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

[Ihor Solodrai <ihor.solodrai@linux.dev>]

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