From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S932307AbbETQwa (ORCPT <rfc822;w@1wt.eu>);
	Wed, 20 May 2015 12:52:30 -0400
Received: from mail.skyhub.de ([78.46.96.112]:45582 "EHLO mail.skyhub.de"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1754315AbbETQw0 (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Wed, 20 May 2015 12:52:26 -0400
Date: Wed, 20 May 2015 18:52:23 +0200
From: Borislav Petkov <bp@alien8.de>
To: Josh Poimboeuf <jpoimboe@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>
Cc: Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Michal Marek <mmarek@suse.cz>, Peter Zijlstra <peterz@infradead.org>,
        X86 ML <x86@kernel.org>, live-patching@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Denys Vlasenko <dvlasenk@redhat.com>, Brian Gerst <brgerst@gmail.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v4 0/3] Compile-time stack frame pointer validation
Message-ID: <20150520165223.GC3424@pd.tnic>
References: <cover.1431966710.git.jpoimboe@redhat.com>
 <20150520103339.GA22205@gmail.com>
 <20150520141331.GA16995@treble.redhat.com>
 <20150520144810.GA10374@gmail.com>
 <CALCETrWAP=7dN8P3dn0JwYTn2SHYtyi-3z7KqGcjV0X6d=s50g@mail.gmail.com>
 <20150520162537.GD16995@treble.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20150520162537.GD16995@treble.redhat.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, May 20, 2015 at 11:25:37AM -0500, Josh Poimboeuf wrote:
> > I've never quite understood what the '?' means.
> 
> It basically means "here's a function address we found on the stack,
> which may or may not have been called."  It's needed because stack
> walking isn't currently 100% reliable.

Yeah, that was not that trivial to figure out at the time:

unsigned long
print_context_stack(struct thread_info *tinfo,
		...

                if (__kernel_text_address(addr)) {
                        if ((unsigned long) stack == bp + sizeof(long)) {
                                ops->address(data, addr, 1);
                                frame = frame->next_frame;
                                bp = (unsigned long) frame;
                        } else {
                                ops->address(data, addr, 0);
                        }

and that ops->address is

print_trace_address()
|-> printk_stack_address()

So if I'm understanding this correctly, if rBP+8 is equal to rSP, i.e.
return address is on the stack, then this frame got called.

Otherwise -> "?".

I might be missing something though...

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--