linux-kernel - Re: [PATCH v4 0/3] Compile-time stack frame pointer validation

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Message-ID: <20150520165223.GC3424@pd.tnic>
Date:	Wed, 20 May 2015 18:52:23 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
	live-patching@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 0/3] Compile-time stack frame pointer validation

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.
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