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1================
2bpftool-gen
3================
4-------------------------------------------------------------------------------
5tool for BPF code-generation
6-------------------------------------------------------------------------------
7
8:Manual section: 8
9
10SYNOPSIS
11========
12
13 **bpftool** [*OPTIONS*] **gen** *COMMAND*
14
15 *OPTIONS* := { { **-j** | **--json** } [{ **-p** | **--pretty** }] }
16
17 *COMMAND* := { **object** | **skeleton** | **help** }
18
19GEN COMMANDS
20=============
21
22| **bpftool** **gen object** *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
23| **bpftool** **gen skeleton** *FILE* [**name** *OBJECT_NAME*]
24| **bpftool** **gen help**
25
26DESCRIPTION
27===========
28 **bpftool gen object** *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
29 Statically link (combine) together one or more *INPUT_FILE*'s
30 into a single resulting *OUTPUT_FILE*. All the files involved
31 are BPF ELF object files.
32
33 The rules of BPF static linking are mostly the same as for
34 user-space object files, but in addition to combining data
35 and instruction sections, .BTF and .BTF.ext (if present in
36 any of the input files) data are combined together. .BTF
37 data is deduplicated, so all the common types across
38 *INPUT_FILE*'s will only be represented once in the resulting
39 BTF information.
40
41 BPF static linking allows to partition BPF source code into
42 individually compiled files that are then linked into
43 a single resulting BPF object file, which can be used to
44 generated BPF skeleton (with **gen skeleton** command) or
45 passed directly into **libbpf** (using **bpf_object__open()**
46 family of APIs).
47
48 **bpftool gen skeleton** *FILE*
49 Generate BPF skeleton C header file for a given *FILE*.
50
51 BPF skeleton is an alternative interface to existing libbpf
52 APIs for working with BPF objects. Skeleton code is intended
53 to significantly shorten and simplify code to load and work
54 with BPF programs from userspace side. Generated code is
55 tailored to specific input BPF object *FILE*, reflecting its
56 structure by listing out available maps, program, variables,
57 etc. Skeleton eliminates the need to lookup mentioned
58 components by name. Instead, if skeleton instantiation
59 succeeds, they are populated in skeleton structure as valid
60 libbpf types (e.g., **struct bpf_map** pointer) and can be
61 passed to existing generic libbpf APIs.
62
63 In addition to simple and reliable access to maps and
64 programs, skeleton provides a storage for BPF links (**struct
65 bpf_link**) for each BPF program within BPF object. When
66 requested, supported BPF programs will be automatically
67 attached and resulting BPF links stored for further use by
68 user in pre-allocated fields in skeleton struct. For BPF
69 programs that can't be automatically attached by libbpf,
70 user can attach them manually, but store resulting BPF link
71 in per-program link field. All such set up links will be
72 automatically destroyed on BPF skeleton destruction. This
73 eliminates the need for users to manage links manually and
74 rely on libbpf support to detach programs and free up
75 resources.
76
77 Another facility provided by BPF skeleton is an interface to
78 global variables of all supported kinds: mutable, read-only,
79 as well as extern ones. This interface allows to pre-setup
80 initial values of variables before BPF object is loaded and
81 verified by kernel. For non-read-only variables, the same
82 interface can be used to fetch values of global variables on
83 userspace side, even if they are modified by BPF code.
84
85 During skeleton generation, contents of source BPF object
86 *FILE* is embedded within generated code and is thus not
87 necessary to keep around. This ensures skeleton and BPF
88 object file are matching 1-to-1 and always stay in sync.
89 Generated code is dual-licensed under LGPL-2.1 and
90 BSD-2-Clause licenses.
91
92 It is a design goal and guarantee that skeleton interfaces
93 are interoperable with generic libbpf APIs. User should
94 always be able to use skeleton API to create and load BPF
95 object, and later use libbpf APIs to keep working with
96 specific maps, programs, etc.
97
98 As part of skeleton, few custom functions are generated.
99 Each of them is prefixed with object name. Object name can
100 either be derived from object file name, i.e., if BPF object
101 file name is **example.o**, BPF object name will be
102 **example**. Object name can be also specified explicitly
103 through **name** *OBJECT_NAME* parameter. The following
104 custom functions are provided (assuming **example** as
105 the object name):
106
107 - **example__open** and **example__open_opts**.
108 These functions are used to instantiate skeleton. It
109 corresponds to libbpf's **bpf_object__open**\ () API.
110 **_opts** variants accepts extra **bpf_object_open_opts**
111 options.
112
113 - **example__load**.
114 This function creates maps, loads and verifies BPF
115 programs, initializes global data maps. It corresponds to
116 libppf's **bpf_object__load**\ () API.
117
118 - **example__open_and_load** combines **example__open** and
119 **example__load** invocations in one commonly used
120 operation.
121
122 - **example__attach** and **example__detach**
123 This pair of functions allow to attach and detach,
124 correspondingly, already loaded BPF object. Only BPF
125 programs of types supported by libbpf for auto-attachment
126 will be auto-attached and their corresponding BPF links
127 instantiated. For other BPF programs, user can manually
128 create a BPF link and assign it to corresponding fields in
129 skeleton struct. **example__detach** will detach both
130 links created automatically, as well as those populated by
131 user manually.
132
133 - **example__destroy**
134 Detach and unload BPF programs, free up all the resources
135 used by skeleton and BPF object.
