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

Josh Poimboeuf
2015-05-20T16:25:43Z

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
- 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()

--
Josh