Date:	Wed, 20 May 2015 09:39:59 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
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>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 0/3] Compile-time stack frame pointer validation

On Wed, May 20, 2015 at 9:25 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Wed, May 20, 2015 at 09:03:37AM -0700, Andy Lutomirski wrote:
>> On Wed, May 20, 2015 at 7:48 AM, Ingo Molnar <mingo@...nel.org> wrote:
>> > Yeah, so many of these seem to be 'leaf only' functions: functions
>> > that don't ever call functions themselves.
>> >
>> > So lets assume we always have CONFIG_FRAME_POINTERS=y.
>> >
>> > If they don't set up a frame pointer then they in essence won't show
>> > up in the call chain - but normally they wouldn't because they call
>> > nothing.
>> >
>> > If they trigger an exception/fault or if they get hit by an interrupt
>> > then I think we'll still correctly walk the stack - just those
>> > functions might be missing from the deterministic call chain, right?
>> > (it will still show up as a '?' entry.)
>>
>> 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.
>
>> > If they crash then we'll see them because the crashing RIP will be
>> > printed.
>> >
>> > So I'm wondering what the x86 policy here should be: to create frame
>> > pointers in them or not. Cc:-ed a few more gents for thoughts.
>> >
>>
>> I think it would be nice to have full DWARF unwind support for
>> everything at some point.  Unfortunately, I don't see any easy path to
>> getting there.  It doesn't help that AFAIK no one has ever proposed a
>> usable in-kernel DWARF unwinder.
>>
>> It also doesn't help that writing correct CFI annotations in inline
>> asm can be very complicated.
>>
>> I think that ia64 manages to have complete unwind support.  How did
>> they manage it?
>>
>> If we had an unwinder, it would be relatively straightforward to write
>> something perf-based that would frequently check that we can unwind
>> all the way out of an NMI back to userspace and warn if we couldn't.
>
> I agree that DWARF unwind support would be nice.  I have some plans
> about how to achieve that in future patch sets.  It includes several
> pieces:
>
> - compile-time DWARF data validation (using some similar approaches to
>   this patch set)
>
> - run time DWARF data validation, including:
>   - a DWARF unwinder which doesn't blindly trust any of the DWARF data

Fantastic!

>   - ensuring DWARF and frame pointer data are consistent with each other
>   - ensuring it can walk all the way to the bottom of the stack
>   - a DEBUG option which validates the stack periodically from an NMI
>     and/or schedule()

We think alike :)

NMI will be much more interesting than schedule.

--Andy
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