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

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

On Tue, Feb 2, 2021 at 3:36 PM Josh Poimboeuf <jpoimboe@...hat.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@...hat.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