1.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
2
3================
4bpftool-gen
5================
6-------------------------------------------------------------------------------
7tool for BPF code-generation
8-------------------------------------------------------------------------------
9
10:Manual section: 8
11
12.. include:: substitutions.rst
13
14SYNOPSIS
15========
16
17	**bpftool** [*OPTIONS*] **gen** *COMMAND*
18
19	*OPTIONS* := { |COMMON_OPTIONS| | { **-L** | **--use-loader** } }
20
21	*COMMAND* := { **object** | **skeleton** | **help** }
22
23GEN COMMANDS
24=============
25
26|	**bpftool** **gen object** *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
27|	**bpftool** **gen skeleton** *FILE* [**name** *OBJECT_NAME*]
28|	**bpftool** **gen subskeleton** *FILE* [**name** *OBJECT_NAME*]
29|	**bpftool** **gen min_core_btf** *INPUT* *OUTPUT* *OBJECT* [*OBJECT*...]
30|	**bpftool** **gen help**
31
32DESCRIPTION
33===========
34	**bpftool gen object** *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
35		  Statically link (combine) together one or more *INPUT_FILE*'s
36		  into a single resulting *OUTPUT_FILE*. All the files involved
37		  are BPF ELF object files.
38
39		  The rules of BPF static linking are mostly the same as for
40		  user-space object files, but in addition to combining data
41		  and instruction sections, .BTF and .BTF.ext (if present in
42		  any of the input files) data are combined together. .BTF
43		  data is deduplicated, so all the common types across
44		  *INPUT_FILE*'s will only be represented once in the resulting
45		  BTF information.
46
47		  BPF static linking allows to partition BPF source code into
48		  individually compiled files that are then linked into
49		  a single resulting BPF object file, which can be used to
50		  generated BPF skeleton (with **gen skeleton** command) or
51		  passed directly into **libbpf** (using **bpf_object__open()**
52		  family of APIs).
53
54	**bpftool gen skeleton** *FILE*
55		  Generate BPF skeleton C header file for a given *FILE*.
56
57		  BPF skeleton is an alternative interface to existing libbpf
58		  APIs for working with BPF objects. Skeleton code is intended
59		  to significantly shorten and simplify code to load and work
60		  with BPF programs from userspace side. Generated code is
61		  tailored to specific input BPF object *FILE*, reflecting its
62		  structure by listing out available maps, program, variables,
63		  etc. Skeleton eliminates the need to lookup mentioned
64		  components by name. Instead, if skeleton instantiation
65		  succeeds, they are populated in skeleton structure as valid
66		  libbpf types (e.g., **struct bpf_map** pointer) and can be
67		  passed to existing generic libbpf APIs.
68
69		  In addition to simple and reliable access to maps and
70		  programs, skeleton provides a storage for BPF links (**struct
71		  bpf_link**) for each BPF program within BPF object. When
72		  requested, supported BPF programs will be automatically
73		  attached and resulting BPF links stored for further use by
74		  user in pre-allocated fields in skeleton struct. For BPF
75		  programs that can't be automatically attached by libbpf,
76		  user can attach them manually, but store resulting BPF link
77		  in per-program link field. All such set up links will be
78		  automatically destroyed on BPF skeleton destruction. This
79		  eliminates the need for users to manage links manually and
80		  rely on libbpf support to detach programs and free up
81		  resources.
82
83		  Another facility provided by BPF skeleton is an interface to
84		  global variables of all supported kinds: mutable, read-only,
85		  as well as extern ones. This interface allows to pre-setup
86		  initial values of variables before BPF object is loaded and
87		  verified by kernel. For non-read-only variables, the same
88		  interface can be used to fetch values of global variables on
89		  userspace side, even if they are modified by BPF code.
90
91		  During skeleton generation, contents of source BPF object
92		  *FILE* is embedded within generated code and is thus not
93		  necessary to keep around. This ensures skeleton and BPF
94		  object file are matching 1-to-1 and always stay in sync.
