Re: [PATCH bpf-next v2 7/9] libbpf: don't free btf if program of multi-link tracing existing

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, Mar 11, 2024 at 2:35 AM Menglong Dong
<dongmenglong.8@bytedance.com> wrote:
>
> By default, the kernel btf that we load during loading program will be
> freed after the programs are loaded in bpf_object_load(). However, we
> still need to use these btf for tracing of multi-link during attaching.
> Therefore, we don't free the btfs until the bpf object is closed if any
> bpf programs of the type multi-link tracing exist.
>
> Meanwhile, introduce the new api bpf_object__free_btf() to manually free
> the btfs after attaching.
>
> Signed-off-by: Menglong Dong <dongmenglong.8@bytedance.com>
> ---
>  tools/lib/bpf/libbpf.c   | 47 ++++++++++++++++++++++++++++++----------
>  tools/lib/bpf/libbpf.h   |  2 ++
>  tools/lib/bpf/libbpf.map |  1 +
>  3 files changed, 38 insertions(+), 12 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 567ad367e7aa..fd5428494a7e 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -8267,6 +8267,39 @@ static int bpf_object_prepare_struct_ops(struct bpf_object *obj)
>         return 0;
>  }
>
> +void bpf_object__free_btfs(struct bpf_object *obj)
> +{
> +       int i;
> +
> +       /* clean up module BTFs */
> +       for (i = 0; i < obj->btf_module_cnt; i++) {
> +               close(obj->btf_modules[i].fd);
> +               btf__free(obj->btf_modules[i].btf);
> +               free(obj->btf_modules[i].name);
> +       }
> +       free(obj->btf_modules);
> +       obj->btf_modules = NULL;
> +       obj->btf_module_cnt = 0;
> +
> +       /* clean up vmlinux BTF */
> +       btf__free(obj->btf_vmlinux);
> +       obj->btf_vmlinux = NULL;
> +}
> +
> +static void bpf_object_early_free_btf(struct bpf_object *obj)
> +{
> +       struct bpf_program *prog;
> +
> +       bpf_object__for_each_program(prog, obj) {
> +               if (prog->expected_attach_type == BPF_TRACE_FENTRY_MULTI ||
> +                   prog->expected_attach_type == BPF_TRACE_FEXIT_MULTI ||
> +                   prog->expected_attach_type == BPF_MODIFY_RETURN_MULTI)
> +                       return;
> +       }
> +
> +       bpf_object__free_btfs(obj);
> +}
> +
>  static int bpf_object_load(struct bpf_object *obj, int extra_log_level, const char *target_btf_path)
>  {
>         int err, i;
> @@ -8307,18 +8340,7 @@ static int bpf_object_load(struct bpf_object *obj, int extra_log_level, const ch
>         /* clean up fd_array */
>         zfree(&obj->fd_array);
>
> -       /* clean up module BTFs */
> -       for (i = 0; i < obj->btf_module_cnt; i++) {
> -               close(obj->btf_modules[i].fd);
> -               btf__free(obj->btf_modules[i].btf);
> -               free(obj->btf_modules[i].name);
> -       }
> -       free(obj->btf_modules);
> -
> -       /* clean up vmlinux BTF */
> -       btf__free(obj->btf_vmlinux);
> -       obj->btf_vmlinux = NULL;
> -
> +       bpf_object_early_free_btf(obj);
>         obj->loaded = true; /* doesn't matter if successfully or not */
>
>         if (err)
> @@ -8791,6 +8813,7 @@ void bpf_object__close(struct bpf_object *obj)
>         usdt_manager_free(obj->usdt_man);
>         obj->usdt_man = NULL;
>
> +       bpf_object__free_btfs(obj);
>         bpf_gen__free(obj->gen_loader);
>         bpf_object__elf_finish(obj);
>         bpf_object_unload(obj);
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 5723cbbfcc41..c41a909ea4c1 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -299,6 +299,8 @@ LIBBPF_API struct bpf_program *
>  bpf_object__find_program_by_name(const struct bpf_object *obj,
>                                  const char *name);
>
> +LIBBPF_API void bpf_object__free_btfs(struct bpf_object *obj);
> +

It shouldn't be exported.
libbpf should clean it up when bpf_object is freed.