On 27/06/2024 18:25, Alan Maguire wrote: > On 27/06/2024 00:01, Dominique Martinet wrote: >> Thanks for the reply! >> >> Alan Maguire wrote on Wed, Jun 26, 2024 at 03:29:37PM +0100: >>> On 26/06/2024 04:22, Dominique Martinet wrote: >>>> The SOURCE_DATE_EPOCH environment variable is used in reproducible >>>> builds to set build timestamps and is generally synonymous with >>>> reproducible builds: instead of making users figure out how to pass >>>> pahole flags (e.g. explicitly setting PAHOLE_FLAGS in linux) just assume >>>> that the user wants a reproducible build if this variable is set. >>> >>> From the kernel perspective it looks like as a result of [1] >>> [1] https://patchwork.kernel.org/project/linux-kbuild/patch/1443741332.2730.75.camel@xxxxxxxxxxxxxxxxx/ >>> the setting of SOURCE_DATE_EPOCH will be converted into >>> KBUILD_BUILD_TIMESTAMP; I thought the plan was to add the >>> BTF feature flag explicitly if KBUILD_BUILD_TIMESTAMP was set? >> >> Hmm, that patch (2015) never got in apparently?... >> And I don't see anythiing that'd make pahole output reproducible if >> either variables are set in the kernel either right now; we still build > > I can send a patch for this to bpf-next if it's needed. But it'd be good > to figure out the right approach first, more below... > >> plenty of older kernels so even if such a patch gets in I'd rather this >> is checked in pahole than kbuild. >> > > I'm not sure that's the right answer though - there's a bunch of > existing infrastructure in the kernel tree that relies on > KBUILD_BUILD_TIMESTAMP, so to integrate properly with the existing > kbuild infrastructure around reproducible builds feels like the right > direction to me. It's a bit more work, but not too much. Arnaldo may > disagree of course, and he's the maintainer so it's his decision - this > is just my personal take. > > We've tried to explicitly enable flags in pahole Makefile.btf so that > it's obvious from pahole invocations which features are wanted, and to > get that support on older kernels it would just be a matter of > backporting the > > if > 1.26 > use --btf_features > > ...patch, and a patch for adding the reproducible build option to stable > kernels. Similar has been done in the past for other pahole features. > > All that's needed here for bpf-next tree is I think is the below 3-line > addition: > > 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, $(pahole-ver), 126) += > --btf_features=distilled_base > endif > > > Can you test at your end if that does the right thing? Thanks! > > Alan > FWIW I verified that with the above change, and export KBUILD_BUILD_TIMESTAMP=$(date -I) ...the feature flag is added to pahole and as a result we get reproducible BTF in vmlinux; .btf.vmlinux.bin.o is identical across multiple kernel builds.
