linux-kernel - Re: [v8,02/12] objtool: Allow alternatives to be ignored

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Message-ID: <20180128210642.xob2win53h2gztak@treble>
Date:   Sun, 28 Jan 2018 15:06:42 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk,
        x86@...nel.org, thomas.lendacky@....com
Subject: Re: [v8,02/12] objtool: Allow alternatives to be ignored

On Mon, Jan 22, 2018 at 12:25:22PM -0800, Guenter Roeck wrote:
> 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.

I've got a pending fix for this, so that objtool doesn't seg fault, and
instead prints out a warning:

  quirks.o: warning: objtool: efi_delete_dummy_variable()+0x99: unsupported intra-function call
  quirks.o: warning: objtool: If this is a retpoline, please patch it in with alternatives and annotate it with ANNOTATE_NOSPEC_ALTERNATIVE.

The code is here, along with a few more fixes:

  https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=TODO-objtool-seg-fault

Will post it soon.

-- 
Josh