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bpftool: Add gen subcommand manpage

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Add bpftool-gen.rst describing skeleton on the high level. Also include
a small, but complete, example BPF app (BPF side, userspace side, generated
skeleton) in example section to demonstrate skeleton API and its usage.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20191218052552.2915188-4-andriin@fb.com
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Andrii Nakryiko 2019-12-17 21:25:52 -08:00 committed by Alexei Starovoitov
parent 3bf3c79b77
commit cb21ac5885
2 changed files with 306 additions and 1 deletions
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tools/bpf/bpftool/Documentation/bpftool-gen.rst Normal file
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================
bpftool-gen
================
-------------------------------------------------------------------------------
tool for BPF code-generation
-------------------------------------------------------------------------------
:Manual section: 8
SYNOPSIS
========
**bpftool** [*OPTIONS*] **gen** *COMMAND*
*OPTIONS* := { { **-j** | **--json** } [{ **-p** | **--pretty** }] }
*COMMAND* := { **skeleton | **help** }
GEN COMMANDS
=============
| **bpftool** **gen skeleton** *FILE*
| **bpftool** **gen help**
DESCRIPTION
===========
**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, derived from
object file name. I.e., if BPF object file name is
**example.o**, BPF object name will be **example**. The
following custom functions are provided in such case:
- **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 .extern 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 help**
Print short help message.
OPTIONS
=======
-h, --help
Print short generic help message (similar to **bpftool help**).
-V, --version
Print version number (similar to **bpftool version**).
-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 from libbpf, including debug-level
information.
EXAMPLES
========
**$ cat example.c**
::
#include <stdbool.h>
#include <linux/ptrace.h>
#include <linux/bpf.h>
#include "bpf_helpers.h"
const volatile int param1 = 42;
bool global_flag = true;
struct { int x; } data = {};
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_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;
}
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.
**$ bpftool gen skeleton example.o**
::
/* 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 <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_user.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_user**
::
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.
SEE ALSO
========
**bpf**\ (2),
**bpf-helpers**\ (7),
**bpftool**\ (8),
**bpftool-map**\ (8),
**bpftool-prog**\ (8),
**bpftool-cgroup**\ (8),
**bpftool-feature**\ (8),
**bpftool-net**\ (8),
**bpftool-perf**\ (8),
**bpftool-btf**\ (8)

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tools/bpf/bpftool/Documentation/bpftool.rst
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@ -81,4 +81,5 @@ SEE ALSO
**bpftool-feature**\ (8),
**bpftool-net**\ (8),
**bpftool-perf**\ (8),
**bpftool-btf**\ (8)
**bpftool-btf**\ (8),
**bpftool-gen**\ (8),