linux-kernel - Re: [PATCH 2/3] objtool: Ignore retpoline alternatives

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Date:   Thu, 11 Jan 2018 17:58:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     David Woodhouse <dwmw2@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        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>,
        Kees Cook <keescook@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH 2/3] objtool: Ignore retpoline alternatives

On Thu, Jan 11, 2018 at 10:48:26AM -0600, Josh Poimboeuf wrote:
> I think I heard that retpolines won't be ported to anything older than
> GCC 4.9, so maybe it's safe to use '%='.  I don't remember when it was
> introduced into GCC though.

root@...erlagos:~/tmp# gcc-4.8 -o test test.c
root@...erlagos:~/tmp# ./test
11

--- test.c

#include <stdio.h>

void main(void)
{
        int val;

        asm ("mov $(%=),%0" : "=A" (val));

        printf("%d\n", val);
}