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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Feb 12, 2020 at 10:07 AM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > 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?

I prefer bpf_program__set_attach_target() approach. Section name is a
program identifier and a *hint* for libbpf to determine program type,
attach type, and whatever else makes sense. But there still should be
an API to set all that manually at runtime, thus
bpf_program__set_attach_target(). Doing same by overriding section
name feels like a hack, plus it doesn't handle overriding
attach_program_fd at all.

>
> 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;