95		  Generated code is dual-licensed under LGPL-2.1 and
96		  BSD-2-Clause licenses.
97
98		  It is a design goal and guarantee that skeleton interfaces
99		  are interoperable with generic libbpf APIs. User should
100		  always be able to use skeleton API to create and load BPF
101		  object, and later use libbpf APIs to keep working with
102		  specific maps, programs, etc.
103
104		  As part of skeleton, few custom functions are generated.
105		  Each of them is prefixed with object name. Object name can
106		  either be derived from object file name, i.e., if BPF object
107		  file name is **example.o**, BPF object name will be
108		  **example**. Object name can be also specified explicitly
109		  through **name** *OBJECT_NAME* parameter. The following
110		  custom functions are provided (assuming **example** as
111		  the object name):
112
113		  - **example__open** and **example__open_opts**.
114		    These functions are used to instantiate skeleton. It
115		    corresponds to libbpf's **bpf_object__open**\ () API.
116		    **_opts** variants accepts extra **bpf_object_open_opts**
117		    options.
118
119		  - **example__load**.
120		    This function creates maps, loads and verifies BPF
121		    programs, initializes global data maps. It corresponds to
122		    libppf's **bpf_object__load**\ () API.
123
124		  - **example__open_and_load** combines **example__open** and
125		    **example__load** invocations in one commonly used
126		    operation.
127
128		  - **example__attach** and **example__detach**
129		    This pair of functions allow to attach and detach,
130		    correspondingly, already loaded BPF object. Only BPF
131		    programs of types supported by libbpf for auto-attachment
132		    will be auto-attached and their corresponding BPF links
133		    instantiated. For other BPF programs, user can manually
134		    create a BPF link and assign it to corresponding fields in
135		    skeleton struct. **example__detach** will detach both
136		    links created automatically, as well as those populated by
137		    user manually.
138
139		  - **example__destroy**
140		    Detach and unload BPF programs, free up all the resources
141		    used by skeleton and BPF object.
142
143		  If BPF object has global variables, corresponding structs
144		  with memory layout corresponding to global data data section
145		  layout will be created. Currently supported ones are: *.data*,
146		  *.bss*, *.rodata*, and *.kconfig* structs/data sections.
147		  These data sections/structs can be used to set up initial
148		  values of variables, if set before **example__load**.
149		  Afterwards, if target kernel supports memory-mapped BPF
150		  arrays, same structs can be used to fetch and update
151		  (non-read-only) data from userspace, with same simplicity
152		  as for BPF side.
153
154	**bpftool gen subskeleton** *FILE*
155		  Generate BPF subskeleton C header file for a given *FILE*.
156
157		  Subskeletons are similar to skeletons, except they do not own
158		  the corresponding maps, programs, or global variables. They
159		  require that the object file used to generate them is already
160		  loaded into a *bpf_object* by some other means.
161
162		  This functionality is useful when a library is included into a
163		  larger BPF program. A subskeleton for the library would have
164		  access to all objects and globals defined in it, without
165		  having to know about the larger program.
166
167		  Consequently, there are only two functions defined
168		  for subskeletons:
169
170		  - **example__open(bpf_object\*)**
171		    Instantiates a subskeleton from an already opened (but not
172		    necessarily loaded) **bpf_object**.
173
174		  - **example__destroy()**
175		    Frees the storage for the subskeleton but *does not* unload
176		    any BPF programs or maps.
177
178	**bpftool** **gen min_core_btf** *INPUT* *OUTPUT* *OBJECT* [*OBJECT*...]
179		  Generate a minimum BTF file as *OUTPUT*, derived from a given
180		  *INPUT* BTF file, containing all needed BTF types so one, or
181		  more, given eBPF objects CO-RE relocations may be satisfied.
182
183		  When kernels aren't compiled with CONFIG_DEBUG_INFO_BTF,
184		  libbpf, when loading an eBPF object, has to rely on external
185		  BTF files to be able to calculate CO-RE relocations.
186
187		  Usually, an external BTF file is built from existing kernel
188		  DWARF data using pahole. It contains all the types used by
189		  its respective kernel image and, because of that, is big.
190
191		  The min_core_btf feature builds smaller BTF files, customized
192		  to one or multiple eBPF objects, so they can be distributed
193		  together with an eBPF CO-RE based application, turning the
194		  application portable to different kernel versions.
