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BPFTOOL-GEN(8) BPFTOOL-GEN(8)

NAME

bpftool-gen - tool for BPF code-generation

SYNOPSIS

bpftool [OPTIONS] gen COMMAND

OPTIONS := { { -j | --json } [{ -p | --pretty }] }

COMMAND := { skeleton | help }



GEN COMMANDS

bpftool gen skeleton FILE
bpftool gen help

DESCRIPTION

bpftool gen skeleton FILE
Generate BPF skeleton C header file for a given FILE.

BPF skeleton is an alternative interface to existing libbpf APIs for working with BPF objects. Skeleton code is intended to significantly shorten and simplify code to load and work with BPF programs from userspace side. Generated code is tailored to specific input BPF object FILE, reflecting its structure by listing out available maps, program, variables, etc. Skeleton eliminates the need to lookup mentioned components by name. Instead, if skeleton instantiation succeeds, they are populated in skeleton structure as valid libbpf types (e.g., struct bpf_map pointer) and can be passed to existing generic libbpf APIs.

In addition to simple and reliable access to maps and programs, skeleton provides a storage for BPF links (struct bpf_link) for each BPF program within BPF object. When requested, supported BPF programs will be automatically attached and resulting BPF links stored for further use by user in pre-allocated fields in skeleton struct. For BPF programs that can't be automatically attached by libbpf, user can attach them manually, but store resulting BPF link in per-program link field. All such set up links will be automatically destroyed on BPF skeleton destruction. This eliminates the need for users to manage links manually and rely on libbpf support to detach programs and free up resources.

Another facility provided by BPF skeleton is an interface to global variables of all supported kinds: mutable, read-only, as well as extern ones. This interface allows to pre-setup initial values of variables before BPF object is loaded and verified by kernel. For non-read-only variables, the same interface can be used to fetch values of global variables on userspace side, even if they are modified by BPF code.

During skeleton generation, contents of source BPF object FILE is embedded within generated code and is thus not necessary to keep around. This ensures skeleton and BPF object file are matching 1-to-1 and always stay in sync. Generated code is dual-licensed under LGPL-2.1 and BSD-2-Clause licenses.

It is a design goal and guarantee that skeleton interfaces are interoperable with generic libbpf APIs. User should always be able to use skeleton API to create and load BPF object, and later use libbpf APIs to keep working with specific maps, programs, etc.

As part of skeleton, few custom functions are generated. Each of them is prefixed with object name, derived from object file name. I.e., if BPF object file name is example.o, BPF object name will be example. The following custom functions are provided in such case:

  • example__open and example__open_opts. These functions are used to instantiate skeleton. It corresponds to libbpf's bpf_object__open() API. _opts variants accepts extra bpf_object_open_opts options.
  • example__load. This function creates maps, loads and verifies BPF programs, initializes global data maps. It corresponds to libppf's bpf_object__load() API.
  • example__open_and_load combines example__open and example__load invocations in one commonly used operation.
  • example__attach and example__detach This pair of functions allow to attach and detach, correspondingly, already loaded BPF object. Only BPF programs of types supported by libbpf for auto-attachment will be auto-attached and their corresponding BPF links instantiated. For other BPF programs, user can manually create a BPF link and assign it to corresponding fields in skeleton struct. example__detach will detach both links created automatically, as well as those populated by user manually.
  • example__destroy Detach and unload BPF programs, free up all the resources used by skeleton and BPF object.

If BPF object has global variables, corresponding structs with memory layout corresponding to global data data section layout will be created. Currently supported ones are: .data, .bss, .rodata, and .kconfig structs/data sections. These data sections/structs can be used to set up initial values of variables, if set before example__load. Afterwards, if target kernel supports memory-mapped BPF arrays, same structs can be used to fetch and update (non-read-only) data from userspace, with same simplicity as for BPF side.

bpftool gen help
Print short help message.



