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

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Date:   Mon, 20 Nov 2017 14:41:47 +0000
From:   Mark Rutland <mark.rutland@....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,
        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 06/18] arm64: kvm: use -fno-jump-tables with clang

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.

Thanks,
Mark.

> Suggested-by: AKASHI Takahiro <takahiro.akashi@...aro.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  arch/arm64/kvm/hyp/Makefile | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hyp/Makefile b/arch/arm64/kvm/hyp/Makefile
> index f04400d494b7..19fa1c6b6b69 100644
> --- a/arch/arm64/kvm/hyp/Makefile
> +++ b/arch/arm64/kvm/hyp/Makefile
> @@ -5,6 +5,10 @@
>  
>  ccflags-y += -fno-stack-protector -DDISABLE_BRANCH_PROFILING
>  
> +ifeq ($(cc-name),clang)
> +ccflags-y += -fno-jump-tables
> +endif
> +
>  KVM=../../../../virt/kvm
>  
>  obj-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/hyp/vgic-v2-sr.o
> -- 
> 2.15.0.448.gf294e3d99a-goog
>