Re: [PATCH] kbuild: pass -fno-pic when probing for -mfentry

[Nathan Chancellor <nathan@kernel.org>]

Hi Ricardo,

On Tue, Oct 20, 2026 at 06:10:52AM +0000, Ricardo Ribalda wrote:
> Some distros (namely Debian) configure their 32-bit toolchain with
> --enable-default-pie.
> 
> To properly probe for -mfentry on them we need to pass -fno-pic,
> otherwise allmodconfig will fail with:
> 
> ./arch/x86/include/asm/ftrace.h:9:3: error: #error Compiler does not support fentry?
> 
> Fixes: 454e00cf8ee5 ("kbuild: avoid re-running compiler and linker probes")
> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> ---
> I found this while testing next with media-ci.
> 
> Full logs:
> https://gitlab.freedesktop.org/linux-media/users/ribalda/-/jobs/113299249#L4729

Thanks a lot for the patch/report. I noticed another problem with this
check and the fix is quite ugly, so I think I would rather just back out
of this move altogether, along with documenting these pitfalls for
future travelers:

  https://lore.kernel.org/20261020-kconfig-fix-mfentry-check-v1-0-428258bc0fc0@kernel.org/

> ---
>  scripts/Kconfig.toolchain | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/scripts/Kconfig.toolchain b/scripts/Kconfig.toolchain
> index b0b9a663ddbe..53b361cf365a 100644
> --- a/scripts/Kconfig.toolchain
> +++ b/scripts/Kconfig.toolchain
> @@ -170,8 +170,10 @@ config CC_HAS_MNOP_MCOUNT
>  	def_bool $(cc-option,-mnop-mcount)
>  
>  # s390-linux-gnu-gcc did not support -mfentry until gcc-9.
> +# 32-bit x86 gcc does not support -mfentry in combination with -fpic, which will
> +# be enabled by default when gcc is configured with --enable-default-pie.
>  config CC_HAS_MFENTRY
> -	def_bool $(cc-option,-mfentry)
> +	def_bool $(cc-option,-fno-pic -mfentry)
>  
>  config LD_OPT_FORCE_GROUP_ALLOCATION
>  	def_string "$(ld-option-str,--force-group-allocation)"
> 
> ---
> base-commit: 3d229ce89862b8e82d71a9e11d6f840ca272a6d6
> change-id: 20261020-pie-6774df20b0fa
> 
> Best regards,
> -- 
> Ricardo Ribalda <ribalda@chromium.org>
> 

-- 
Cheers,
Nathan