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

[Eduard Zingerman <eddyz87@gmail.com>]

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?