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

From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Guenter Roeck <linux@roeck-us.net>
Cc: David Woodhouse <dwmw2@infradead.org>,
	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
Date: Sun, 28 Jan 2018 15:06:42 -0600	[thread overview]
Message-ID: <20180128210642.xob2win53h2gztak@treble> (raw)
In-Reply-To: <20180122202522.GA29081@roeck-us.net>

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