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

From: Ingo Molnar
Date: Wed Jul 12 2017 - 15:28:00 EST

* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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:
> >
> >
> > 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

Ok, you are right.

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


... 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!