Provided by: linux-tools-common_5.19.0-21.21_all 
NAME
bpftool-gen - tool for BPF code-generation
SYNOPSIS
bpftool [OPTIONS] gen COMMAND
OPTIONS := { { -j | --json } [{ -p | --pretty }] | { -d | --debug } | { -l | --legacy }
| { -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
-h, --help
Print short help message (similar to bpftool help).
-V, --version
Print bpftool's version number (similar to bpftool version), the number of the
libbpf version in use, and optional features that were included when bpftool was
compiled. Optional features include linking against libbfd to provide the
disassembler for JIT-ted programs (bpftool prog dump jited) and usage of BPF
skeletons (some features like bpftool prog profile or showing pids associated to
BPF objects may rely on it).
-j, --json
Generate JSON output. For commands that cannot produce JSON, this option has no
effect.
-p, --pretty
Generate human-readable JSON output. Implies -j.
-d, --debug
Print all logs available, even debug-level information. This includes logs from
libbpf as well as from the verifier, when attempting to load programs.
-l, --legacy
Use legacy libbpf mode which has more relaxed BPF program requirements. By
default, bpftool has more strict requirements about section names, changes
pinning logic and doesn't support some of the older non-BTF map declarations.
See https://github.com/libbpf/libbpf/wiki/Libbpf:-the-road-to-v1.0 for details.
-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);
...
SEE ALSO
bpf(2), bpf-helpers(7), bpftool(8), bpftool-btf(8), bpftool-cgroup(8),
bpftool-feature(8), bpftool-iter(8), bpftool-link(8), bpftool-map(8), bpftool-net(8),
bpftool-perf(8), bpftool-prog(8), bpftool-struct_ops(8)
BPFTOOL-GEN(8)