linux-kernel - Re: [PATCH bpf-next] kbuild, bpf: Enable reproducible BTF generation

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Message-ID: <3b834807-9f20-4f04-b788-f45dfac5cb1f@t-8ch.de>
Date: Wed, 11 Dec 2024 07:24:40 +0100
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Ihor Solodrai <ihor.solodrai@...me>
Cc: Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, 
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, Kui-Feng Lee <kuifeng@...com>, 
	Alan Maguire <alan.maguire@...cle.com>, Martin Rodriguez Reboredo <yakoyoku@...il.com>, 
	Miguel Ojeda <ojeda@...nel.org>, bpf@...r.kernel.org, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] kbuild, bpf: Enable reproducible BTF generation

Hi Ihor,

On 2024-12-11 00:17:02+0000, Ihor Solodrai wrote:
> On Tuesday, December 10th, 2024 at 3:23 PM, Thomas Weißschuh <linux@...ssschuh.net> 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@gentoo.org/
> > Link: https://lore.kernel.org/lkml/20240322-pahole-reprodicible-v1-1-3eaafb1842da@weissschuh.net/
> > Signed-off-by: Thomas Weißschuh linux@...ssschuh.net
> > 
> > ---
> > 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.

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.

> 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@...ssschuh.net
> > 
> > 
>