Messages in this thread | | | Subject | Re: [PATCH bpf-next 3/5] libbpf: add low level TC-BPF API | From | Daniel Borkmann <> | Date | Fri, 16 Apr 2021 01:10:01 +0200 |
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On 4/16/21 12:22 AM, Andrii Nakryiko wrote: > 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().
I think that's okay. I guess question is what do we define as initial scope for the low-level TC API. cls_bpf w/ fixed direct-action mode + fixed eth_p_all, allowing to flexibly specify handle / priority or a block_index feels reasonable.
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