Re: [PATCH v2] x86/boot: define CC_HAVE_ASM_GOTO

[Borislav Petkov]

On Thu, Sep 27, 2018 at 01:47:58PM -0700, ndesaulniers@xxxxxxxxxx wrote:
> Early prototypes of Clang with asm goto support produce 6 instances of
> the following warning:
> 
> In file included from arch/x86/boot/compressed/misc.h:20:
> In file included from ./include/linux/elf.h:5:
> In file included from ./arch/x86/include/asm/elf.h:8:
> In file included from ./include/linux/thread_info.h:38:
> In file included from ./arch/x86/include/asm/thread_info.h:53:
> ./arch/x86/include/asm/cpufeature.h:150:2: warning: "Compiler lacks
> ASM_GOTO support. Add -D __BPF_TRACING__ to your compiler arguments"
> [-W#warnings]
> your compiler arguments"
>  ^
> 
> Since 6 files under arch/x86/boot/compressed/ include
> arch/x86/boot/compressed/misc.h AND
> arch/x86/boot/compressed/Makefile happens to redefine KBUILD_CFLAGS,
> which set these variables in the top level MAKEFILE.
> 
> Suggested-by: Borislav Petkov <bp@xxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> v1 -> v2:
>   Updated commit message to provide more context as per Borislav.
> 
>  arch/x86/boot/compressed/Makefile | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 28764dacf018..158c0b4e178a 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -56,6 +56,13 @@ KBUILD_LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "\-z noreloc-overflow" \
>  endif
>  LDFLAGS_vmlinux := -T
>  
> +# check for 'asm goto'
> +ifeq ($(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-goto.sh $(CC) $(KBUILD_CFLAGS)), y)
> +  CC_HAVE_ASM_GOTO := 1
> +  KBUILD_CFLAGS += -DCC_HAVE_ASM_GOTO
> +  KBUILD_AFLAGS += -DCC_HAVE_ASM_GOTO
> +endif

I would still like to know why can't we do the -D_SETUP thing here:
https://lkml.kernel.org/r/20180926090841.GC5745@xxxxxxx

instead of polluting this Makefile with defines which are not really
needed in the compressed kernel build, except to silence build warnings.

I mean, we can perpetuate that ugly hack and do:

#define __BPF_TRACING__

here in arch/x86/boot/compressed/misc.h which we could kill once clang
can do asm goto...

Hmm.

-- 
Regards/Gruss,
    Boris.

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