Re: [PATCH bpf-next v3 2/3] libbpf: add low level TC-BPF API

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 and removal of
filters. Some additional features like TCA_BPF_POLICE and TCA_RATE for
the API have been omitted. These can added on top later by extending the
bpf_tc_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_attach may be used to attach, and replace SCHED_CLS bpf
classifier. The protocol is always set as ETH_P_ALL. The replace option
in bpf_tc_opts is used to control replacement behavior.  Attachment
fails if filter with existing attributes already exists.

bpf_tc_detach may be used to detach existing SCHED_CLS filter. The
bpf_tc_attach_id object filled in during attach must be passed in to the
detach functions for them to remove the filter and its attached
classififer correctly.

bpf_tc_get_info is a helper that can be used to obtain attributes
for the filter and classififer.

Examples:

        struct bpf_tc_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_attach(fd, if_nametoindex("lo"),
                          BPF_TC_CLSACT_INGRESS,
                          NULL, &id);
        if (r < 0)
                return r;

... which is roughly equivalent to:
  # tc qdisc add dev lo clsact
  # tc filter add dev lo ingress bpf obj foo.o sec classifier da

... as direct action mode is always enabled.

To replace an existing filter:

        DECLARE_LIBBPF_OPTS(bpf_tc_opts, opts, .handle = id.handle,
                            .priority = id.priority, .replace = true);
        r = bpf_tc_attach(fd, if_nametoindex("lo"),
                          BPF_TC_CLSACT_INGRESS,
                          &opts, &id);
        if (r < 0)
                return r;

To obtain info of a particular filter, the example above can be extended
as follows:

        struct bpf_tc_info info = {};

        r = bpf_tc_get_info(if_nametoindex("lo"),
                            BPF_TC_CLSACT_INGRESS,
                            &id, &info);
        if (r < 0)
                return r;

... where id corresponds to the bpf_tc_attach_id filled in during an
attach operation.

Reviewed-by: Toke Høiland-Jørgensen <redacted>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 tools/lib/bpf/libbpf.h   |  44 ++++++
 tools/lib/bpf/libbpf.map |   3 +
 tools/lib/bpf/netlink.c  | 319 ++++++++++++++++++++++++++++++++++++++-
 3 files changed, 360 insertions(+), 6 deletions(-)

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index bec4e6a6e31d..b4ed6a41ea70 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h

@@ -16,6 +16,8 @@
 #include <stdbool.h>
 #include <sys/types.h>  // for size_t
 #include <linux/bpf.h>
+#include <linux/pkt_sched.h>
+#include <linux/tc_act/tc_bpf.h>

 #include "libbpf_common.h"

@@ -775,6 +777,48 @@ LIBBPF_API int bpf_linker__add_file(struct bpf_linker *linker, const char *filen
 LIBBPF_API int bpf_linker__finalize(struct bpf_linker *linker);
 LIBBPF_API void bpf_linker__free(struct bpf_linker *linker);

+/* Convenience macros for the clsact attach hooks */
+#define BPF_TC_CLSACT_INGRESS TC_H_MAKE(TC_H_CLSACT, TC_H_MIN_INGRESS)
+#define BPF_TC_CLSACT_EGRESS TC_H_MAKE(TC_H_CLSACT, TC_H_MIN_EGRESS)

+                              const struct bpf_tc_attach_id *id,
+                              struct bpf_tc_info *info);
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif

diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index b9b29baf1df8..686444fbb838 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map

@@ -361,4 +361,7 @@ LIBBPF_0.4.0 {
                bpf_linker__new;
                bpf_map__inner_map;
                bpf_object__set_kversion;
+               bpf_tc_attach;
+               bpf_tc_detach;
+               bpf_tc_get_info;
 } LIBBPF_0.3.0;