xref: /openbmc/linux/tools/bpf/bpftool/Documentation/bpftool-gen.rst (revision bbaf1ff0)

.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)

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

:Manual section: 8

.. include:: substitutions.rst

SYNOPSIS
========

	**bpftool** [*OPTIONS*] **gen** *COMMAND*

	*OPTIONS* := { |COMMON_OPTIONS| | { **-L** | **--use-loader** } }

	*COMMAND* := { **object** | **skeleton** | **help** }

GEN COMMANDS
=============

|	**bpftool** **gen object** *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
|	**bpftool** **gen skeleton** *FILE* [**name** *OBJECT_NAME*]
|	**bpftool** **gen subskeleton** *FILE* [**name** *OBJECT_NAME*]
|	**bpftool** **gen min_core_btf** *INPUT* *OUTPUT* *OBJECT* [*OBJECT*...]
|	**bpftool** **gen help**

DESCRIPTION
===========
	**bpftool gen object** *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
		  Statically link (combine) together one or more *INPUT_FILE*'s
		  into a single resulting *OUTPUT_FILE*. All the files involved
		  are BPF ELF object files.

		  The rules of BPF static linking are mostly the same as for
		  user-space object files, but in addition to combining data
		  and instruction sections, .BTF and .BTF.ext (if present in
		  any of the input files) data are combined together. .BTF
		  data is deduplicated, so all the common types across
		  *INPUT_FILE*'s will only be represented once in the resulting
		  BTF information.

		  BPF static linking allows to partition BPF source code into
		  individually compiled files that are then linked into
		  a single resulting BPF object file, which can be used to
		  generated BPF skeleton (with **gen skeleton** command) or
		  passed directly into **libbpf** (using **bpf_object__open()**
		  family of APIs).

	**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. Object name can
		  either be derived from object file name, i.e., if BPF object
		  file name is **example.o**, BPF object name will be
		  **example**. Object name can be also specified explicitly
		  through **name** *OBJECT_NAME* parameter. The following
		  custom functions are provided (assuming **example** as
		  the object name):

		  - **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 subskeleton** *FILE*
		  Generate BPF subskeleton C header file for a given *FILE*.

		  Subskeletons are similar to skeletons, except they do not own
		  the corresponding maps, programs, or global variables. They
		  require that the object file used to generate them is already
		  loaded into a *bpf_object* by some other means.

		  This functionality is useful when a library is included into a
		  larger BPF program. A subskeleton for the library would have
		  access to all objects and globals defined in it, without
		  having to know about the larger program.

		  Consequently, there are only two functions defined
		  for subskeletons:

		  - **example__open(bpf_object\*)**
		    Instantiates a subskeleton from an already opened (but not
		    necessarily loaded) **bpf_object**.

		  - **example__destroy()**
		    Frees the storage for the subskeleton but *does not* unload
		    any BPF programs or maps.

	**bpftool** **gen min_core_btf** *INPUT* *OUTPUT* *OBJECT* [*OBJECT*...]
		  Generate a minimum BTF file as *OUTPUT*, derived from a given
		  *INPUT* BTF file, containing all needed BTF types so one, or
		  more, given eBPF objects CO-RE relocations may be satisfied.

		  When kernels aren't compiled with CONFIG_DEBUG_INFO_BTF,
		  libbpf, when loading an eBPF object, has to rely on external
		  BTF files to be able to calculate CO-RE relocations.

		  Usually, an external BTF file is built from existing kernel
		  DWARF data using pahole. It contains all the types used by
		  its respective kernel image and, because of that, is big.

		  The min_core_btf feature builds smaller BTF files, customized
		  to one or multiple eBPF objects, so they can be distributed
		  together with an eBPF CO-RE based application, turning the
		  application portable to different kernel versions.

		  Check examples bellow for more information how to use it.

	**bpftool gen help**
		  Print short help message.

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

	-L, --use-loader
		  For skeletons, generate a "light" skeleton (also known as "loader"
		  skeleton). A light skeleton contains a loader eBPF program. It does
		  not use the majority of the libbpf infrastructure, and does not need
		  libelf.

EXAMPLES
========
**$ cat example1.bpf.c**

::

  #include <stdbool.h>
  #include <linux/ptrace.h>
  #include <linux/bpf.h>
  #include <bpf/bpf_helpers.h>

  const volatile int param1 = 42;
  bool global_flag = true;
  struct { int x; } data = {};

  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;
  }

**$ cat example2.bpf.c**

::

  #include <linux/ptrace.h>
  #include <linux/bpf.h>
  #include <bpf/bpf_helpers.h>

  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_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. Source code is split across two source code files.

**$ clang --target=bpf -g example1.bpf.c -o example1.bpf.o**

**$ clang --target=bpf -g example2.bpf.c -o example2.bpf.o**

**$ bpftool gen object example.bpf.o example1.bpf.o example2.bpf.o**

This set of commands compiles *example1.bpf.c* and *example2.bpf.c*
individually and then statically links respective object files into the final
BPF ELF object file *example.bpf.o*.

**$ bpftool gen skeleton example.bpf.o name example | tee example.skel.h**

::

  /* 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.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**

::

  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.

min_core_btf
------------

**$ bpftool btf dump file 5.4.0-example.btf format raw**

::

  [1] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
  [2] CONST '(anon)' type_id=1
  [3] VOLATILE '(anon)' type_id=1
  [4] ARRAY '(anon)' type_id=1 index_type_id=21 nr_elems=2
  [5] PTR '(anon)' type_id=8
  [6] CONST '(anon)' type_id=5
  [7] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=(none)
  [8] CONST '(anon)' type_id=7
  [9] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none)
  <long output>

**$ bpftool btf dump file one.bpf.o format raw**

::

  [1] PTR '(anon)' type_id=2
  [2] STRUCT 'trace_event_raw_sys_enter' size=64 vlen=4
        'ent' type_id=3 bits_offset=0
        'id' type_id=7 bits_offset=64
        'args' type_id=9 bits_offset=128
        '__data' type_id=12 bits_offset=512
  [3] STRUCT 'trace_entry' size=8 vlen=4
        'type' type_id=4 bits_offset=0
        'flags' type_id=5 bits_offset=16
        'preempt_count' type_id=5 bits_offset=24
  <long output>

**$ bpftool gen min_core_btf 5.4.0-example.btf 5.4.0-smaller.btf one.bpf.o**

**$ bpftool btf dump file 5.4.0-smaller.btf format raw**

::

  [1] TYPEDEF 'pid_t' type_id=6
  [2] STRUCT 'trace_event_raw_sys_enter' size=64 vlen=1
        'args' type_id=4 bits_offset=128
  [3] STRUCT 'task_struct' size=9216 vlen=2
        'pid' type_id=1 bits_offset=17920
        'real_parent' type_id=7 bits_offset=18048
  [4] ARRAY '(anon)' type_id=5 index_type_id=8 nr_elems=6
  [5] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
  [6] TYPEDEF '__kernel_pid_t' type_id=8
  [7] PTR '(anon)' type_id=3
  [8] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
  <end>

Now, the "5.4.0-smaller.btf" file may be used by libbpf as an external BTF file
when loading the "one.bpf.o" object into the "5.4.0-example" kernel. Note that
the generated BTF file won't allow other eBPF objects to be loaded, just the
ones given to min_core_btf.

::

  LIBBPF_OPTS(bpf_object_open_opts, opts, .btf_custom_path = "5.4.0-smaller.btf");
  struct bpf_object *obj;

  obj = bpf_object__open_file("one.bpf.o", &opts);

  ...