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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 16 Apr 2021 11:01:18 +0200

Daniel Borkmann <[email-masked]> writes:

> On 4/16/21 12:22 AM, Andrii Nakryiko wrote:
>> On Thu, Apr 15, 2021 at 3:10 PM Daniel Borkmann <[email-masked]> wrote:
>>> On 4/15/21 1:58 AM, Andrii Nakryiko wrote:
>>>> On Wed, Apr 14, 2021 at 4:32 PM Daniel Borkmann <[email-masked]> wrote:
>>>>> On 4/15/21 1:19 AM, Andrii Nakryiko wrote:
>>>>>> On Wed, Apr 14, 2021 at 3:51 PM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>>>>>> Andrii Nakryiko <[email-masked]> writes:
>>>>>>>> On Wed, Apr 14, 2021 at 3:58 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>>>>>>>> Andrii Nakryiko <[email-masked]> writes:
>>>>>>>>>> On Tue, Apr 6, 2021 at 3:06 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>>>>>>>>>> Andrii Nakryiko <[email-masked]> writes:
>>>>>>>>>>>> On Sat, Apr 3, 2021 at 10:47 AM Alexei Starovoitov
>>>>>>>>>>>> <[email-masked]> 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 <[email-masked]> 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.

Sounds reasonable to me, with the addition of 'parent' to the things you
can specify.

So snipping a few bits from Kumar's patch and paring it down a bit, we'd
end up with something like this?

+struct bpf_tc_cls_opts {
+	size_t sz;
+	__u32 chain_index;
+	__u32 handle;
+	__u32 priority;
+	__u32 class_id;
+};
+#define bpf_tc_cls_opts__last_field class_id
+
+/* Acts as a handle for an attached filter */
+struct bpf_tc_cls_attach_id {
+	__u32 ifindex;
+	union {
+		__u32 block_index;
+		__u32 parent_id;
+	};
+	__u32 protocol;
+	__u32 chain_index;
+	__u32 handle;
+	__u32 priority;
+};
+
+struct bpf_tc_cls_info {
+	struct bpf_tc_cls_attach_id id;
+	__u32 class_id;
+	__u32 bpf_flags;
+	__u32 bpf_flags_gen;
+};
+
+LIBBPF_API int bpf_tc_cls_attach_dev(int fd, __u32 ifindex, __u32 parent_id,
+				     const struct bpf_tc_cls_opts *opts,
+				     struct bpf_tc_cls_attach_id *id);
+LIBBPF_API int bpf_tc_cls_detach_dev(const struct bpf_tc_cls_attach_id *id);
+LIBBPF_API int bpf_tc_cls_get_info_dev(int fd, __u32 ifindex, __u32 parent_id,
+				       const struct bpf_tc_cls_opts *opts,
+				       struct bpf_tc_cls_info *info);

What about change and replace? I guess we could do without those, right?

-Toke