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

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Message-ID: <1516649644.9521.61.camel@infradead.org>
Date:   Mon, 22 Jan 2018 19:34:04 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Guenter Roeck <linux@...ck-us.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     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 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. 
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