Re: [PATCH bpf-next v2 12/18] libbpf: don't free btf if tracing_multi progs existing

[Menglong Dong <menglong.dong@linux.dev>]

On 7/15/25 06:07, Andrii Nakryiko wrote:
> On Thu, Jul 3, 2025 at 5:21 AM Menglong Dong <menglong8.dong@gmail.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 <dongml2@chinatelecom.cn>
>> ---
>>   tools/lib/bpf/libbpf.c   | 24 +++++++++++++++++++++++-
>>   tools/lib/bpf/libbpf.h   |  2 ++
>>   tools/lib/bpf/libbpf.map |  1 +
>>   3 files changed, 26 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index aee36402f0a3..530c29f2f5fc 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -8583,6 +8583,28 @@ static void bpf_object_post_load_cleanup(struct bpf_object *obj)
>>          obj->btf_vmlinux = NULL;
>>   }
>>
>> +void bpf_object__free_btfs(struct bpf_object *obj)
> let's not add this as a new API. We'll keep BTF fds open, if
> necessary, but not (yet) give user full control of when those FDs will
> be closed, I'm not convinced yet we need that much user control over
> this


Okay! I previously thought that this would take up a certain amount of
memory, but it seems I was overthinking :/

I'll remove this API in the next version.

Thanks!
Menglong Dong


>
>
>> +{
>> +       if (!obj->btf_vmlinux || obj->state != OBJ_LOADED)
>> +               return;
>> +
>> +       bpf_object_post_load_cleanup(obj);
>> +}
>> +
>> +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_post_load_cleanup(obj);
>> +}
>> +
>>   static int bpf_object_prepare(struct bpf_object *obj, const char *target_btf_path)
>>   {
>>          int err;
>> @@ -8654,7 +8676,7 @@ static int bpf_object_load(struct bpf_object *obj, int extra_log_level, const ch
>>                          err = bpf_gen__finish(obj->gen_loader, obj->nr_programs, obj->nr_maps);
>>          }
>>
>> -       bpf_object_post_load_cleanup(obj);
>> +       bpf_object_early_free_btf(obj);
>>          obj->state = OBJ_LOADED; /* doesn't matter if successfully or not */
>>
>>          if (err) {
>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>> index d1cf813a057b..7cc810aa7967 100644
>> --- a/tools/lib/bpf/libbpf.h
>> +++ b/tools/lib/bpf/libbpf.h
>> @@ -323,6 +323,8 @@ 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);
>> +
>>   LIBBPF_API int
>>   libbpf_prog_type_by_name(const char *name, enum bpf_prog_type *prog_type,
>>                           enum bpf_attach_type *expected_attach_type);
>> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
>> index c7fc0bde5648..4a0c993221a5 100644
>> --- a/tools/lib/bpf/libbpf.map
>> +++ b/tools/lib/bpf/libbpf.map
>> @@ -444,4 +444,5 @@ LIBBPF_1.6.0 {
>>                  bpf_program__line_info_cnt;
>>                  btf__add_decl_attr;
>>                  btf__add_type_attr;
>> +               bpf_object__free_btfs;
>>   } LIBBPF_1.5.0;
>> --
>> 2.39.5
>>
>>