Re: [PATCH] kbuild: Set KBUILD_CFLAGS before incl. arch Makefile

[Matthias Kaehlcke <mka@chromium.org>]

El Thu, Nov 02, 2017 at 02:26:48PM -0700 Nick Desaulniers ha dit:

> From: Chris Fries <cfries@google.com>
> 
> Set the clang KBUILD_CFLAGS up before including arch/ Makefiles,
> so that ld-options (etc.) can work correctly.
> 
> This fixes errors with clang such as ld-options trying to CC
> against your host architecture, but LD trying to link against
> your target architecture.
> 
> We didn't notice this problem on Android, because we took the original
> LLVMLinux patch into our 4.4 kernels, which did not have this issue. We
> ran into this taking the proper upstream patch on newer kernel versions.
> The original LLVMLinux patch can be seen at:
> 
> http://git.linuxfoundation.org/?p=llvmlinux/kernel.git;a=blobdiff;f=Makefile;h=389006c4ef494cda3a1ee52bf355618673ab4f31;hp=e41a3356abee83f08288362950bfceebd25ec3c2;hb=ef9126da11b18ff34eb1f01561f53c378860336c;hpb=f800c25b7a762d445ba1439a2428c8362157eba6
> 
> It seems that when the patch was re-upstreamed, a V2 was requested that
> moved the definition of Clang's target triple to be later in the top
> level Makefile than the inclusion of the arch specific Makefile,
> breaking macros like ld-option when cross compiling. V2 was requested
> at:
> 
> https://lkml.org/lkml/2017/4/21/116
> 
> Signed-off-by: Chris Fries <cfries@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
>  Makefile | 64 ++++++++++++++++++++++++++++++++--------------------------------
>  1 file changed, 32 insertions(+), 32 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 5f91a28a3cea..72ea86157114 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -512,6 +512,38 @@ ifneq ($(filter install,$(MAKECMDGOALS)),)
>          endif
>  endif
>  
> +ifeq ($(cc-name),clang)
> +ifneq ($(CROSS_COMPILE),)
> +CLANG_TARGET	:= --target=$(notdir $(CROSS_COMPILE:%-=%))
> +GCC_TOOLCHAIN	:= $(realpath $(dir $(shell which $(LD)))/..)
> +endif
> +ifneq ($(GCC_TOOLCHAIN),)
> +CLANG_GCC_TC	:= --gcc-toolchain=$(GCC_TOOLCHAIN)
> +endif
> +KBUILD_CFLAGS += $(CLANG_TARGET) $(CLANG_GCC_TC)
> +KBUILD_AFLAGS += $(CLANG_TARGET) $(CLANG_GCC_TC)
> +KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
> +KBUILD_CFLAGS += $(call cc-disable-warning, unused-variable)
> +KBUILD_CFLAGS += $(call cc-disable-warning, format-invalid-specifier)
> +KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
> +KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
> +# Quiet clang warning: comparison of unsigned expression < 0 is always false
> +KBUILD_CFLAGS += $(call cc-disable-warning, tautological-compare)
> +# CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
> +# source of a reference will be _MergedGlobals and not on of the whitelisted names.
> +# See modpost pattern 2
> +KBUILD_CFLAGS += $(call cc-option, -mno-global-merge,)
> +KBUILD_CFLAGS += $(call cc-option, -fcatch-undefined-behavior)
> +KBUILD_CFLAGS += $(call cc-option, -no-integrated-as)
> +KBUILD_AFLAGS += $(call cc-option, -no-integrated-as)
> +else
> +
> +# These warnings generated too much noise in a regular build.
> +# Use make W=1 to enable them (see scripts/Makefile.extrawarn)
> +KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
> +KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
> +endif
> +
>  ifeq ($(mixed-targets),1)
>  # ===========================================================================
>  # We're called with mixed targets (*config and build targets).
> @@ -695,38 +727,6 @@ ifdef CONFIG_CC_STACKPROTECTOR
>  endif
>  KBUILD_CFLAGS += $(stackp-flag)
>  
> -ifeq ($(cc-name),clang)
> -ifneq ($(CROSS_COMPILE),)
> -CLANG_TARGET	:= --target=$(notdir $(CROSS_COMPILE:%-=%))
> -GCC_TOOLCHAIN	:= $(realpath $(dir $(shell which $(LD)))/..)
> -endif
> -ifneq ($(GCC_TOOLCHAIN),)
> -CLANG_GCC_TC	:= --gcc-toolchain=$(GCC_TOOLCHAIN)
> -endif
> -KBUILD_CFLAGS += $(CLANG_TARGET) $(CLANG_GCC_TC)
> -KBUILD_AFLAGS += $(CLANG_TARGET) $(CLANG_GCC_TC)
> -KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
> -KBUILD_CFLAGS += $(call cc-disable-warning, unused-variable)
> -KBUILD_CFLAGS += $(call cc-disable-warning, format-invalid-specifier)
> -KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
> -KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
> -# Quiet clang warning: comparison of unsigned expression < 0 is always false
> -KBUILD_CFLAGS += $(call cc-disable-warning, tautological-compare)
> -# CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
> -# source of a reference will be _MergedGlobals and not on of the whitelisted names.
> -# See modpost pattern 2
> -KBUILD_CFLAGS += $(call cc-option, -mno-global-merge,)
> -KBUILD_CFLAGS += $(call cc-option, -fcatch-undefined-behavior)
> -KBUILD_CFLAGS += $(call cc-option, -no-integrated-as)
> -KBUILD_AFLAGS += $(call cc-option, -no-integrated-as)
> -else
> -
> -# These warnings generated too much noise in a regular build.
> -# Use make W=1 to enable them (see scripts/Makefile.extrawarn)
> -KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
> -KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
> -endif
> -
>  ifdef CONFIG_FRAME_POINTER
>  KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls
>  else

FWIW

Tested-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>

On the Chrome OS side we missed this because by default our kernel
builds use an architecture aware wrapper instead of bare clang, which
masks this issue.