OPTIONS

-h, --help
Print short help message (similar to bpftool help).
-V, --version
Print version number (similar to bpftool version), and optional features that were included when bpftool was compiled. Optional features include linking against libbfd to provide the disassembler for JIT-ted programs (bpftool prog dump jited) and usage of BPF skeletons (some features like bpftool prog profile or showing pids associated to BPF objects may rely on it).
-j, --json
Generate JSON output. For commands that cannot produce JSON, this option has no effect.
-p, --pretty
Generate human-readable JSON output. Implies -j.
-d, --debug
Print all logs available, even debug-level information. This includes logs from libbpf as well as from the verifier, when attempting to load programs.



EXAMPLES

$ cat example.c 

#include <stdbool.h>
#include <linux/ptrace.h>
#include <linux/bpf.h>
#include "bpf_helpers.h"
const volatile int param1 = 42;
bool global_flag = true;
struct { int x; } data = {};
struct {
      __uint(type, BPF_MAP_TYPE_HASH);
      __uint(max_entries, 128);
      __type(key, int);
      __type(value, long);
} my_map SEC(".maps");
SEC("raw_tp/sys_enter")
int handle_sys_enter(struct pt_regs *ctx)
{
      static long my_static_var;
      if (global_flag)
              my_static_var++;
      else
              data.x += param1;
      return 0;
}
SEC("raw_tp/sys_exit")
int handle_sys_exit(struct pt_regs *ctx)
{
      int zero = 0;
      bpf_map_lookup_elem(&my_map, &zero);
      return 0;
}


This is example BPF application with two BPF programs and a mix of BPF maps and global variables.

$ bpftool gen skeleton example.o

/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
/* THIS FILE IS AUTOGENERATED! */
#ifndef __EXAMPLE_SKEL_H__
#define __EXAMPLE_SKEL_H__
#include <stdlib.h>
#include <bpf/libbpf.h>
struct example {
      struct bpf_object_skeleton *skeleton;
      struct bpf_object *obj;
      struct {
              struct bpf_map *rodata;
              struct bpf_map *data;
              struct bpf_map *bss;
              struct bpf_map *my_map;
      } maps;
      struct {
              struct bpf_program *handle_sys_enter;
              struct bpf_program *handle_sys_exit;
      } progs;
      struct {
              struct bpf_link *handle_sys_enter;
              struct bpf_link *handle_sys_exit;
      } links;
      struct example__bss {
              struct {
                      int x;
              } data;
      } *bss;
      struct example__data {
              _Bool global_flag;
              long int handle_sys_enter_my_static_var;
      } *data;
      struct example__rodata {
              int param1;
      } *rodata;
};
static void example__destroy(struct example *obj);
static inline struct example *example__open_opts(
              const struct bpf_object_open_opts *opts);
static inline struct example *example__open();
static inline int example__load(struct example *obj);
static inline struct example *example__open_and_load();
static inline int example__attach(struct example *obj);
static inline void example__detach(struct example *obj);
#endif /* __EXAMPLE_SKEL_H__ */


$ cat example_user.c

#include "example.skel.h"
int main()
{
      struct example *skel;
      int err = 0;
      skel = example__open();
      if (!skel)
              goto cleanup;
      skel->rodata->param1 = 128;
      err = example__load(skel);
      if (err)
              goto cleanup;
      err = example__attach(skel);
      if (err)
              goto cleanup;
      /* all libbpf APIs are usable */
      printf("my_map name: %s\n", bpf_map__name(skel->maps.my_map));
      printf("sys_enter prog FD: %d\n",
             bpf_program__fd(skel->progs.handle_sys_enter));
      /* detach and re-attach sys_exit program */
      bpf_link__destroy(skel->links.handle_sys_exit);
      skel->links.handle_sys_exit =
              bpf_program__attach(skel->progs.handle_sys_exit);
      printf("my_static_var: %ld\n",
             skel->bss->handle_sys_enter_my_static_var);
cleanup:
      example__destroy(skel);
      return err;
}


# ./example_user

my_map name: my_map
sys_enter prog FD: 8
my_static_var: 7


This is a stripped-out version of skeleton generated for above example code.

SEE ALSO

bpf(2), bpf-helpers(7), bpftool(8), bpftool-btf(8), bpftool-cgroup(8), bpftool-feature(8), bpftool-iter(8), bpftool-link(8), bpftool-map(8), bpftool-net(8), bpftool-perf(8), bpftool-prog(8), bpftool-struct_ops(8)