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

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Message-ID: <20180129171526.GA17121@roeck-us.net>
Date:   Mon, 29 Jan 2018 09:15:26 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 Sun, Jan 28, 2018 at 03:06:42PM -0600, Josh Poimboeuf wrote:
> 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
> 

'objtool: Improve retpoline alternative handling' works for me.

Thanks!
Guenter