1 .. SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
2
3 ================
4 bpftool-gen
5 ================
6 -------------------------------------------------------------------------------
7 tool for BPF code-generation
8 -------------------------------------------------------------------------------
9
10 :Manual section: 8
11
12 .. include:: substitutions.rst
13
14 SYNOPSIS
15 ========
16
17 **bpftool** [*OPTIONS*] **gen** *COMMAND*
18
19 *OPTIONS* := { |COMMON_OPTIONS| | { **-L** | **--use-loader** } }
20
21 *COMMAND* := { **object** | **skeleton** | **help** }
22
23 GEN 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
32 DESCRIPTION
33 ===========
34 bpftool gen object *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
35 Statically link (combine) together one or more *INPUT_FILE*'s into a single
36 resulting *OUTPUT_FILE*. All the files involved are BPF ELF object files.
37
38 The rules of BPF static linking are mostly the same as for user-space
39 object files, but in addition to combining data and instruction sections,
40 .BTF and .BTF.ext (if present in any of the input files) data are combined
41 together. .BTF data is deduplicated, so all the common types across
42 *INPUT_FILE*'s will only be represented once in the resulting BTF
43 information.
44
45 BPF static linking allows to partition BPF source code into individually
46 compiled files that are then linked into a single resulting BPF object
47 file, which can be used to generated BPF skeleton (with **gen skeleton**
48 command) or passed directly into **libbpf** (using **bpf_object__open()**
49 family of APIs).
50
51 bpftool gen skeleton *FILE*
52 Generate BPF skeleton C header file for a given *FILE*.
53
54 BPF skeleton is an alternative interface to existing libbpf APIs for
55 working with BPF objects. Skeleton code is intended to significantly
56 shorten and simplify code to load and work with BPF programs from userspace
57 side. Generated code is tailored to specific input BPF object *FILE*,
58 reflecting its structure by listing out available maps, program, variables,
59 etc. Skeleton eliminates the need to lookup mentioned components by name.
60 Instead, if skeleton instantiation succeeds, they are populated in skeleton
61 structure as valid libbpf types (e.g., **struct bpf_map** pointer) and can
62 be passed to existing generic libbpf APIs.
63
64 In addition to simple and reliable access to maps and programs, skeleton
65 provides a storage for BPF links (**struct bpf_link**) for each BPF program
66 within BPF object. When requested, supported BPF programs will be
67 automatically attached and resulting BPF links stored for further use by
68 user in pre-allocated fields in skeleton struct. For BPF programs that
69 can't be automatically attached by libbpf, user can attach them manually,
70 but store resulting BPF link in per-program link field. All such set up
71 links will be automatically destroyed on BPF skeleton destruction. This
72 eliminates the need for users to manage links manually and rely on libbpf
73 support to detach programs and free up resources.
74
75 Another facility provided by BPF skeleton is an interface to global
76 variables of all supported kinds: mutable, read-only, as well as extern
77 ones. This interface allows to pre-setup initial values of variables before
78 BPF object is loaded and verified by kernel. For non-read-only variables,
79 the same interface can be used to fetch values of global variables on
80 userspace side, even if they are modified by BPF code.
81
82 During skeleton generation, contents of source BPF object *FILE* is
83 embedded within generated code and is thus not necessary to keep around.
84 This ensures skeleton and BPF object file are matching 1-to-1 and always
85 stay in sync. Generated code is dual-licensed under LGPL-2.1 and
86 BSD-2-Clause licenses.
87
88 It is a design goal and guarantee that skeleton interfaces are
89 interoperable with generic libbpf APIs. User should always be able to use
90 skeleton API to create and load BPF object, and later use libbpf APIs to
91 keep working with specific maps, programs, etc.
92
93 As part of skeleton, few custom functions are generated. Each of them is
94 prefixed with object name. Object name can either be derived from object
95 file name, i.e., if BPF object file name is **example.o**, BPF object name
96 will be **example**. Object name can be also specified explicitly through
97 **name** *OBJECT_NAME* parameter. The following custom functions are
98 provided (assuming **example** as the object name):
99
100 - **example__open** and **example__open_opts**.
101 These functions are used to instantiate skeleton. It corresponds to
102 libbpf's **bpf_object__open**\ () API. **_opts** variants accepts extra
103 **bpf_object_open_opts** options.
