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: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20240229064523.2091270-5-thinker.li@gmail.com

diff --git a/tools/bpf/bpftool/Documentation/bpftool-gen.rst b/tools/bpf/bpftool/Documentation/bpftool-gen.rst
index 5006e724d1bc01c1afe5e13fb4ca29a33725a3ea..5e60825818dd37867c80f90b25ea6e63c521ff92 100644 (file)
--- a/tools/bpf/bpftool/Documentation/bpftool-gen.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool-gen.rst
@@ -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**
+
+**$ bpftool gen object example.bpf.o example1.bpf.o example2.bpf.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*.
+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