linux-kernel - Re: [PATCH v2] x86/build: fix compiler support check for CONFIG

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Date:   Tue, 18 Dec 2018 14:20:44 +0100
From:   Gi-Oh Kim <gi-oh.kim@...ud.ionos.com>
To:     zhenzhong.duan@...cle.com
Cc:     yamada.masahiro@...ionext.com, mingo@...hat.com,
        tglx@...utronix.de, x86@...nel.org, mroos@...ux.ee, hpa@...or.com,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/build: fix compiler support check for CONFIG_RETPOLINE

On Wed, Dec 5, 2018 at 8:11 AM Zhenzhong Duan <zhenzhong.duan@...cle.com> wrote:
>
>
> On 2018/12/5 14:27, Masahiro Yamada wrote:
> > It is troublesome to add a diagnostic like this to the Makefile
> > parse stage because the top-level Makefile could be parsed with
> > a stale include/config/auto.conf.
> >
> > 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
> > Fixes: 4cd24de3a098 ("x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support")
> > Reported-by: Meelis Roos <mroos@...ux.ee>
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> > ---
> >
> > Changes in v2:
> >    - Revive ifdef CONFIG_RETPOLINE surrounding the KBUILD_CFLAGS addition
> >    - Rephase the commit log a bit, hoping the cause of the issue will be clearer
> >
> >   arch/x86/Makefile | 10 +++++++---
> >   1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> > index f5d7f41..75ef499 100644
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -220,9 +220,6 @@ 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 += $(RETPOLINE_CFLAGS)
> >   endif
> >
> > @@ -307,6 +304,13 @@ ifndef CC_HAVE_ASM_GOTO
> >       @echo Compiler lacks asm-goto support.
> >       @exit 1
> >   endif
> > +ifdef CONFIG_RETPOLINE
> > +ifeq ($(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
> Acked-by: Zhenzhong Duan <zhenzhong.duan@...cle.com>
>
> --
> Regards
> Zhenzhong
>

Hi,

I think this patch needs to be merged to linux-stable 4.14.y branch.
Following is what I did to reproduce the problem.
The problem is gone after applying the patch.

1. I created and installed a headers package as following.
make -j7 deb-pkg
cd ..
sudo dpkg -i linux-headers-4.14.89+_4.14.89+-18_amd64.deb

2. I managed to re-produce the same problem as following.
gohkim@...0837:~/study/drv$ make -C /usr/src/linux-headers-4.14.89+/
kernelrelease
make: Entering directory '/usr/src/linux-headers-4.14.89+'
arch/x86/Makefile:245: *** You are building kernel with non-retpoline
compiler, please update your compiler.. Stop.
make: Leaving directory '/usr/src/linux-headers-4.14.89+'

3. Weird thing is that it works fine if I use sudo command.
gohkim@...0837:~/study/drv$ sudo make -C
/usr/src/linux-headers-4.14.89+/ kernelrelease
make: Entering directory '/usr/src/linux-headers-4.14.89+'
  HOSTCC  scripts/kconfig/conf.o
  HOSTCC  scripts/kconfig/zconf.tab.o
  HOSTLD  scripts/kconfig/conf
scripts/kconfig/conf  --silentoldconfig Kconfig
4.14.89
make: Leaving directory '/usr/src/linux-headers-4.14.89+'


-- 
GIOH KIM
Linux Kernel Entwickler

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