Re: Questions about the state of some BTF features

[Alan Maguire <alan.maguire@oracle.com>]

On 25/10/2024 17:19, Alexei Starovoitov wrote:
> On Fri, Oct 25, 2024 at 9:15 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>>
>> On 25/10/2024 17:09, Alexei Starovoitov wrote:
>>> On Thu, Oct 24, 2024 at 4:26 PM Andrii Nakryiko
>>> <andrii.nakryiko@gmail.com> wrote:
>>>>
>>>>>
>>>>> The good news is that already happens, provided you have the updated
>>>>> pahole to handle distilled base generation. After building selftests I see
>>>>>
>>>>> $ objdump -h bpf_testmod.ko |grep BTF
>>>>>   7 .BTF_ids      000001c8  0000000000000000  0000000000000000  00002c50
>>>>>  2**0
>>>>>  50 .BTF          000036f4  0000000000000000  0000000000000000  0006e048
>>>>>  2**0
>>>>>  51 .BTF.base     000004cc  0000000000000000  0000000000000000  0007173c
>>>>>  2**0
>>>>>
>>>>
>>>> Indeed, after updating to the latest pahole master now I get
>>>> .BTF.base, very nice.
>>>
>>> I pulled the latest pahole, rebuilt everything,
>>> but still cannot get it to generate BTF.base.
>>>
>>> Any special trick needed?
>>
>> Hmm, should just work for bpf_testmod.ko as long as "pahole
>> --supported_btf_features" reports "distilled_base" among the set of
>> features. scripts/Makefile.btf should add that feature if KBUILD_EXTMOD
>> is set, as it should be in the case of building bpf_testmod.ko. I'll
>> double-check at my end with latest bpf-next, but it was working
>> yesterday for me.
> 
> There must be something else necessary:
> 
> pahole -J -j --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
> --lang_exclude=rust --btf_features=distilled_base --btf_base vmlinux
> .../bpf/bpf_testmod/bpf_testmod.ko; .../resolve_btfids -b vmlinux
> .../selftests/bpf/bpf_testmod/bpf_testmod.ko;
> 
> objdump -h .../testing/selftests/bpf/bpf_testmod/bpf_testmod.ko|grep BTF
>   7 .BTF_ids      000001c8  0000000000000000  0000000000000000  00001d94  2**0
>  50 .BTF          00002ea7  0000000000000000  0000000000000000  00062e30  2**0
> 

Not sure what's going on for you here to be honest. I just tried pulling
latest bpf-next and dwarves master branch, rebuilding pahole and
selftests. I see .BTF.base sections for each .ko in selftests/bpf.
Can you provide the output of

pahole --supported_btf_features

? If it contains distilled_base things _should_ be working. The only
other reason I can think of that you might not get .BTF.base sections is
if dwarves was built against a local libbpf (rather than the git
submodule) or local libbpf headers _and_ that libbpf did not include
distilled base BTF support; in that case we skip .BTF.base generation
since the APIs are not available.