Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

[Ingo Molnar <mingo@kernel.org>]

* Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> On Wed, Jul 12, 2017 at 10:27:10AM +0200, Ingo Molnar wrote:
> > > Create a new "ORC" unwinder, enabled by CONFIG_ORC_UNWINDER, and plug it
> > > into the x86 unwinder framework.  Objtool is used to generate the ORC
> > > debuginfo.  The ORC debuginfo format is basically a simplified version
> > > of DWARF CFI.  More details below.
> > 
> > BTW., we should perhaps consolidate our unwinder related Kconfig space, 
> > hierarchically:
> > 
> > 	CONFIG_UNWINDER
> > 	CONFIG_UNWINDER_ORC
> > 	CONFIG_UNWINDER_FRAME_POINTERS
> > 
> > Note that as a side effect it would be a valid small systems build option to have 
> > no unwinder at all, if CONFIG_EXPERT=y is set and such: !CONFIG_UNWINDER=n would 
> > be a sibling to !CONFIG_BUG.
> 
> So is the idea that CONFIG_UNWINDER=n means "use the 'guess' unwinder"?
> Or should it mean that the unwind API isn't available?
> 
> Without frame pointers and orc, it defaults to the 'guess' unwinder, for
> which the only overhead is a tiny amount of code.  It's still
> technically considered an unwinder because it plugs into the unwind
> interfaces (unwind_start(), unwind_next_frame(), etc) and is used for
> things like /proc/<pid>/stack.
> 
> So I'm not really sure CONFIG_UNWINDER=n would make sense.  Maybe there
> should just be a multiple-choice where you have to choose one of
> CONFIG_UNWINDER_{ORC,FRAME_POINTER,GUESS}.

Ok, you are right.

Maybe we could offer a menu of unwinders - i.e. make the whole Kconfig interface a 
bit nicer:

  CONFIG_UNWINDER_FRAME_POINTER
  CONFIG_UNWINDER_ORC
  CONFIG_UNWINDER_GUESS

... or so?

Default would be the historic FRAME_POINTER, at least initially, I think.

I wouldn't mind making CONFIG_UNWINDER_ORC the new default either, due to the 
non-trivial speedup it offers - but maybe folks would object?

> > CONFIG_FRAME_POINTERS et al would be left for architectures where it has a meaning 
> > beyond backtrace generation. (Not sure whether there's any such architectures.)
> 
> Well, on x86, hardened usercopy relies on frame pointers, but not the
> unwinder.  It does the frame pointer walk manually to avoid the full
> unwinder overhead.  See arch_within_stack_frames().

Oh well...

> Ok, how about:
> 
>   "Orc unwind tables take up ~50% more RAM (+1.3MB on an x86 defconfig
>   kernel) than DWARF eh_frame tables."
> 
> (My previous 1MB number was from my distro-based config, and it also
> forgot to take into account the fast lookup table (".orc_lookup")).

Sounds good to me!

Thanks,

	Ingo