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

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

On Thu, Apr 15, 2021 at 3:10 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 4/15/21 1:58 AM, Andrii Nakryiko wrote:
> > On Wed, Apr 14, 2021 at 4:32 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
> >> On 4/15/21 1:19 AM, Andrii Nakryiko wrote:
> >>> 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.
> >>
> >> [...]
> >>
> >>   >>> 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 think the problem here is though that this would need to be deterministic
> >> when upgrading from one kernel version to another where we don't use the
> >> fallback anymore, e.g. in case of Cilium we always want to keep the progs
> >> attached to allow headless updates on the agent, meaning, traffic keeps
> >> flowing through the BPF datapath while in user space, our agent restarts
> >> after upgrade, and atomically replaces the BPF progs once up and running
> >> (we're doing this for the whole range of 4.9 to 5.x kernels that we support).
> >> While we use the 'simple' api that is discussed here internally in Cilium,
> >> this attach behavior would have to be consistent, so transparent fallback
> >> inside libbpf on link vs non-link availability won't work (at least in our
> >> case).
> >
> > What about pinning? It's not exactly the same, but bpf_link could
> > actually pin a BPF program, if using legacy TC, and pin bpf_link, if
> > using bpf_link-based APIs. Of course before switching from iproute2 to
> > libbpf APIs you'd need to design your applications to use pinning
> > instead of relying implicitly on permanently attached BPF program.
>
> All the progs we load from Cilium in a K8s setting w/ Pods, we could have easily
> over 100 loaded at the same time on a node, and we template the per Pod ones, so
> the complexity of managing those pinned lifecycles from the agent and dealing with
> the semantic/fallback differences between kernels feels probably not worth the
> gain. So if there would be a libbpf tc simplified attach API, I'd for the time
> being stick to the existing aka legacy means.

Sure. Then what do you think about keeping only low-level TC APIs, and
in the future add bpf_program__attach_tc(), which will use
bpf_link-based one. It seems like it's not worth it to pretend we have
bpf_link-based semantics with "legacy" current TC APIs. Similarly how
we have a low-level XDP attach API, and bpf_link-based (only)
bpf_program__attach_xdp().

>
> Thanks,
> Daniel