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 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.

>
> > 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
>