104
105 - **example__load**.
106 This function creates maps, loads and verifies BPF programs, initializes
107 global data maps. It corresponds to libbpf's **bpf_object__load**\ ()
108 API.
109
110 - **example__open_and_load** combines **example__open** and
111 **example__load** invocations in one commonly used operation.
112
113 - **example__attach** and **example__detach**.
114 This pair of functions allow to attach and detach, correspondingly,
115 already loaded BPF object. Only BPF programs of types supported by libbpf
116 for auto-attachment will be auto-attached and their corresponding BPF
117 links instantiated. For other BPF programs, user can manually create a
118 BPF link and assign it to corresponding fields in skeleton struct.
119 **example__detach** will detach both links created automatically, as well
120 as those populated by user manually.
121
122 - **example__destroy**.
123 Detach and unload BPF programs, free up all the resources used by
124 skeleton and BPF object.
125
126 If BPF object has global variables, corresponding structs with memory
127 layout corresponding to global data data section layout will be created.
128 Currently supported ones are: *.data*, *.bss*, *.rodata*, and *.kconfig*
129 structs/data sections. These data sections/structs can be used to set up
130 initial values of variables, if set before **example__load**. Afterwards,
131 if target kernel supports memory-mapped BPF arrays, same structs can be
132 used to fetch and update (non-read-only) data from userspace, with same
133 simplicity as for BPF side.
134
135 bpftool gen subskeleton *FILE*
136 Generate BPF subskeleton C header file for a given *FILE*.
137
138 Subskeletons are similar to skeletons, except they do not own the
139 corresponding maps, programs, or global variables. They require that the
140 object file used to generate them is already loaded into a *bpf_object* by
141 some other means.
142
143 This functionality is useful when a library is included into a larger BPF
144 program. A subskeleton for the library would have access to all objects and
145 globals defined in it, without having to know about the larger program.
146
147 Consequently, there are only two functions defined for subskeletons:
148
149 - **example__open(bpf_object\*)**.
150 Instantiates a subskeleton from an already opened (but not necessarily
151 loaded) **bpf_object**.
152
153 - **example__destroy()**.
154 Frees the storage for the subskeleton but *does not* unload any BPF
155 programs or maps.
156
157 bpftool gen min_core_btf *INPUT* *OUTPUT* *OBJECT* [*OBJECT*...]
158 Generate a minimum BTF file as *OUTPUT*, derived from a given *INPUT* BTF
159 file, containing all needed BTF types so one, or more, given eBPF objects
160 CO-RE relocations may be satisfied.
161
162 When kernels aren't compiled with CONFIG_DEBUG_INFO_BTF, libbpf, when
163 loading an eBPF object, has to rely on external BTF files to be able to
164 calculate CO-RE relocations.
165
166 Usually, an external BTF file is built from existing kernel DWARF data
167 using pahole. It contains all the types used by its respective kernel image
168 and, because of that, is big.
169
170 The min_core_btf feature builds smaller BTF files, customized to one or
171 multiple eBPF objects, so they can be distributed together with an eBPF
172 CO-RE based application, turning the application portable to different
173 kernel versions.
174
175 Check examples below for more information on how to use it.
176
177 bpftool gen help
178 Print short help message.
179
180 OPTIONS
181 =======
182 .. include:: common_options.rst
183
184 -L, --use-loader
185 For skeletons, generate a "light" skeleton (also known as "loader"
186 skeleton). A light skeleton contains a loader eBPF program. It does not use
187 the majority of the libbpf infrastructure, and does not need libelf.
