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

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Message-ID: <nycvar.YFH.7.76.1801111804240.11852@cbobk.fhfr.pm>
Date:   Thu, 11 Jan 2018 18:05:24 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        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>,
        gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH 2/3] objtool: Ignore retpoline alternatives

On Thu, 11 Jan 2018, Peter Zijlstra 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);
> }

	# gcc --version
	gcc (SUSE Linux) 4.3.4 [gcc-4_3-branch revision 152973]
	# ./test 
	8

So this part is OK. The asm-goto hard dependency would be worse with that 
compiler though.

-- 
Jiri Kosina
SUSE Labs