linux-kernel - Re: new objtool unreachable instruction warnings

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Message-ID: <20170726204102.ms3pz47hgw25j27v@treble>
Date:   Wed, 26 Jul 2017 15:41:02 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: new objtool unreachable instruction warnings

On Wed, Jul 26, 2017 at 09:52:36PM +0200, Arnd Bergmann wrote:
> On Wed, Jul 26, 2017 at 9:10 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > On Wed, Jul 26, 2017 at 04:46:40PM +0200, Arnd Bergmann wrote:
> >> I only saw these warnings once, they are either very rare, or were introduced
> >> recently:
> >>
> >> arch/x86/kvm/vmx.o: warning: objtool: .altinstr_replacement+0x0:
> >> unreachable instruction
> >> arch/x86/kvm/svm.o: warning: objtool: .altinstr_replacement+0x6e:
> >> unreachable instruction
> >>
> >> I see this with gcc-4.6 though gcc-7, but not with gcc-4.3.
> >>
> >> The configuration file that triggered it is
> >> https://pastebin.com/aMn45GYP
> >>
> >> I managed to trace the problem down to the CC_HAVE_ASM_GOTO
> >> macro, without that, we don't run into the problem.
> >
> > Thanks for reporting all these!
> >
> > This should fix all the warnings you reported.  I'll split it up into
> > real patches and submit them soon.  Let me know if you find anything
> > else.
> 
> Looks good. I tried ten different randconfigs that found warnings in the
> past few days, and all the warnings are gone now.
> 
> For the warnings that showed up only with gcc-7, I think we may want
> a backport to stable kernels, the other ones seem to be recent additions
> (after 4.12).

Yeah, I think you're right.

The GCC 7 one (where GCC inserts 'ud2' instead of dividing by zero) was
introduced with:

  d1091c7fa3d5 ("objtool: Improve detection of BUG() and other dead ends")

The atom one is only in -next.

And the one above (arch/x86/kvm/vmx.o .altinstr_replacement unreachable
instruction) has always been possible, but has only recently been made
visible with some new objtool whitelisting in -next.

-- 
Josh