linux-kernel - Re: [PATCH] x86/build: fix compiler support check for CONFIG

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Date:   Wed, 5 Dec 2018 15:10:49 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     zhenzhong.duan@...cle.com
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        Meelis Roos <mroos@...ux.ee>, "H. Peter Anvin" <hpa@...or.com>,
        Borislav Petkov <bp@...en8.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/build: fix compiler support check for CONFIG_RETPOLINE

On Wed, Dec 5, 2018 at 3:08 PM Zhenzhong Duan <zhenzhong.duan@...cle.com> wrote:
>
>
> On 2018/12/5 11:00, Masahiro Yamada wrote:
> > It is wrong to add CONFIG option diagnostic to the Makefile parse
> > stage.
> >
> > Once you are hit by the error about non-retpoline compiler, the
> > compilation still breaks even after disabling CONFIG_RETPOLINE.
> >
> > The easiest fix is to move this check to the "archprepare" like commit
> > 829fe4aa9ac1 ("x86: Allow generating user-space headers without a
> > compiler") did.
> >
> > Link: https://lkml.org/lkml/2018/12/4/206
> > Reported-by: Meelis Roos <mroos@...ux.ee>
> > Fixes: 4cd24de3a098 ("x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support")
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> > ---
> >
> >   arch/x86/Makefile | 14 ++++++++------
> >   1 file changed, 8 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> > index f5d7f41..ae0148b 100644
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -219,12 +219,7 @@ KBUILD_CFLAGS += -Wno-sign-compare
> >   KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> >
> >   # Avoid indirect branches in kernel to deal with Spectre
> > -ifdef CONFIG_RETPOLINE
> > -ifeq ($(RETPOLINE_CFLAGS),)
> > -  $(error You are building kernel with non-retpoline compiler, please update your compiler.)
> > -endif
> > -  KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
> > -endif
> > +KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
>
> Is it better to also move "# Avoid indirect branches in kernel to deal
> with Spectre"
>
> and "KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)"?
>
> Seems unconditionaly using "KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)" will
> have compiler
>
> using "__x86_indirect_thunk_\reg" call even ifCONFIG_RETPOLINE is
> disabled. I guess
>
> link process will fail?


Good catch.

I will send v2.

Thanks.



> Thanks
>
> Zhenzhong
>
> >
> >   archscripts: scripts_basic
> >       $(Q)$(MAKE) $(build)=arch/x86/tools relocs
> > @@ -307,6 +302,13 @@ ifndef CC_HAVE_ASM_GOTO
> >       @echo Compiler lacks asm-goto support.
> >       @exit 1
> >   endif
> > +ifdef CONFIG_RETPOLINE
> > +ifeq ($(RETPOLINE_CFLAGS),)
> > +     @echo "You are building kernel with non-retpoline compiler." >&2
> > +     @echo "Please update your compiler." >&2
> > +     @false
> > +endif
> > +endif
> >
> >   archclean:
> >       $(Q)rm -rf $(objtree)/arch/i386



-- 
Best Regards
Masahiro Yamada