Re: [PATCH bpf-next v2 3/4] libbpf: add low level TC-BPF API

[Daniel Borkmann]

On 4/19/21 2:18 PM, Kumar Kartikeya Dwivedi wrote:
> This adds functions that wrap the netlink API used for adding,
> manipulating, and removing traffic control filters. These functions
> operate directly on the loaded prog's fd, and return a handle to the
> filter using an out parameter named id.
>
> The basic featureset is covered to allow for attaching, manipulation of
> properties, and removal of filters. Some additional features like
> TCA_BPF_POLICE and TCA_RATE for tc_cls have been omitted. These can
> added on top later by extending the bpf_tc_cls_opts struct.
>
> Support for binding actions directly to a classifier by passing them in
> during filter creation has also been omitted for now. These actions have
> an auto clean up property because their lifetime is bound to the filter
> they are attached to. This can be added later, but was omitted for now
> as direct action mode is a better alternative to it, which is enabled by
> default.
>
> An API summary:
>
> bpf_tc_act_{attach, change, replace} may be used to attach, change, and

typo on bpf_tc_act_{...} ?
               ^^^
> replace SCHED_CLS bpf classifier. The protocol field can be set as 0, in
> which case it is subsitituted as ETH_P_ALL by default.

Do you have an actual user that needs anything other than ETH_P_ALL? Why is it
even needed? Why not stick to just ETH_P_ALL?

> The behavior of the three functions is as follows:
>
> attach = create filter if it does not exist, fail otherwise
> change = change properties of the classifier of existing filter
> replace = create filter, and replace any existing filter

This touches on tc oddities quite a bit. Why do we need to expose them? Can't we
simplify/abstract this e.g. i) create or update instance, ii) delete instance,
iii) get instance ? What concrete use case do you have that you need those three
above?

> bpf_tc_cls_detach may be used to detach existing SCHED_CLS
> filter. The bpf_tc_cls_attach_id object filled in during attach,
> change, or replace must be passed in to the detach functions for them to
> remove the filter and its attached classififer correctly.
>
> bpf_tc_cls_get_info is a helper that can be used to obtain attributes
> for the filter and classififer. The opts structure may be used to
> choose the granularity of search, such that info for a specific filter
> corresponding to the same loaded bpf program can be obtained. By
> default, the first match is returned to the user.
>
> Examples:
>
> 	struct bpf_tc_cls_attach_id id = {};
> 	struct bpf_object *obj;
> 	struct bpf_program *p;
> 	int fd, r;
>
> 	obj = bpf_object_open("foo.o");
> 	if (IS_ERR_OR_NULL(obj))
> 		return PTR_ERR(obj);
>
> 	p = bpf_object__find_program_by_title(obj, "classifier");
> 	if (IS_ERR_OR_NULL(p))
> 		return PTR_ERR(p);
>
> 	if (bpf_object__load(obj) < 0)
> 		return -1;
>
> 	fd = bpf_program__fd(p);
>
> 	r = bpf_tc_cls_attach(fd, if_nametoindex("lo"),
> 			      BPF_TC_CLSACT_INGRESS,
> 			      NULL, &id);
> 	if (r < 0)
> 		return r;
>
> ... which is roughly equivalent to (after clsact qdisc setup):
>    # tc filter add dev lo ingress bpf obj foo.o sec classifier da
>
> ... as direct action mode is always enabled.
>
> If a user wishes to modify existing options on an attached classifier,
> bpf_tc_cls_change API may be used.
>
> Only parameters class_id can be modified, the rest are filled in to
> identify the correct filter. protocol can be left out if it was not
> chosen explicitly (defaulting to ETH_P_ALL).
>
> Example:
>
> 	/* Optional parameters necessary to select the right filter */
> 	DECLARE_LIBBPF_OPTS(bpf_tc_cls_opts, opts,
> 			    .handle = id.handle,
> 			    .priority = id.priority,
> 			    .chain_index = id.chain_index)

Why do we need chain_index as part of the basic API?

> 	opts.class_id = TC_H_MAKE(1UL << 16, 12);
> 	r = bpf_tc_cls_change(fd, if_nametoindex("lo"),
> 			      BPF_TC_CLSACT_INGRESS,
> 			      &opts, &id);

Also, I'm not sure whether the prefix should even be named  bpf_tc_cls_*() tbh,
yes, despite being "low level" api. I think in the context of bpf we should stop
regarding this as 'classifier' and 'action' objects since it's really just a
single entity and not separate ones. It's weird enough to explain this concept
to new users and if a libbpf based api could cleanly abstract it, I would be all
for it. I don't think we need to map 1:1 the old tc legacy even in the low level
api, tbh, as it feels backwards. I think the 'handle' & 'priority' bits are okay,
but I would remove the others.

> 	if (r < 0)
> 		return r;
>
> 	struct bpf_tc_cls_info info = {};
> 	r = bpf_tc_cls_get_info(fd, if_nametoindex("lo"),
> 			        BPF_TC_CLSACT_INGRESS,
> 				&opts, &info);
> 	if (r < 0)
> 		return r;
>
> 	assert(info.class_id == TC_H_MAKE(1UL << 16, 12));
>
> This would be roughly equivalent to doing:
>    # tc filter change dev lo egress prio <p> handle <h> bpf obj foo.o sec \
>      classifier classid 1:12

Why even bother to support !da mode, what are you trying to solve with it? I
don't think official libbpf api should support something that doesn't scale.

> ... except a new bpf program will be loaded and replace existing one.
>
> Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>