linux-kernel - Re: [kbuild:kbuild 5/19] drivers/atm/eni.o: warning: objtool: eni_init_one()+0xe42: indirect call found in RETPOLINE build

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Date:   Wed, 17 Jul 2019 11:52:07 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Seth Forshee <seth.forshee@...onical.com>, kbuild-all@...org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kbuild test robot <lkp@...el.com>
Subject: Re: [kbuild:kbuild 5/19] drivers/atm/eni.o: warning: objtool:
 eni_init_one()+0xe42: indirect call found in RETPOLINE build

On Wed, Jul 17, 2019 at 1:20 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Tue, Jul 16, 2019 at 07:42:49AM -0500, Seth Forshee wrote:
> > On Tue, Jul 16, 2019 at 03:57:24PM +0900, Masahiro Yamada wrote:
> > > (+ Josh Poimboeuf)
> > >
> > > On Tue, Jul 16, 2019 at 8:44 AM kbuild test robot <lkp@...el.com> wrote:
> > > >
> > > > tree:   https://kernel.googlesource.com/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git kbuild
> > > > head:   0ff0c3753e06c0420c80dac1b0187a442b372acb
> > > > commit: 2eaf4e87ba258cc3f27e486cdf32d5ba76303c6f [5/19] kbuild: add -fcf-protection=none to retpoline flags
> > > > config: x86_64-randconfig-s2-07160214 (attached as .config)
> > > > compiler: gcc-4.9 (Debian 4.9.4-2) 4.9.4
> > > > reproduce:
> > > >         git checkout 2eaf4e87ba258cc3f27e486cdf32d5ba76303c6f
> > > >         # save the attached .config to linux build tree
> > > >         make ARCH=x86_64
> > >
> > > 0-day bot reports objtool warnings with the following applied:
> > > https://patchwork.kernel.org/patch/11037379/
> > >
> > > I have no idea about objtool.
> > >
> > > Is it better to drop this patch for now?
> >
> > I'm surprised that the change would have any impact on a build with
> > gcc-4.9, since -fcf-protection seems to have been introduced in gcc-8. I
> > guess there's no full build log that would let us see the actual flags
> > passed to the compiler.
> >
> > I'll try to reproduce this result. If you think the patch should be
> > dropped in the meantime, that's fine.
>
> The problem with this patch is that it's breaking the following check in
> arch/x86/Makefile.  GCC 4.9 doesn't support retpolines, so it's supposed
> to fail with the below error.
>
> 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
>
> Maybe the flags should be placed in another variable other than
> RETPOLINE_CFLAGS.



Josh,
Thanks. You are right.


Seth,
I think you can add the flag to KBUILD_CFLAGS.

If you want to make sure this does not affect non-retpoline
build, you can surround the code with ifdef.

ifdef CONFIG_RETPOLINE
KBUILD_CFLAGS  += $(call cc-option,-fcf-protection=none)
endif



-- 
Best Regards
Masahiro Yamada