Re: [PATCH bpf-next] kbuild, bpf: reproducible BTF from pahole when KBUILD_BUILD_TIMESTAMP set

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

On 02/07/2024 08:58, Masahiro Yamada wrote:
> On Tue, Jul 2, 2024 at 2:32 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>>
>> Reproducible builds [1] require that the same source code with
>> the same set of tools can build identical objects each time,
>> but pahole in parallel mode was non-deterministic in
>> BTF generation prior to
>>
>> dba7b5e ("pahole: Encode BTF serially in a reproducible build")
>>
>> This was a problem since said BTF is baked into kernels and modules in
>> .BTF sections, so parallel pahole was causing non-reproducible binary
>> generation.  Now with the above commit we have support for parallel
>> reproducible BTF generation in pahole.
>>
>> KBUILD_BUILD_TIMESTAMP is set for reproducible builds, so if it
>> is set, add reproducible_build to --btf_features.
>>
>> [1] Documentation/kbuild/reproducible-builds.rst
>>
>> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> 
> 
> 
> 
> Does not make sense.
> 
> 
> 
> KBUILD_BUILD_TIMESTAMP is not a switch for
> "please enable the reproducible build".
> 
> 
> KBUILD_BUILD_TIMESTAMP requires the build code
> to use the given time in the output where timestamps are used.
> 
> Your patch does not use the timestamp at all.
>

No, and that's not the intention. It is used as a signal to pahole to
enable reproducibility in parallel build. There is a cost to this so
it's not advisable in all scenarios [1]. Is there a kbuild-approved way
to determine if reproducible builds are in operation?

Alan

[1]
https://lore.kernel.org/dwarves/20240412211604.789632-12-acme@kernel.org/
> 
> If --btf_features=reproducible_build has no downside,
> please add it whenever supported.
> 
> 
> 
> 
> 
> 
> 
>> ---
>>  scripts/Makefile.btf | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
>> index b75f09f3f424..40bb72662967 100644
>> --- a/scripts/Makefile.btf
>> +++ b/scripts/Makefile.btf
>> @@ -21,6 +21,10 @@ else
>>  # Switch to using --btf_features for v1.26 and later.
>>  pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
>>
>> +ifneq ($(KBUILD_BUILD_TIMESTAMP),)
>> +pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=reproducible_build
>> +endif
>> +
>>  ifneq ($(KBUILD_EXTMOD),)
>>  module-pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=distilled_base
>>  endif
>> --
>> 2.31.1
>>
> 
>