Re: [PATCH bpf-next] libbpf: Add support for dynamic program attach target

[Song Liu <songliubraving@fb.com>]

> On Feb 12, 2020, at 9:34 AM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> On Wed, Feb 12, 2020 at 4:32 AM Eelco Chaudron <echaudro@redhat.com> wrote:
>> 
>> Currently when you want to attach a trace program to a bpf program
>> the section name needs to match the tracepoint/function semantics.
>> 
>> However the addition of the bpf_program__set_attach_target() API
>> allows you to specify the tracepoint/function dynamically.
>> 
>> The call flow would look something like this:
>> 
>>  xdp_fd = bpf_prog_get_fd_by_id(id);
>>  trace_obj = bpf_object__open_file("func.o", NULL);
>>  prog = bpf_object__find_program_by_title(trace_obj,
>>                                           "fentry/myfunc");
>>  bpf_program__set_attach_target(prog, xdp_fd,
>>                                 "fentry/xdpfilt_blk_all");
>>  bpf_object__load(trace_obj)
>> 
>> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>


I am trying to solve the same problem with slightly different approach. 

It works as the following (with skeleton):

	obj = myobject_bpf__open_opts(&opts);
	bpf_object__for_each_program(prog, obj->obj)
		bpf_program__overwrite_section_name(prog, new_names[id++]);
	err = myobject_bpf__load(obj);

I don't have very strong preference. But I think my approach is simpler? 

Thanks,
Song


diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 514b1a524abb..4c29a7181d57 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -238,6 +238,8 @@ struct bpf_program {
        __u32 line_info_rec_size;
        __u32 line_info_cnt;
        __u32 prog_flags;
+
+       char *overwritten_section_name;
 };

 struct bpf_struct_ops {
@@ -442,6 +444,7 @@ static void bpf_program__exit(struct bpf_program *prog)
        zfree(&prog->pin_name);
        zfree(&prog->insns);
        zfree(&prog->reloc_desc);
+       zfree(&prog->overwritten_section_name);

        prog->nr_reloc = 0;
        prog->insns_cnt = 0;
@@ -6637,7 +6640,7 @@ static int libbpf_find_attach_btf_id(struct bpf_program *prog)
 {
        enum bpf_attach_type attach_type = prog->expected_attach_type;
        __u32 attach_prog_fd = prog->attach_prog_fd;
-       const char *name = prog->section_name;
+       const char *name = prog->overwritten_section_name ? : prog->section_name;
        int i, err;

        if (!name)
@@ -8396,3 +8399,11 @@ void bpf_object__destroy_skeleton(struct bpf_object_skeleton *s)
        free(s->progs);
        free(s);
 }
+
+char *bpf_program__overwrite_section_name(struct bpf_program *prog,
+                                         const char *sec_name)
+{
+       prog->overwritten_section_name = strdup(sec_name);
+
+       return prog->overwritten_section_name;
+}
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 3fe12c9d1f92..02f0d8b57cc4 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -595,6 +595,10 @@ bpf_program__bpil_addr_to_offs(struct bpf_prog_info_linear *info_linear);
 LIBBPF_API void
 bpf_program__bpil_offs_to_addr(struct bpf_prog_info_linear *info_linear);

+LIBBPF_API char *
+bpf_program__overwrite_section_name(struct bpf_program *prog,
+                                   const char *sec_name);
+
 /*
  * A helper function to get the number of possible CPUs before looking up
  * per-CPU maps. Negative errno is returned on failure.
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index b035122142bb..ed26c20729db 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -235,3 +235,8 @@ LIBBPF_0.0.7 {
                btf__align_of;
                libbpf_find_kernel_btf;
 } LIBBPF_0.0.6;
+
+LIBBPF_0.0.8 {
+       global:
+               bpf_program__overwrite_section_name;
+} LIBBPF_0.0.7;