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

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Date:   Fri, 4 Mar 2022 14:51:16 +0800
From:   David Gow <davidgow@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
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 <linux-um@...ts.infradead.org>,
        linux-kbuild@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        KUnit Development <kunit-dev@...glegroups.com>,
        llvm@...ts.linux.dev,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

On Fri, Mar 4, 2022 at 1:30 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> 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?

Yeah: this only seems to show up with UML source compiled as "user"
files (i.e., talking to the host system). Most of the kernel source
doesn't show the warning because -Qunused-arguments is set somewhere,
probably in KBUILD_CPPFLAGS, which doesn't filter down into
USER_CFLAGS:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile?id=7e57714cd0ad2d5bb90e50b5096a0e671dec1ef3#n784

Is this flag supposed to only work on ARM/AArch64, or is it just
currently not implemented anywhere else. If it's actually
ARM-specific, then moving it to the arch/arm{,64} makefile as in Kees'
patch definitely makes more sense. If it's going to show up on other
architectures at some point anyway, maybe something like this makes
sense.

>
> 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?
>
No: it works fine without the ifdef: I just wanted to limit the damage
I could do playing with things I didn't fully understand. :-)

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

Cheers,
-- David

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