195
196		  Check examples bellow for more information how to use it.
197
198	**bpftool gen help**
199		  Print short help message.
200
201OPTIONS
202=======
203	.. include:: common_options.rst
204
205	-L, --use-loader
206		  For skeletons, generate a "light" skeleton (also known as "loader"
207		  skeleton). A light skeleton contains a loader eBPF program. It does
208		  not use the majority of the libbpf infrastructure, and does not need
209		  libelf.
210
211EXAMPLES
212========
213**$ cat example1.bpf.c**
214
215::
216
217  #include <stdbool.h>
218  #include <linux/ptrace.h>
219  #include <linux/bpf.h>
220  #include <bpf/bpf_helpers.h>
221
222  const volatile int param1 = 42;
223  bool global_flag = true;
224  struct { int x; } data = {};
225
226  SEC("raw_tp/sys_enter")
227  int handle_sys_enter(struct pt_regs *ctx)
228  {
229  	static long my_static_var;
230  	if (global_flag)
231  		my_static_var++;
232  	else
233  		data.x += param1;
234  	return 0;
235  }
236
237**$ cat example2.bpf.c**
238
239::
240
241  #include <linux/ptrace.h>
242  #include <linux/bpf.h>
243  #include <bpf/bpf_helpers.h>
244
245  struct {
246  	__uint(type, BPF_MAP_TYPE_HASH);
247  	__uint(max_entries, 128);
248  	__type(key, int);
249  	__type(value, long);
250  } my_map SEC(".maps");
251
252  SEC("raw_tp/sys_exit")
253  int handle_sys_exit(struct pt_regs *ctx)
254  {
255  	int zero = 0;
256  	bpf_map_lookup_elem(&my_map, &zero);
257  	return 0;
258  }
259
260This is example BPF application with two BPF programs and a mix of BPF maps
261and global variables. Source code is split across two source code files.
262
263**$ clang --target=bpf -g example1.bpf.c -o example1.bpf.o**
264
265**$ clang --target=bpf -g example2.bpf.c -o example2.bpf.o**
266
267**$ bpftool gen object example.bpf.o example1.bpf.o example2.bpf.o**
268
269This set of commands compiles *example1.bpf.c* and *example2.bpf.c*
270individually and then statically links respective object files into the final
271BPF ELF object file *example.bpf.o*.
272
273**$ bpftool gen skeleton example.bpf.o name example | tee example.skel.h**
274
275::
276
277  /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
278
279  /* THIS FILE IS AUTOGENERATED! */
280  #ifndef __EXAMPLE_SKEL_H__
281  #define __EXAMPLE_SKEL_H__
282
283  #include <stdlib.h>
284  #include <bpf/libbpf.h>
285
286  struct example {
287  	struct bpf_object_skeleton *skeleton;
288  	struct bpf_object *obj;
289  	struct {
290  		struct bpf_map *rodata;
291  		struct bpf_map *data;
292  		struct bpf_map *bss;
293  		struct bpf_map *my_map;
294  	} maps;
295  	struct {
296  		struct bpf_program *handle_sys_enter;
297  		struct bpf_program *handle_sys_exit;
298  	} progs;
299  	struct {
300  		struct bpf_link *handle_sys_enter;
301  		struct bpf_link *handle_sys_exit;
302  	} links;
303  	struct example__bss {
304  		struct {
305  			int x;
306  		} data;
307  	} *bss;
308  	struct example__data {
309  		_Bool global_flag;
310  		long int handle_sys_enter_my_static_var;
311  	} *data;
312  	struct example__rodata {
313  		int param1;
314  	} *rodata;
315  };
316
317  static void example__destroy(struct example *obj);
318  static inline struct example *example__open_opts(
319                const struct bpf_object_open_opts *opts);
320  static inline struct example *example__open();
321  static inline int example__load(struct example *obj);
322  static inline struct example *example__open_and_load();
323  static inline int example__attach(struct example *obj);
324  static inline void example__detach(struct example *obj);
