[RFC PATCH 12/17] gcc-plugins: objtool: Add plugin to detect switch table on arm64

[Nick Desaulniers]

On Mon, Feb 1, 2021 at 4:02 PM Josh Poimboeuf <jpoimboe at redhat.com> wrote:
>
> On Mon, Feb 01, 2021 at 03:17:40PM -0800, Nick Desaulniers wrote:
> > On the earlier thread, Julien writes:
> >
> > >> I think most people interested in livepatching are using GCC built
> > >> kernels, but I could be mistaken (althought in the long run, both
> > >> compilers should be supported, and yes, I realize the objtool solution
> > >> currently only would support GCC).
> >
> > Google's production kernels are using livepatching and are built with
> > Clang.  Getting similar functionality working for arm64 would be of
> > interest.
>
> Well, that's cool.  I had no idea.
>
> I'm curious how they're generating livepatch modules?  Because
> kpatch-build doesn't support Clang (AFAIK), and if they're not using
> kpatch-build then there are some traps to look out for.

Ok, I just met with a bunch of folks that are actively working on
this.  Let me intro
Yonghyun Hwang <yonghyun at google.com>
Pete Swain <swine at google.com>
who will be the folks on point for this from Google.

My understanding after some clarifications today is that Google is
currently using a proprietary kernel patching mechanism that developed
around a decade ago, "pre-ksplice Oracle acquisition."  But we are
looking to transition to kpatch, and help towards supporting arm64.
Live patching is important for deploying kernel fixes faster than
predetermined scheduled draining of jobs in clusters.

The first steps for kpatch transition is supporting builds with Clang.
Yonghyun is working on that and my hope is he will have patches for
you for that soon.

Curiously, the proprietary mechanism doesn't rely on stack validation.
I think that such dependency was questioned on the cover letter
patch's thread as well.  Maybe there's "some traps to look out for"
you're referring to there?  I'm not privy to the details, though I
would guess it has to do with ensuring kernel threads aren't executing
(or planning to return through) code regions that are trying to be
patched/unpatched.  I am curious about frame pointers never being
omitted for arm64; is frame pointer chasing is unreliable in certain
contexts?

The internal functionality has been used heavily in production for
almost a decade, though without it being public or supporting arm64;
I'm not sure precisely how they solve such issues (or how others might
review such an approach).

Either way, the dependencies for live patching are less important, so
long as they are toolchain portable.  The ability to live patch kernel
images is ___important___ to Google.

> > Objtool support on arm64 is interesting to me though, because it has
> > found bugs in LLVM codegen. That alone is extremely valuable.  But not
> > it's not helpful if it's predicated or tightly coupled to GCC, as this
> > series appears to do.
>
> I agree 100%, if there are actual Clang livepatch users (which it sounds
> like there are) then we should target both compilers.

Or will be. (Sorry, I didn't know we hadn't completed the transition
to kpatch yet.  It is "the opposite side of the house" from where I
work; I literally have 8 bosses, not kidding).

Though if kpatch moves to requiring GCC plugins for architectures we
use extensively or would like to use more of, that's probably going to
throw a wrench in multiple transition plans.  (The fleet's transition
to Clang is done, I'm not worried about that).

> And yes, objtool has been pretty good at finding compiler bugs, so the
> more coverage the better.
> > The idea of rebuilding control flow from binary analysis and using
> > that to find codegen bugs is a really cool idea (novel, even? idk),
> > and I wish we had some analog for userspace binaries that could
> > perform similar checks.
>
> Objtool is generic in many ways -- in fact I recently heard from a PhD
> candidate who used it successfully on another kernel for an ORC
> unwinder.

That's pretty cool!  Reuse outside the initial context is always a
good sign that something was designed right.

> It could probably be used on user space without much effort.  That was
> an early original stated goal but I definitely don't have the bandwidth
> or incentive to work on it.

Heh.  I'm a big fan of game theory; carrot or stick, right?
-- 
Thanks,
~Nick Desaulniers