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

Eduard Zingerman
2025-12-18T21:12:59Z

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?