanism targeting
> >> a 5.12 kernel, I'll get netlink attachment, which can stick around when
> >> I do bpf_link__disconnect(). But then if the kernel gets upgraded to
> >> support bpf_link for TC programs I'll suddenly transparently get
> >> bpf_link and the attachments will go away unless I pin them. This
> >> seems... less than ideal?
> >
> > That's what we are doing with bpf_program__attach_kprobe(), though.
> > And so far I've only seen people (privately) saying how good it would
> > be to have bpf_link-based TC APIs, doesn't seem like anyone with a
> > realistic use case prefers the current APIs. So I suspect it's not
> > going to be a problem in practice. But at least I'd start there and
> > see how people are using it and if they need anything else.
>
> *sigh* - I really wish you would stop arbitrarily declaring your own use
> cases "realistic" and mine (implied) "unrealistic". Makes it really hard
> to have a productive discussion...

Well (sigh?..), this wasn't my intention, sorry you read it this way.
But we had similar discussions when I was adding bpf_link-based XDP
attach APIs. And guess what, now I see that samples/bpf/whatever_xdp
is switched to bpf_link-based XDP, because that makes everything
simpler and more reliable. What I also know is that in production we
ran into multiple issues with anything that doesn't auto-detach on
process exit/crash (unless pinned explicitly, of course). And that
people that are trying to use TC right now are saying how having
bpf_link-based TC APIs would make everything *simpler* and *safer*. So
I don't know... I understand it might be convenient in some cases to
not care about a lifetime of BPF programs you are attaching, but then
there are usually explicit and intentional ways to achieve at least
similar behavior with safety by default.

So I guess call me unconvinced (yet? still?). Give it another shot, though.

>
> >> If we expose the low-level API I can elect to just use this if I know I
> >> want netlink behaviour, but if bpf_program__attach_tc() is the only API
> >> available it would at least need a flag to enforce one mode or the other
> >> (I can see someone wanting to enforce kernel bpf_link semantics as well,
> >> so a flag for either mode seems reasonable?).
> >
> > Sophisticated enough users can also do feature detection to know if
> > it's going to work or not.
>
> Sure, but that won't help if there's no API to pick the attach mode they
> want.

I'm not intending to allow legacy kprobe APIs to be "chosen", for
instance. Because I'm convinced it's a bad API that no one should use
if they can use an FD-based one. It might be a different case for TC,
who knows. I'd just start with safer APIs and then evaluate whether
there is a real demand for less safe ones. It's just some minor
refactoring and exposing more APIs, when/if we need them.

>
> > There are many ways to skin this cat. I'd prioritize bpf_link-based TC
> > APIs to be added with legacy TC API as a fallback.
>
> I'm fine with adding that; I just want the functions implementing the TC
> API to also be exported so users can use those if they prefer...
>
> -Toke
>