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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 ...