325
326  #endif /* __EXAMPLE_SKEL_H__ */
327
328**$ cat example.c**
329
330::
331
332  #include "example.skel.h"
333
334  int main()
335  {
336  	struct example *skel;
337  	int err = 0;
338
339  	skel = example__open();
340  	if (!skel)
341  		goto cleanup;
342
343  	skel->rodata->param1 = 128;
344
345  	err = example__load(skel);
346  	if (err)
347  		goto cleanup;
348
349  	err = example__attach(skel);
350  	if (err)
351  		goto cleanup;
352
353  	/* all libbpf APIs are usable */
354  	printf("my_map name: %s\n", bpf_map__name(skel->maps.my_map));
355  	printf("sys_enter prog FD: %d\n",
356  	       bpf_program__fd(skel->progs.handle_sys_enter));
357
358  	/* detach and re-attach sys_exit program */
359  	bpf_link__destroy(skel->links.handle_sys_exit);
360  	skel->links.handle_sys_exit =
361  		bpf_program__attach(skel->progs.handle_sys_exit);
362
363  	printf("my_static_var: %ld\n",
364  	       skel->bss->handle_sys_enter_my_static_var);
365
366  cleanup:
367  	example__destroy(skel);
368  	return err;
369  }
370
371**# ./example**
372
373::
374
375  my_map name: my_map
376  sys_enter prog FD: 8
377  my_static_var: 7
378
379This is a stripped-out version of skeleton generated for above example code.
380
381min_core_btf
382------------
383
384**$ bpftool btf dump file 5.4.0-example.btf format raw**
385
386::
387
388  [1] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
389  [2] CONST '(anon)' type_id=1
390  [3] VOLATILE '(anon)' type_id=1
391  [4] ARRAY '(anon)' type_id=1 index_type_id=21 nr_elems=2
392  [5] PTR '(anon)' type_id=8
393  [6] CONST '(anon)' type_id=5
394  [7] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=(none)
395  [8] CONST '(anon)' type_id=7
396  [9] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none)
397  <long output>
398
399**$ bpftool btf dump file one.bpf.o format raw**
400
401::
402
403  [1] PTR '(anon)' type_id=2
404  [2] STRUCT 'trace_event_raw_sys_enter' size=64 vlen=4
405        'ent' type_id=3 bits_offset=0
406        'id' type_id=7 bits_offset=64
407        'args' type_id=9 bits_offset=128
408        '__data' type_id=12 bits_offset=512
409  [3] STRUCT 'trace_entry' size=8 vlen=4
410        'type' type_id=4 bits_offset=0
411        'flags' type_id=5 bits_offset=16
412        'preempt_count' type_id=5 bits_offset=24
413  <long output>
414
415**$ bpftool gen min_core_btf 5.4.0-example.btf 5.4.0-smaller.btf one.bpf.o**
416
417**$ bpftool btf dump file 5.4.0-smaller.btf format raw**
418
419::
420
421  [1] TYPEDEF 'pid_t' type_id=6
422  [2] STRUCT 'trace_event_raw_sys_enter' size=64 vlen=1
423        'args' type_id=4 bits_offset=128
424  [3] STRUCT 'task_struct' size=9216 vlen=2
425        'pid' type_id=1 bits_offset=17920
426        'real_parent' type_id=7 bits_offset=18048
427  [4] ARRAY '(anon)' type_id=5 index_type_id=8 nr_elems=6
428  [5] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
429  [6] TYPEDEF '__kernel_pid_t' type_id=8
430  [7] PTR '(anon)' type_id=3
431  [8] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
432  <end>
433
434Now, the "5.4.0-smaller.btf" file may be used by libbpf as an external BTF file
435when loading the "one.bpf.o" object into the "5.4.0-example" kernel. Note that
436the generated BTF file won't allow other eBPF objects to be loaded, just the
437ones given to min_core_btf.
438
439::
440
441  LIBBPF_OPTS(bpf_object_open_opts, opts, .btf_custom_path = "5.4.0-smaller.btf");
442  struct bpf_object *obj;
443
444  obj = bpf_object__open_file("one.bpf.o", &opts);
445
446  ...
447