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

Andy Lutomirski
2015-05-20T16:40:27Z

On Wed, May 20, 2015 at 9:25 AM, Josh Poimboeuf <jpoimboe@redhat.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@kernel.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