Commit f2e81192 authored by Kui-Feng Lee's avatar Kui-Feng Lee Committed by Andrii Nakryiko
Browse files

bpftool: Add an example for struct_ops map and shadow type. 



The example in bpftool-gen.8 explains how to use the pointer of the shadow
type to change the value of a field of a struct_ops map.

Signed-off-by: default avatarKui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Reviewed-by: default avatarQuentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20240229064523.2091270-5-thinker.li@gmail.com
parent a7b0fa35

tools/bpf/bpftool/Documentation/bpftool-gen.rst

+52 −6
Original line number Diff line number Diff line
@@ -257,18 +257,48 @@ EXAMPLES 
  	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.

**$ cat example3.bpf.c**



::



  #include <linux/ptrace.h>

  #include <linux/bpf.h>

  #include <bpf/bpf_helpers.h>

  /* This header file is provided by the bpf_testmod module. */

  #include "bpf_testmod.h"



  int test_2_result = 0;



  /* bpf_Testmod.ko calls this function, passing a "4"

   * and testmod_map->data.

   */

  SEC("struct_ops/test_2")

  void BPF_PROG(test_2, int a, int b)

  {

	test_2_result = a + b;

  }



  SEC(".struct_ops")

  struct bpf_testmod_ops testmod_map = {

	.test_2 = (void *)test_2,

	.data = 0x1,

  };



This is example BPF application with three BPF programs and a mix of BPF

maps and global variables. Source code is split across three 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**

**$ clang --target=bpf -g example3.bpf.c -o example3.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 object example.bpf.o example1.bpf.o example2.bpf.o example3.bpf.o**



This set of commands compiles *example1.bpf.c*, *example2.bpf.c* and

*example3.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**
@@ -291,7 +321,15 @@ BPF ELF object file *example.bpf.o*. 
  		struct bpf_map *data;

  		struct bpf_map *bss;

  		struct bpf_map *my_map;

		struct bpf_map *testmod_map;

  	} maps;

	struct {

		struct example__testmod_map__bpf_testmod_ops {

			const struct bpf_program *test_1;

			const struct bpf_program *test_2;

			int data;

		} *testmod_map;

	} struct_ops;

  	struct {

  		struct bpf_program *handle_sys_enter;

  		struct bpf_program *handle_sys_exit;
@@ -304,6 +342,7 @@ BPF ELF object file *example.bpf.o*. 
  		struct {

  			int x;

  		} data;

		int test_2_result;

  	} *bss;

  	struct example__data {

  		_Bool global_flag;
@@ -342,10 +381,16 @@ BPF ELF object file *example.bpf.o*. 


  	skel->rodata->param1 = 128;



	/* Change the value through the pointer of shadow type */

	skel->struct_ops.testmod_map->data = 13;



  	err = example__load(skel);

  	if (err)

  		goto cleanup;



	/* The result of the function test_2() */

	printf("test_2_result: %d\n", skel->bss->test_2_result);



  	err = example__attach(skel);

  	if (err)

  		goto cleanup;
@@ -372,6 +417,7 @@ BPF ELF object file *example.bpf.o*. 


::



  test_2_result: 17

  my_map name: my_map

  sys_enter prog FD: 8

  my_static_var: 7