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

[Vlad Buslov]

On Mon 29 Mar 2021 at 15:32, Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
> Vlad Buslov <vladbu@xxxxxxxxxx> writes:
>
>> On Thu 25 Mar 2021 at 14:00, Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote:
>>> This adds functions that wrap the netlink API used for adding,
>>> manipulating, and removing filters and actions. These functions operate
>>> directly on the loaded prog's fd, and return a handle to the filter and
>>> action using an out parameter (id for tc_cls, and index for tc_act).
>>>
>>> The basic featureset is covered to allow for attaching, manipulation of
>>> properties, and removal of filters and actions. 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.
>>>
>>> An API summary:
>>>
>>> The BPF TC-CLS API
>>>
>>> bpf_tc_act_{attach, change, replace}_{dev, block} may be used to attach,
>>> change, and replace SCHED_CLS bpf classifiers. Separate set of functions
>>> are provided for network interfaces and shared filter blocks.
>>>
>>> bpf_tc_cls_detach_{dev, block} 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_dev(fd, if_nametoindex("lo"),
>>> 				  BPF_TC_CLSACT_INGRESS, ETH_P_IP,
>>> 				  NULL, &id);
>>> 	if (r < 0)
>>> 		return r;
>>>
>>> ... which is roughly equivalent to (after clsact qdisc setup):
>>>   # tc filter add dev lo ingress bpf obj /home/kkd/foo.o sec classifier
>>>
>>> If a user wishes to modify existing options on an attached filter, the
>>> bpf_tc_cls_change_{dev, block} API may be used. Parameters like
>>> chain_index, priority, and handle are ignored in the bpf_tc_cls_opts
>>> struct as they cannot be modified after attaching a filter.
>>>
>>> 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)
>>> 	/* Turn on direct action mode */
>>> 	opts.direct_action = true;
>>> 	r = bpf_tc_cls_change_dev(fd, id.ifindex, id.parent_id,
>>> 			          id.protocol, &opts, &id);
>>> 	if (r < 0)
>>> 		return r;
>>>
>>> 	/* Verify that the direct action mode has been set */
>>> 	struct bpf_tc_cls_info info = {};
>>> 	r = bpf_tc_cls_get_info_dev(fd, id.ifindex, id.parent_id,
>>> 			            id.protocol, &opts, &info);
>>> 	if (r < 0)
>>> 		return r;
>>>
>>> 	assert(info.bpf_flags & TCA_BPF_FLAG_ACT_DIRECT);
>>>
>>> This would be roughly equivalent to doing:
>>>   # tc filter change dev lo egress prio <p> handle <h> bpf obj /home/kkd/foo.o section classifier da
>>>
>>> ... except a new bpf program will be loaded and replace existing one.
>>>
>>> If a user wishes to either replace an existing filter, or create a new
>>> one with the same properties, they can use bpf_tc_cls_replace_dev. The
>>> benefit of bpf_tc_cls_change is that it fails if no matching filter
>>> exists.
>>>
>>> The BPF TC-ACT API
>>>
>>> bpf_tc_act_{attach, replace} may be used to attach and replace already
>>> attached SCHED_ACT actions. Passing an index of 0 has special meaning,
>>> in that an index will be automatically chosen by the kernel. The index
>>> chosen by the kernel is the return value of these functions in case of
>>> success.
>>>
>>> bpf_tc_act_detach may be used to detach a SCHED_ACT action prog
>>> identified by the index parameter. The index 0 again has a special
>>> meaning, in that passing it will flush all existing SCHED_ACT actions
>>> loaded using the ACT API.
>>>
>>> bpf_tc_act_get_info is a helper to get the required attributes of a
>>> loaded program to be able to manipulate it futher, by passing them
>>> into the aforementioned functions.
>>>
>>> Example:
>>>
>>> 	struct bpf_object *obj;
>>> 	struct bpf_program *p;
>>> 	__u32 index;
>>> 	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, "action");
>>> 	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_act_attach(fd, NULL, &index);
>>> 	if (r < 0)
>>> 		return r;
>>>
>>> 	if (bpf_tc_act_detach(index))
>>> 		return -1;
>>>
>>> ... which is equivalent to the following sequence:
>>> 	tc action add action bpf obj /home/kkd/foo.o sec action
>>> 	tc action del action bpf index <idx>
>>
>> How do you handle the locking here? Please note that while
>> RTM_{NEW|GET|DEL}FILTER API has been refactored to handle its own
>> locking internally (and registered with RTNL_FLAG_DOIT_UNLOCKED flag),
>> RTM_{NEW|GET|DEL}ACTION API still expects to be called with rtnl lock
>> taken.
>
> Huh, locking? This is all userspace code that uses the netlink API...
>
> -Toke

Thanks for the clarification. I'm not familiar with libbpf internals and
it wasn't obvious to me that this functionality is not for creating
classifiers/actions from BPF program executing in kernel-space.