Re: [PATCH] x86: Use correct CHECKFLAGS for realmode code

[Mark Bloch]

On 22/05/2026 10:03, Thomas Weißschuh wrote:
> Currently the CHECKFLAGS from the normal build are also used for the
> realmode code. This leads to inconsistent flags confusing sparse and
> the logic in the kernel headers. Since recently this inconsistent state
> is detected by asm-generic/bitsperlong.h, triggering a warning.
> 
> Fix the CHECKFLAGS to let sparse properly check this code and
> also avoid the warning.
> 
> Technically the replacement of -D__x86_64__ is not necessary, as it will
> only be added to CHECKFLAGS later in the file.
> Replace it anyways for consistency.
> 
> Reported-by: Mark Bloch <mbloch@xxxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/3bbcf456-322c-46f9-b238-88fb8ad227b2@xxxxxxxxxx/
> Fixes: 62357a5888ea ("asm-generic/bitsperlong.h: Add sanity checks for __BITS_PER_LONG")
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
> ---
>  arch/x86/Makefile             | 5 +++++
>  arch/x86/boot/Makefile        | 1 +
>  arch/x86/realmode/rm/Makefile | 1 +
>  3 files changed, 7 insertions(+)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 46fec0b08487..bdafa3635dd1 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -61,6 +61,11 @@ REALMODE_CFLAGS += $(cc_stack_align4)
>  REALMODE_CFLAGS += $(CLANG_FLAGS)
>  export REALMODE_CFLAGS
>  
> +REALMODE_CHECKFLAGS := $(subst -m64, -m16, \
> +		       $(subst -m32, -m16, \
> +		       $(subst -D__x86_64__, -D__i386__, \
> +		       $(CHECKFLAGS))))
> +
>  # BITS is used as extension for files which are available in a 32 bit
>  # and a 64 bit version to simplify shared Makefiles.
>  # e.g.: obj-y += foo_$(BITS).o
> diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> index 3f9fb3698d66..20b569930229 100644
> --- a/arch/x86/boot/Makefile
> +++ b/arch/x86/boot/Makefile
> @@ -55,6 +55,7 @@ KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP
>  KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
>  KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
>  KBUILD_CFLAGS	+= $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
> +CHECKFLAGS	:= $(REALMODE_CHECKFLAGS)
>  
>  $(obj)/bzImage: asflags-y  := $(SVGA_MODE)
>  
> diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
> index a0fb39abc5c8..725c5ef6cdf1 100644
> --- a/arch/x86/realmode/rm/Makefile
> +++ b/arch/x86/realmode/rm/Makefile
> @@ -67,3 +67,4 @@ KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP -D_WAKEUP \
>  		   -I$(srctree)/arch/x86/boot
>  KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
>  KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
> +CHECKFLAGS	:= $(REALMODE_CHECKFLAGS)
> 
> ---
> base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
> change-id: 20260521-x86-sparse-boot-cb427318607d
> 
> Best regards,
> --  
> Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
> 

Tested-by: Mark Bloch <mbloch@xxxxxxxxxx>