linux/tools/bpf/bpftool/Documentation/bpftool-gen.rst

================
bpftool-gen
================
-------------------------------------------------------------------------------
tool for BPF code-generation
-------------------------------------------------------------------------------

:Manual section: 8

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
=======
	.. include:: common_options.rst

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.