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

[Toke Høiland-Jørgensen]

Daniel Borkmann <daniel@xxxxxxxxxxxxx> writes:

> 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.

Hmm, I'm OK with dropping the TC oddities (including the cls_ in the
name), but I think we should be documenting it so that users that do
come from TC will not be completely lost :)

-Toke