On Thu, Dec 12, 2024 at 1:07 PM Thomas Weißschuh <linux@xxxxxxxxxxxxxx> wrote: > > Hi Andrii, > > On 2024-12-12 11:23:03-0800, Andrii Nakryiko wrote: > > On Tue, Dec 10, 2024 at 10:24 PM Thomas Weißschuh <linux@xxxxxxxxxxxxxx> wrote: > > > On 2024-12-11 00:17:02+0000, Ihor Solodrai wrote: > > > > On Tuesday, December 10th, 2024 at 3:23 PM, Thomas Weißschuh <linux@xxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > > Pahole v1.27 added a new BTF generation feature to support > > > > > reproducibility in the face of multithreading. > > > > > Enable it if supported and reproducible builds are requested. > > > > > > > > > > As unknown --btf_features are ignored, avoid the test for the pahole > > > > > version to keep the line readable. > > > > > > > > > > Fixes: b4f72786429c ("scripts/pahole-flags.sh: Parse DWARF and generate BTF with multithreading.") > > > > > Fixes: 72d091846de9 ("kbuild: avoid too many execution of scripts/pahole-flags.sh") > > > > > Link: https://lore.kernel.org/lkml/4154d202-5c72-493e-bf3f-bce882a296c6@xxxxxxxxxx/ > > > > > Link: https://lore.kernel.org/lkml/20240322-pahole-reprodicible-v1-1-3eaafb1842da@xxxxxxxxxxxxxx/ > > > > > Signed-off-by: Thomas Weißschuh linux@xxxxxxxxxxxxxx > > > > > > > > > > --- > > > > > scripts/Makefile.btf | 1 + > > > > > 1 file changed, 1 insertion(+) > > > > > > > > > > diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf > > > > > index c3cbeb13de503555adcf00029a0b328e74381f13..da23265bc8b3cf43c0a1c89fbc4f53815a290e13 100644 > > > > > --- a/scripts/Makefile.btf > > > > > +++ b/scripts/Makefile.btf > > > > > @@ -22,6 +22,7 @@ else > > > > > > > > > > # Switch to using --btf_features for v1.26 and later. > > > > > pahole-flags-$(call test-ge, $(pahole-ver), 126) = -j$(JOBS) --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs > > > > > +pahole-flags-$(if $(KBUILD_BUILD_TIMESTAMP),y) += --btf_features=reproducible_build > > > > > > > > Hi Thomas, > > > > > > > > There are a couple of issues with reproducible_build flag which I > > > > think are worth mentioning here. I don't know all the reasons behind > > > > adding this now, and it's optional too, so feel free to discard my > > > > comments. > > > > > > > > Currently with this flag, the BTF output is deterministic for a given > > > > order of DWARF compilation units. So the BTF will be the same for the > > > > same vmlinux binary. However, if the vmlinux is rebuilt due to an > > > > incremental change in a source code, my understanding is that there is > > > > no guarantee that DWARF CUs will be in the same order in the binary. > > > > > > The goal behind reproducible builds is to produce bit-by-bit idential > > > binaries. If the CUs are in a different order then that requirement > > > would have been broken there already. > > > > I'm curious, how do we guarantee that we get bit-by-bit identical > > DWARF? Do we enforce the order of linking of .o files into the final > > vmlinux? Is this described anywhere? > > The CU order has to be fixed, otherwise the non-debugging parts of the > binary would not be reproducible either. > For docs is Documentation/kbuild/reproducible-builds.rst, the linked > reproducible-builds.org project has much more information. > > Also besides reproducible builds, lots of kernel components rely > (accidentally or intentionally) on a stable initialization order, which > is also defined by linking order. > > From Documentation/kbuild/makefiles.rst: > > Link order is significant, because certain functions > (module_init() / __initcall) will be called during boot in the > order they appear. So keep in mind that changing the link > order may e.g. change the order in which your SCSI > controllers are detected, and thus your disks are renumbered. > > > > For an incremental build a full relink with *all* CUs is done, not only > > > the changed once, so the order should always be the same. > > > > The concern here is whether linker guarantees that CUs' DWARF data > > will be appended in exactly the same order in such case? > > Otherwise it wouldn't be reproducible in general. > The pahole developers specifically implemented > --btf_features=reproducible_build for use in the kernel; after I sent > a precursor patch to this one (also linked in the patch): > > https://lore.kernel.org/lkml/20240322-pahole-reprodicible-v1-1-3eaafb1842da@xxxxxxxxxxxxxx/ > > In general the kernel already supports reproducible builds. Great, thanks for all the info! I do agree with Ihor that KBUILD_BUILD_TIMESTAMP is a non-obvious and surprising way to enable this behavior, but if that's what's used for other aspects of kernel build I guess it's fine by me. Ihor's work on making BTF generation more deterministic w.r.t. CU order would automatically benefit --btf_features=reproducible_build in the end and might make it unnecessary, but there is no need to block on a completion of that work. > > For my personal kernel builds only two incompatibilities/rough edges remain: > > * (parallel) BTF generation, which is fixed with this patch > * CONFIG_MODULE_SIG with non-precreated keys (which I am working on) > > > > > At the same time, reproducible_build slows down BTF generation by > > > > 30-50%, maybe more depending on the kernel config. > > > > > > If a user explicitly requests reproducibility then they should get it, > > > even if it is slower. > > > > > > > Hopefully these problems will be solved in upcoming pahole releases. > > > > > > I don't see it as big problem. This is used for release builds, not > > > during development. > > > > > > > Question: why KBUILD_BUILD_TIMESTAMP flag? Isn't it more appropriate > > > > to use a separate flag for this particular feature? > > > > > > Adding an additional variable would need to be documented and would > > > makes the feature harder to use. KBUILD_BUILD_TIMESTAMP already needs to > > > be set by the user if they are building for reproducibility. > > > > > > > > ifneq ($(KBUILD_EXTMOD),) > > > > > module-pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=distilled_base > > > > > > > > > > --- > > > > > base-commit: 7cb1b466315004af98f6ba6c2546bb713ca3c237 > > > > > change-id: 20241124-pahole-reproducible-2b879ac8bdab > > > > > > > > > > Best regards, > > > > > -- > > > > > Thomas Weißschuh linux@xxxxxxxxxxxxxx