linux-kernel - Re: [PATCH v2 10/18] arm64: add a workaround for GNU gold with ARM64_MODULE

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Date:   Thu, 16 Nov 2017 18:47:15 +0000
From:   Will Deacon <will.deacon@....com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Alex Matveev <alxmtvv@...il.com>, Andi Kleen <ak@...ux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: Re: [PATCH v2 10/18] arm64: add a workaround for GNU gold with
 ARM64_MODULE_PLTS

On Thu, Nov 16, 2017 at 08:41:01AM -0800, Sami Tolvanen wrote:
> On Thu, Nov 16, 2017 at 11:50:12AM +0000, Will Deacon wrote:
> > Why don't we just not do LTO if the toolchain is busted?
> 
> Because LTO can not only potentially improve performance, especially
> when combined with PGO (Profile Guided Optimization), but it also
> makes it possible to enable features like Control Flow Integrity that
> can make kernel vulnerabilities more difficult to exploit:
> 
>   https://clang.llvm.org/docs/ControlFlowIntegrity.html
> 
> > This feels like it will end up being a game of whack-a-mole as code
> > could be introduced that tickles known bugs on older toolchains.
> 
> I'm not sure this is much different from dealing with older versions
> of the existing toolchain. Otherwise, we wouldn't need cc-version or
> other similar macros, for example.

I think the big difference is that we have no compelling need to support
older versions of clang or gold.

Will