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:   Tue, 16 Jul 2019 11:20:14 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Seth Forshee <seth.forshee@...onical.com>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.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 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