linux-kernel - Re: [PATCH] um: clang: Strip out -mno-global-merge from USER

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Date:   Thu, 3 Mar 2022 10:30:47 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     David Gow <davidgow@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>, Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-um@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

Hi David,

On Thu, Mar 03, 2022 at 05:06:42PM +0800, David Gow wrote:
> The things built with USER_CFLAGS don't seem to recognise it as a
> compiler option, and print a warning:
> clang: warning: argument unused during compilation: '-mno-global-merge' [-Wunused-command-line-argument]
> 
> Fixes: 744814d2fa ("um: Allow builds with Clang")
> Signed-off-by: David Gow <davidgow@...gle.com>
> ---
> 
> This warning shows up after merging:
> https://lore.kernel.org/lkml/20220227184517.504931-6-keescook@chromium.org/
> 
> I'm not 100% sure why this is necessary, but it does seem to work. All
> the attempts to get rid of -mno-global-merge entirely have been met with
> skepticism, but I'm guessing that it's not a problem for just the UML
> "user" files, as they shouldn't(?) interact too much with modules.

Thank you for the patch! I think it is correct, as this flag only works
for AArch64 and ARM, as it is only used in Clang::AddAArch64TargetArgs()
and Clang::AddARMTargetArgs() in clang/lib/Driver/ToolChains/Clang.cpp,
which are obviously never called with UML. I am not sure why we do not
see warning during regular kernel builds, maybe something about how UML
objects are compiled exposes this?

Regardless, I would definitely like to clean up this instance of the
warning because I would like to make this warning a hard error so that
we do not get cryptic cc-option failures:

https://github.com/ClangBuiltLinux/linux/issues/1587

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

One small comment below.

>  arch/um/Makefile | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/um/Makefile b/arch/um/Makefile
> index f2fe63bfd819..320b09cd513c 100644
> --- a/arch/um/Makefile
> +++ b/arch/um/Makefile
> @@ -75,6 +75,10 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
>  		-D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
>  		-idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__
>  
> +ifdef CONFIG_CC_IS_CLANG

Is this ifdef needed?

> +USER_CFLAGS := $(patsubst -mno-global-merge,,$(USER_CFLAGS))
> +endif
> +
>  #This will adjust *FLAGS accordingly to the platform.
>  include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
>  
> -- 
> 2.35.1.616.g0bdcbb4464-goog
>