Re: [PATCH bpf-next 2/8] libbpf: extract BTF loading and simplify ELF parsing logic

[Song Liu <songliubraving@fb.com>]

> On Jun 17, 2019, at 10:24 AM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> On Sat, Jun 15, 2019 at 1:26 PM Song Liu <liu.song.a23@gmail.com> wrote:
>> 
>> On Mon, Jun 10, 2019 at 9:49 PM Andrii Nakryiko <andriin@fb.com> wrote:
>>> 
>>> As a preparation for adding BTF-based BPF map loading, extract .BTF and
>>> .BTF.ext loading logic. Also simplify error handling in
>>> bpf_object__elf_collect() by returning early, as there is no common
>>> clean up to be done.
>>> 
>>> Signed-off-by: Andrii Nakryiko <andriin@fb.com>
>>> ---
>>> tools/lib/bpf/libbpf.c | 137 ++++++++++++++++++++++-------------------
>>> 1 file changed, 75 insertions(+), 62 deletions(-)
>>> 
>>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>>> index ba89d9727137..9e39a0a33aeb 100644
>>> --- a/tools/lib/bpf/libbpf.c
>>> +++ b/tools/lib/bpf/libbpf.c
>>> @@ -1078,6 +1078,58 @@ static void bpf_object__sanitize_btf_ext(struct bpf_object *obj)
>>>        }
>>> }
>>> 
>>> +static int bpf_object__load_btf(struct bpf_object *obj,
>>> +                               Elf_Data *btf_data,
>>> +                               Elf_Data *btf_ext_data)
>>> +{
>>> +       int err = 0;
>>> +
>>> +       if (btf_data) {
>>> +               obj->btf = btf__new(btf_data->d_buf, btf_data->d_size);
>>> +               if (IS_ERR(obj->btf)) {
>>> +                       pr_warning("Error loading ELF section %s: %d.\n",
>>> +                                  BTF_ELF_SEC, err);
>>> +                       goto out;
>> 
>> If we goto out here, we will return 0.
> 
> 
> Yes, it's intentional. BTF is treated as optional, so if we fail to
> load it, libbpf will emit warning, but will proceed as nothing
> happened and no BTF was supposed to be loaded.
> 
>> 
>>> +               }
>>> +               err = btf__finalize_data(obj, obj->btf);
>>> +               if (err) {
>>> +                       pr_warning("Error finalizing %s: %d.\n",
>>> +                                  BTF_ELF_SEC, err);
>>> +                       goto out;
>>> +               }
>>> +               bpf_object__sanitize_btf(obj);
>>> +               err = btf__load(obj->btf);
>>> +               if (err) {
>>> +                       pr_warning("Error loading %s into kernel: %d.\n",
>>> +                                  BTF_ELF_SEC, err);
>>> +                       goto out;
>>> +               }
>>> +       }
>>> +       if (btf_ext_data) {
>>> +               if (!obj->btf) {
>>> +                       pr_debug("Ignore ELF section %s because its depending ELF section %s is not found.\n",
>>> +                                BTF_EXT_ELF_SEC, BTF_ELF_SEC);
>>> +                       goto out;
>> 
>> We will also return 0 when goto out here.
> 
> 
> See above, it's original behavior of libbpf.
> 
>> 
>>> +               }
>>> +               obj->btf_ext = btf_ext__new(btf_ext_data->d_buf,
>>> +                                           btf_ext_data->d_size);
>>> +               if (IS_ERR(obj->btf_ext)) {
>>> +                       pr_warning("Error loading ELF section %s: %ld. Ignored and continue.\n",
>>> +                                  BTF_EXT_ELF_SEC, PTR_ERR(obj->btf_ext));
>>> +                       obj->btf_ext = NULL;
>>> +                       goto out;
>> And, here. And we will not free obj->btf.
> 
> This is situation in which we successfully loaded .BTF, but failed to
> load .BTF.ext. In that case we'll warn about .BTF.ext, but will drop
> it and continue with .BTF only.
> 

Yeah, that makes sense. 

Shall we let bpf_object__load_btf() return void? Since it always 
returns 0?

Thanks,
Song

<snip>