136
137 If BPF object has global variables, corresponding structs
138 with memory layout corresponding to global data data section
139 layout will be created. Currently supported ones are: *.data*,
140 *.bss*, *.rodata*, and *.kconfig* structs/data sections.
141 These data sections/structs can be used to set up initial
142 values of variables, if set before **example__load**.
143 Afterwards, if target kernel supports memory-mapped BPF
144 arrays, same structs can be used to fetch and update
145 (non-read-only) data from userspace, with same simplicity
146 as for BPF side.
147
148 **bpftool gen help**
149 Print short help message.
150
151OPTIONS
152=======
153 .. include:: common_options.rst
154
155EXAMPLES
156========
157**$ cat example1.bpf.c**
158
159::
160
161 #include <stdbool.h>
162 #include <linux/ptrace.h>
163 #include <linux/bpf.h>
164 #include <bpf/bpf_helpers.h>
165
166 const volatile int param1 = 42;
167 bool global_flag = true;
168 struct { int x; } data = {};
169
170 SEC("raw_tp/sys_enter")
171 int handle_sys_enter(struct pt_regs *ctx)
172 {
173 static long my_static_var;
174 if (global_flag)
175 my_static_var++;
176 else
177 data.x += param1;
178 return 0;
179 }
180
181**$ cat example2.bpf.c**
182
183::
184
185 #include <linux/ptrace.h>
186 #include <linux/bpf.h>
187 #include <bpf/bpf_helpers.h>
188
189 struct {
190 __uint(type, BPF_MAP_TYPE_HASH);
191 __uint(max_entries, 128);
192 __type(key, int);
193 __type(value, long);
194 } my_map SEC(".maps");
195
196 SEC("raw_tp/sys_exit")
197 int handle_sys_exit(struct pt_regs *ctx)
198 {
199 int zero = 0;
200 bpf_map_lookup_elem(&my_map, &zero);
201 return 0;
202 }
203
204This is example BPF application with two BPF programs and a mix of BPF maps
205and global variables. Source code is split across two source code files.
206
207**$ clang -target bpf -g example1.bpf.c -o example1.bpf.o**
208**$ clang -target bpf -g example2.bpf.c -o example2.bpf.o**
209**$ bpftool gen object example.bpf.o example1.bpf.o example2.bpf.o**
210
211This set of commands compiles *example1.bpf.c* and *example2.bpf.c*
212individually and then statically links respective object files into the final
213BPF ELF object file *example.bpf.o*.
214
215**$ bpftool gen skeleton example.bpf.o name example | tee example.skel.h**
216
217::
218
219 /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
220
221 /* THIS FILE IS AUTOGENERATED! */
222 #ifndef __EXAMPLE_SKEL_H__
223 #define __EXAMPLE_SKEL_H__
224
225 #include <stdlib.h>
226 #include <bpf/libbpf.h>
227
228 struct example {
229 struct bpf_object_skeleton *skeleton;
230 struct bpf_object *obj;
231 struct {
232 struct bpf_map *rodata;
233 struct bpf_map *data;
234 struct bpf_map *bss;
235 struct bpf_map *my_map;
236 } maps;
237 struct {
238 struct bpf_program *handle_sys_enter;
239 struct bpf_program *handle_sys_exit;
240 } progs;
241 struct {
242 struct bpf_link *handle_sys_enter;
243 struct bpf_link *handle_sys_exit;
244 } links;
245 struct example__bss {
246 struct {
247 int x;
248 } data;
249 } *bss;
250 struct example__data {
251 _Bool global_flag;
252 long int handle_sys_enter_my_static_var;
253 } *data;
254 struct example__rodata {
255 int param1;
256 } *rodata;
257 };
258
259 static void example__destroy(struct example *obj);
260 static inline struct example *example__open_opts(
261 const struct bpf_object_open_opts *opts);
262 static inline struct example *example__open();
263 static inline int example__load(struct example *obj);
264 static inline struct example *example__open_and_load();
265 static inline int example__attach(struct example *obj);
266 static inline void example__detach(struct example *obj);
267
268 #endif /* __EXAMPLE_SKEL_H__ */
269
270**$ cat example.c**
271
272::
273
274 #include "example.skel.h"
275
276 int main()
277 {
278 struct example *skel;
279 int err = 0;
280
281 skel = example__open();
282 if (!skel)
283 goto cleanup;
284
285 skel->rodata->param1 = 128;
286
287 err = example__load(skel);
288 if (err)
289 goto cleanup;
290
291 err = example__attach(skel);
292 if (err)
293 goto cleanup;
294
295 /* all libbpf APIs are usable */
296 printf("my_map name: %s\n", bpf_map__name(skel->maps.my_map));
297 printf("sys_enter prog FD: %d\n",
298 bpf_program__fd(skel->progs.handle_sys_enter));
299
300 /* detach and re-attach sys_exit program */
301 bpf_link__destroy(skel->links.handle_sys_exit);
302 skel->links.handle_sys_exit =
303 bpf_program__attach(skel->progs.handle_sys_exit);
304
305 printf("my_static_var: %ld\n",
306 skel->bss->handle_sys_enter_my_static_var);
307
308 cleanup:
309 example__destroy(skel);
310 return err;
311 }
312
313**# ./example**
314
315::
316
317 my_map name: my_map
318 sys_enter prog FD: 8
319 my_static_var: 7
320
321This is a stripped-out version of skeleton generated for above example code.