Date:   Fri, 16 Apr 2021 00:10:20 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        bpf <bpf@...r.kernel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Shuah Khan <shuah@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        open list <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next 3/5] libbpf: add low level TC-BPF API

On 4/15/21 1:58 AM, Andrii Nakryiko wrote:
> On Wed, Apr 14, 2021 at 4:32 PM Daniel Borkmann <daniel@...earbox.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@...hat.com> wrote:
>>>> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>>>>> On Wed, Apr 14, 2021 at 3:58 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>>>>> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>>>>>>> On Tue, Apr 6, 2021 at 3:06 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>>>>>>> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>>>>>>>>> On Sat, Apr 3, 2021 at 10:47 AM Alexei Starovoitov
>>>>>>>>> <alexei.starovoitov@...il.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@...il.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.

Thanks,
Daniel

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