linux-kernel - Re: [PATCH v2 06/18] arm64: kvm: use -fno-jump-tables with clang

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Date:   Mon, 20 Nov 2017 14:43:06 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Andi Kleen <ak@...ux.intel.com>, Kees Cook <keescook@...omium.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-kbuild@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, Greg Hackmann <ghackmann@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Alex Matveev <alxmtvv@...il.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        linux-arm-kernel@...ts.infradead.org,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>
Subject: Re: [PATCH v2 06/18] arm64: kvm: use -fno-jump-tables with clang

On Mon, Nov 20, 2017 at 02:41:47PM +0000, Mark Rutland wrote:
> On Wed, Nov 15, 2017 at 01:34:16PM -0800, Sami Tolvanen wrote:
> > Use -fno-jump-tables to make sure clang doesn't generate branches
> > to EL1 virtual addresses.
> 
> Are there any other reasons that clang might generate absolute
> references/relocations?
> 
> It would be nice if there was the option to disable that more generally,
> rather than disabling individual optimizations. Is there any PIC/PIE
> option that we could use?
> 
> We might need something simnilar for GCC, even if we're not seeing
> problems today.

... and likewise for the EFI stub, which needs to be
position-independent much like the hyp code.

Thanks,
Mark.