linux-kernel - Re: [PATCH v2] kbuild: Set KBUILD_CFLAGS before incl. arch Makefile

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Date:   Thu, 9 Nov 2017 13:31:08 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Behan Webster <behanw@...verseincode.com>,
        Jan-Simon Möller <dl9pf@....de>,
        Mark Charlebois <charlebm@...il.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Chris Fries <cfries@...gle.com>,
        Michal Marek <mmarek@...e.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] kbuild: Set KBUILD_CFLAGS before incl. arch Makefile

Hi Nick

2017-11-08 4:46 GMT+09:00 Nick Desaulniers <ndesaulniers@...gle.com>:
> From: Chris Fries <cfries@...gle.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.

OK.
Your previous patch was applied (and we have not received 0-day report so far),
so this makes sense now.




> 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

IMO, this description is not helpful in any way in upstream.

Moreover,
785f11aa595bc3d4e74096cbd598ada54ecc0d81
did not break anything.   It sound unfair to me.






> diff --git a/Makefile b/Makefile
> index a7476e6934f1..d349734c7ef7 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -472,6 +472,38 @@ ifneq ($(KBUILD_SRC),)
>             $(srctree) $(objtree) $(VERSION) $(PATCHLEVEL)
>  endif
>
> +ifeq ($(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
> +


Could you move this a bit later, please?



# 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

  << INSERT HERE >>

# The arch Makefile can set ARCH_{CPP,A,C}FLAGS to override the default
# values of the respective KBUILD_* variables
ARCH_CPPFLAGS :=
ARCH_AFLAGS :=
ARCH_CFLAGS :=
include arch/$(SRCARCH)/Makefile






arch/$(SRCARCH)/Makefile is included for config targets
for KBUILD_DEFCONFIG, but no reason to compute compiler flags.
I want to cut unnecessary cc-option parsing.




-- 
Best Regards
Masahiro Yamada