linux-kernel - Re: [PATCH bpf-next v4 5/8] kbuild: Sync kconfig when PAHOLE

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Message-ID: <78acd4cd0cb8ed6b4c0c36cb79294efe606a4366.camel@gmail.com>
Date: Thu, 18 Dec 2025 14:36:47 -0800
From: Eduard Zingerman <eddyz87@...il.com>
To: Ihor Solodrai <ihor.solodrai@...ux.dev>, Alan Maguire	
 <alan.maguire@...cle.com>, Alexei Starovoitov <ast@...nel.org>, Andrea
 Righi	 <arighi@...dia.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Andrii Nakryiko	 <andrii@...nel.org>, Bill Wendling <morbo@...gle.com>,
 Changwoo Min	 <changwoo@...lia.com>, Daniel Borkmann
 <daniel@...earbox.net>, David Vernet	 <void@...ifault.com>, Donglin Peng
 <dolinux.peng@...il.com>, Hao Luo	 <haoluo@...gle.com>, Jiri Olsa
 <jolsa@...nel.org>, John Fastabend	 <john.fastabend@...il.com>, Jonathan
 Corbet <corbet@....net>, Justin Stitt	 <justinstitt@...gle.com>, KP Singh
 <kpsingh@...nel.org>, Martin KaFai Lau	 <martin.lau@...ux.dev>, Nathan
 Chancellor <nathan@...nel.org>, Nick Desaulniers	
 <nick.desaulniers+lkml@...il.com>, Nicolas Schier <nsc@...nel.org>, Shuah
 Khan	 <shuah@...nel.org>, Song Liu <song@...nel.org>, Stanislav Fomichev	
 <sdf@...ichev.me>, Tejun Heo <tj@...nel.org>, Yonghong Song	
 <yonghong.song@...ux.dev>
Cc: bpf@...r.kernel.org, dwarves@...r.kernel.org,
 linux-kbuild@...r.kernel.org, 	linux-kernel@...r.kernel.org,
 sched-ext@...ts.linux.dev
Subject: Re: [PATCH bpf-next v4 5/8] kbuild: Sync kconfig when
 PAHOLE_VERSION changes

