Re: [PATCH bpf-next v4 5/8] kbuild: Sync kconfig when PAHOLE_VERSION changes

[Eduard Zingerman <eddyz87@gmail.com>]

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@linux.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@gmail.com>