188
189 EXAMPLES
190 ========
191 **$ cat example1.bpf.c**
192
193 ::
194
195 #include <stdbool.h>
196 #include <linux/ptrace.h>
197 #include <linux/bpf.h>
198 #include <bpf/bpf_helpers.h>
199
200 const volatile int param1 = 42;
201 bool global_flag = true;
202 struct { int x; } data = {};
203
204 SEC("raw_tp/sys_enter")
205 int handle_sys_enter(struct pt_regs *ctx)
206 {
207 static long my_static_var;
208 if (global_flag)
209 my_static_var++;
210 else
211 data.x += param1;
212 return 0;
213 }
214
215 **$ cat example2.bpf.c**
216
217 ::
218
219 #include <linux/ptrace.h>
220 #include <linux/bpf.h>
221 #include <bpf/bpf_helpers.h>
222
223 struct {
224 __uint(type, BPF_MAP_TYPE_HASH);
225 __uint(max_entries, 128);
226 __type(key, int);
227 __type(value, long);
228 } my_map SEC(".maps");
229
230 SEC("raw_tp/sys_exit")
231 int handle_sys_exit(struct pt_regs *ctx)
232 {
233 int zero = 0;
234 bpf_map_lookup_elem(&my_map, &zero);
235 return 0;
236 }
237
238 **$ cat example3.bpf.c**
239
240 ::
241
242 #include <linux/ptrace.h>
243 #include <linux/bpf.h>
244 #include <bpf/bpf_helpers.h>
245 /* This header file is provided by the bpf_testmod module. */
246 #include "bpf_testmod.h"
247
248 int test_2_result = 0;
249
250 /* bpf_Testmod.ko calls this function, passing a "4"
251 * and testmod_map->data.
252 */
253 SEC("struct_ops/test_2")
254 void BPF_PROG(test_2, int a, int b)
255 {
256 test_2_result = a + b;
257 }
258
259 SEC(".struct_ops")
260 struct bpf_testmod_ops testmod_map = {
261 .test_2 = (void *)test_2,
262 .data = 0x1,
263 };
264
265 This is example BPF application with three BPF programs and a mix of BPF
266 maps and global variables. Source code is split across three source code
267 files.
268
269 **$ clang --target=bpf -g example1.bpf.c -o example1.bpf.o**
270
271 **$ clang --target=bpf -g example2.bpf.c -o example2.bpf.o**
272
273 **$ clang --target=bpf -g example3.bpf.c -o example3.bpf.o**
274
275 **$ bpftool gen object example.bpf.o example1.bpf.o example2.bpf.o example3.bpf.o**
276
277 This set of commands compiles *example1.bpf.c*, *example2.bpf.c* and
278 *example3.bpf.c* individually and then statically links respective object
279 files into the final BPF ELF object file *example.bpf.o*.
280
281 **$ bpftool gen skeleton example.bpf.o name example | tee example.skel.h**
282
283 ::
284
285 /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
286
287 /* THIS FILE IS AUTOGENERATED! */
288 #ifndef __EXAMPLE_SKEL_H__
289 #define __EXAMPLE_SKEL_H__
290
291 #include <stdlib.h>
292 #include <bpf/libbpf.h>
293
294 struct example {
295 struct bpf_object_skeleton *skeleton;
296 struct bpf_object *obj;
297 struct {
298 struct bpf_map *rodata;
299 struct bpf_map *data;
300 struct bpf_map *bss;
301 struct bpf_map *my_map;
302 struct bpf_map *testmod_map;
303 } maps;
304 struct {
305 struct example__testmod_map__bpf_testmod_ops {
306 const struct bpf_program *test_1;
307 const struct bpf_program *test_2;
308 int data;
309 } *testmod_map;
310 } struct_ops;
311 struct {
312 struct bpf_program *handle_sys_enter;
313 struct bpf_program *handle_sys_exit;
314 } progs;
315 struct {
316 struct bpf_link *handle_sys_enter;
317 struct bpf_link *handle_sys_exit;
318 } links;
319 struct example__bss {
320 struct {
321 int x;
322 } data;
323 int test_2_result;
324 } *bss;
325 struct example__data {
326 _Bool global_flag;
327 long int handle_sys_enter_my_static_var;
328 } *data;
329 struct example__rodata {
330 int param1;
331 } *rodata;
332 };
333
334 static void example__destroy(struct example *obj);
335 static inline struct example *example__open_opts(
336 const struct bpf_object_open_opts *opts);
337 static inline struct example *example__open();
338 static inline int example__load(struct example *obj);
339 static inline struct example *example__open_and_load();