On Thu, 2025-12-18 at 13:33 -0800, Ihor Solodrai wrote:
> On 12/18/25 11:21 AM, Eduard Zingerman wrote:
> > On Wed, 2025-12-17 at 16:33 -0800, Ihor Solodrai wrote:
> > > This patch implements kconfig re-sync when the pahole version changes
> > > between builds, similar to how it happens for compiler version change
> > > via CC_VERSION_TEXT.
> > > 
> > > Define PAHOLE_VERSION in the top-level Makefile and export it for
> > > config builds. Set CONFIG_PAHOLE_VERSION default to the exported
> > > variable.
> > > 
> > > Kconfig records the PAHOLE_VERSION value in
> > > include/config/auto.conf.cmd [1].
> > > 
> > > The Makefile includes auto.conf.cmd, so if PAHOLE_VERSION changes
> > > between builds, make detects a dependency change and triggers
> > > syncconfig to update the kconfig [2].
> > > 
> > > For external module builds, add a warning message in the prepare
> > > target, similar to the existing compiler version mismatch warning.
> > > 
> > > Note that if pahole is not installed or available, PAHOLE_VERSION is
> > > set to 0 by pahole-version.sh, so the (un)installation of pahole is
> > > treated as a version change.
> > > 
> > > See previous discussions for context [3].
> > > 
> > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/kconfig/preprocess.c?h=v6.18#n91
> > > [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile?h=v6.18#n815
> > > [3] https://lore.kernel.org/bpf/8f946abf-dd88-4fac-8bb4-84fcd8d81cf0@oracle.com/
> > > 
> > > Signed-off-by: Ihor Solodrai <ihor.solodrai@...ux.dev>
> > > ---
> > 
> > When building BPF selftest modules the pahole version change was
> > detected, but it seems that BTF rebuild was not triggered:
> > 
> >   $ (cd ./tools/testing/selftests/bpf/test_kmods/; make -j)
> >   make[1]: Entering directory '/home/ezingerman/bpf-next'
> >   make[2]: Entering directory '/home/ezingerman/bpf-next/tools/testing/selftests/bpf/test_kmods'
> >     CC [M]  bpf_testmod.o
> >     CC [M]  bpf_test_no_cfi.o
> >     CC [M]  bpf_test_modorder_x.o
> >     CC [M]  bpf_test_modorder_y.o
> >     CC [M]  bpf_test_rqspinlock.o
> >     MODPOST Module.symvers
> >     CC [M]  bpf_testmod.mod.o
> >     CC [M]  .module-common.o
> >     CC [M]  bpf_test_no_cfi.mod.o
> >     CC [M]  bpf_test_modorder_x.mod.o
> >     CC [M]  bpf_test_modorder_y.mod.o
> >     CC [M]  bpf_test_rqspinlock.mod.o
> >     LD [M]  bpf_test_modorder_x.ko
> >     LD [M]  bpf_testmod.ko
> >     LD [M]  bpf_test_modorder_y.ko
> >     LD [M]  bpf_test_no_cfi.ko
> >     BTF [M] bpf_test_modorder_x.ko
> >     LD [M]  bpf_test_rqspinlock.ko
> >     BTF     bpf_test_modorder_x.ko
> >     BTF [M] bpf_test_no_cfi.ko
> >     BTF [M] bpf_test_modorder_y.ko
> >     BTF [M] bpf_testmod.ko
> >     BTF     bpf_test_no_cfi.ko
> >     BTF     bpf_test_modorder_y.ko
> >     BTF [M] bpf_test_rqspinlock.ko
> >     BTF     bpf_testmod.ko
> >     BTF     bpf_test_rqspinlock.ko
> >     BTFIDS  bpf_test_modorder_x.ko
> >     BTFIDS  bpf_test_modorder_y.ko
> >     BTFIDS  bpf_test_no_cfi.ko
> >     BTFIDS  bpf_testmod.ko
> >     OBJCOPY bpf_test_modorder_x.ko.BTF
> >     BTFIDS  bpf_test_rqspinlock.ko
> >     OBJCOPY bpf_test_no_cfi.ko.BTF
> >     OBJCOPY bpf_test_modorder_y.ko.BTF
> >     OBJCOPY bpf_testmod.ko.BTF
> >     OBJCOPY bpf_test_rqspinlock.ko.BTF
> >   make[2]: Leaving directory '/home/ezingerman/bpf-next/tools/testing/selftests/bpf/test_kmods'
> >   make[1]: Leaving directory '/home/ezingerman/bpf-next'
> >   [~/bpf-next]
> >   $ (cd ./tools/testing/selftests/bpf/test_kmods/; make -j)
> >   make[1]: Entering directory '/home/ezingerman/bpf-next'
> >   make[2]: Entering directory '/home/ezingerman/bpf-next/tools/testing/selftests/bpf/test_kmods'
> >   make[2]: Leaving directory '/home/ezingerman/bpf-next/tools/testing/selftests/bpf/test_kmods'
> >   make[1]: Leaving directory '/home/ezingerman/bpf-next'
> > 
> > ... update pahole from version 131 to 132 ...
> > 
> >   [~/bpf-next]
> >   $ (cd ./tools/testing/selftests/bpf/test_kmods/; make -j)
> >   make[1]: Entering directory '/home/ezingerman/bpf-next'
> >   make[2]: Entering directory '/home/ezingerman/bpf-next/tools/testing/selftests/bpf/test_kmods'
> >   warning: pahole version differs from the one used to build the kernel
> >     The kernel was built with: 131
> >     You are using:             132
> >   make[2]: Leaving directory '/home/ezingerman/bpf-next/tools/testing/selftests/bpf/test_kmods'
> >   make[1]: Leaving directory '/home/ezingerman/bpf-next'
> > 
> > Is this an expected behavior?
> 
> Yes, it's expected.
> 
> I simply repeated the logic used for compiler version change: for
> external modules only the warning is printed.
> 
> See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile?h=v6.18#n1857
> 

Ok, it does rebuild BTF for the kernel itself.

Tested-by: Eduard Zingerman <eddyz87@...il.com>