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Date:   Mon, 22 Jan 2018 12:25:22 -0800
From:   Guenter Roeck <linux@roeck-us.net>
To:     David Woodhouse <dwmw2@infradead.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Andi Kleen <ak@linux.intel.com>, Paul Turner <pjt@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>, tglx@linutronix.de,
        Kees Cook <keescook@google.com>,
        Rik van Riel <riel@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Jiri Kosina <jikos@kernel.org>, gnomes@lxorguk.ukuu.org.uk,
        x86@kernel.org, thomas.lendacky@amd.com
Subject: Re: [v8,02/12] objtool: Allow alternatives to be ignored
Message-ID: <20180122202522.GA29081@roeck-us.net>
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Hi David,

On Mon, Jan 22, 2018 at 07:34:04PM +0000, David Woodhouse wrote:
> On Thu, 2018-01-18 at 11:41 -0800, Guenter Roeck wrote:
> > 
> > > Not sure, does your gcc have retpolines?? Give me your .o file and I can
> > > diagnose it.
> > >?
> > Yes, it does, only it is the gcc from the Google toolchain which may
> > generate different code than the upstream version.
> > 
> > I attached an affected object file. Please let me know if there is anything else
> > I can do to help.
> Disassembly of section .text.__x86.indirect_thunk:
> 
> 0000000000000000 <__x86.indirect_thunk>:
> ???0:	e8 04 00 00 00???????	callq??9 <__x86.indirect_thunk+0x9>
> ???5:	f3 90????????????????	pause??
> ???7:	eb fc????????????????	jmp????5 <__x86.indirect_thunk+0x5>
> ???9:	48 8d 64 24 08???????	lea????0x8(%rsp),%rsp
> ???e:	c3???????????????????	retq???
> 
> That has the old-style CET-incompatible retpoline in a COMDAT section
> in the .o file. What compiler options are being used for that? The
> kernel should only use retpoline if GCC supports both of
> -mindirect-branch=thunk-extern and -mindirect-branch-register, and this
> compiler is doing *neither* of those.?

It uses "-mindirect-branch=thunk -mindirect-branch-loop=pause
-fno-jump-tables", though I don't know if that even exists in
upstream gcc (it is the gcc use for Chrome OS builds). I'll pass
your feedback to our compiler team.

Either case, I think it is less than optimal that objtool crashes
with _any_ object code.

Thanks,
Guenter