Date:   Mon, 29 Mar 2021 14:32:16 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Vlad Buslov <vladbu@...dia.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf@...r.kernel.org, brouer@...hat.com,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        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>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next 3/5] libbpf: add low level TC-BPF API

Vlad Buslov <vladbu@...dia.com> writes:

> On Thu 25 Mar 2021 at 14:00, Kumar Kartikeya Dwivedi <memxor@...il.com> 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

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