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

From: Nick Desaulniers <ndesaulniers@google.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Julien Thierry <jthierry@redhat.com>,
	Ard Biesheuvel <ardb@kernel.org>, Mark Brown <broonie@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Kees Cook <keescook@chromium.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	linux-efi <linux-efi@vger.kernel.org>,
	linux-hardening@vger.kernel.org,
	LKML <linux-kernel@vger.kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Peter Zijlstra <peterz@infradead.org>,
	raphael.gault@arm.com, Will Deacon <will@kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Bill Wendling <morbo@google.com>, Pete Swain <swine@google.com>,
	Yonghyun Hwang <yonghyun@google.com>,
	live-patching@vger.kernel.org
Subject: Re: [RFC PATCH 12/17] gcc-plugins: objtool: Add plugin to detect switch table on arm64
Date: Tue, 2 Feb 2021 15:52:04 -0800	[thread overview]
Message-ID: <CAKwvOdnr5LcVbv2=2h+j8-ekvoB6PezmSOWhPzZdMaDG6eniag@mail.gmail.com> (raw)
In-Reply-To: <20210202233636.nvbl6wivgnhacbvg@treble>

On Tue, Feb 2, 2021 at 3:36 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Tue, Feb 02, 2021 at 02:33:38PM -0800, Nick Desaulniers wrote:
> > On Mon, Feb 1, 2021 at 4:02 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > >
> > > On Mon, Feb 01, 2021 at 03:17:40PM -0800, Nick Desaulniers wrote:
> > > 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.
>
> So basically you're saying objtool is both useful and well-designed.  I
> will quote you on that!

Haha, all I'm saying is that while I'm not proud that it did find bugs
in LLVM (and I do have existing bugs found by it to fix on my plate),
I don't see who else or how else those would have been spotted, and I
can appreciate that.  I think the tools given to us are broken (by
design, perhaps), so anything that can help us spot issues might help
our code live longer than we do.

I also think that there's room for improvement and experimentation in
debug info formats, though there is currently a proliferation to
support.  Live patching and eBPF seem to have some functional overlap
IIUC, strengths/weaknesses, and their own unique debug info formats to
go with it.  Supporting each one does require some level of toolchain
support or coordination (or complexity, even).
-- 
Thanks,
~Nick Desaulniers