340 static inline int example__attach(struct example *obj);
341 static inline void example__detach(struct example *obj);
342
343 #endif /* __EXAMPLE_SKEL_H__ */
344
345 **$ cat example.c**
346
347 ::
348
349 #include "example.skel.h"
350
351 int main()
352 {
353 struct example *skel;
354 int err = 0;
355
356 skel = example__open();
357 if (!skel)
358 goto cleanup;
359
360 skel->rodata->param1 = 128;
361
362 /* Change the value through the pointer of shadow type */
363 skel->struct_ops.testmod_map->data = 13;
364
365 err = example__load(skel);
366 if (err)
367 goto cleanup;
368
369 /* The result of the function test_2() */
370 printf("test_2_result: %d\n", skel->bss->test_2_result);
371
372 err = example__attach(skel);
373 if (err)
374 goto cleanup;
375
376 /* all libbpf APIs are usable */
377 printf("my_map name: %s\n", bpf_map__name(skel->maps.my_map));
378 printf("sys_enter prog FD: %d\n",
379 bpf_program__fd(skel->progs.handle_sys_enter));
380
381 /* detach and re-attach sys_exit program */
382 bpf_link__destroy(skel->links.handle_sys_exit);
383 skel->links.handle_sys_exit =
384 bpf_program__attach(skel->progs.handle_sys_exit);
385
386 printf("my_static_var: %ld\n",
387 skel->bss->handle_sys_enter_my_static_var);
388
389 cleanup:
390 example__destroy(skel);
391 return err;
392 }
393
394 **# ./example**
395
396 ::
397
398 test_2_result: 17
399 my_map name: my_map
400 sys_enter prog FD: 8
401 my_static_var: 7
402
403 This is a stripped-out version of skeleton generated for above example code.
404
405 min_core_btf
406 ------------
407
408 **$ bpftool btf dump file 5.4.0-example.btf format raw**
409
410 ::
411
412 [1] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
413 [2] CONST '(anon)' type_id=1
414 [3] VOLATILE '(anon)' type_id=1
415 [4] ARRAY '(anon)' type_id=1 index_type_id=21 nr_elems=2
416 [5] PTR '(anon)' type_id=8
417 [6] CONST '(anon)' type_id=5
418 [7] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=(none)
419 [8] CONST '(anon)' type_id=7
420 [9] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none)
421 <long output>
422
423 **$ bpftool btf dump file one.bpf.o format raw**
424
425 ::
426
427 [1] PTR '(anon)' type_id=2
428 [2] STRUCT 'trace_event_raw_sys_enter' size=64 vlen=4
429 'ent' type_id=3 bits_offset=0
430 'id' type_id=7 bits_offset=64
431 'args' type_id=9 bits_offset=128
432 '__data' type_id=12 bits_offset=512
433 [3] STRUCT 'trace_entry' size=8 vlen=4
434 'type' type_id=4 bits_offset=0
435 'flags' type_id=5 bits_offset=16
436 'preempt_count' type_id=5 bits_offset=24
437 <long output>
438
439 **$ bpftool gen min_core_btf 5.4.0-example.btf 5.4.0-smaller.btf one.bpf.o**
440
441 **$ bpftool btf dump file 5.4.0-smaller.btf format raw**
442
443 ::
444
445 [1] TYPEDEF 'pid_t' type_id=6
446 [2] STRUCT 'trace_event_raw_sys_enter' size=64 vlen=1
447 'args' type_id=4 bits_offset=128
448 [3] STRUCT 'task_struct' size=9216 vlen=2
449 'pid' type_id=1 bits_offset=17920
450 'real_parent' type_id=7 bits_offset=18048
451 [4] ARRAY '(anon)' type_id=5 index_type_id=8 nr_elems=6
452 [5] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
453 [6] TYPEDEF '__kernel_pid_t' type_id=8
454 [7] PTR '(anon)' type_id=3
455 [8] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
456 <end>
457
458 Now, the "5.4.0-smaller.btf" file may be used by libbpf as an external BTF file
459 when loading the "one.bpf.o" object into the "5.4.0-example" kernel. Note that
460 the generated BTF file won't allow other eBPF objects to be loaded, just the
461 ones given to min_core_btf.
462
463 ::
464
465 LIBBPF_OPTS(bpf_object_open_opts, opts, .btf_custom_path = "5.4.0-smaller.btf");
466 struct bpf_object *obj;
467
468 obj = bpf_object__open_file("one.bpf.o", &opts);
469
470 ...
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.