linux-kernel - Re: [PATCH 2/4] arm64: prevent instrumentation of LL/SC atomics

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Date:   Tue, 17 Oct 2017 11:03:15 +0100
From:   Will Deacon <will.deacon@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        adech.fo@...il.com, aryabinin@...tuozzo.com,
        catalin.marinas@....com, christoffer.dall@...aro.org,
        dvyukov@...gle.com, linux-kbuild@...r.kernel.org,
        mark.zyngier@....com, mmarek@...e.com,
        yamada.masahiro@...ionext.com
Subject: Re: [PATCH 2/4] arm64: prevent instrumentation of LL/SC atomics

On Mon, Oct 16, 2017 at 02:24:38PM +0100, Mark Rutland wrote:
> While we build the LL/SC atomics as a C object file, this does not
> follow the AAPCS. This does not interoperate with other C code, and can
> only be called from special wrapper assembly.
> 
> Bulding a kernel with CONFIG_KCOV and CONFIG_ARM64_LSE_ATOMICS results
> in the cmopiler inserting calls to __sanitizer_cov_trace_pc within the

cmopiler

> LL/SC atomics. As __sanitizer_cov_trace_pc is built per the AAPCS, these
> calls corrupt register values, resulting in failures at boot time.
> 
> Avoid this (and other similar issues) by opting out of all compiler
> instrumentation. We can opt-in to specific instrumentation in future if
> we want to.
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> ---
>  arch/arm64/lib/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/lib/Makefile b/arch/arm64/lib/Makefile
> index a0abc142c92b..af77516f71b2 100644
> --- a/arch/arm64/lib/Makefile
> +++ b/arch/arm64/lib/Makefile
> @@ -17,5 +17,6 @@ CFLAGS_atomic_ll_sc.o	:= -fcall-used-x0 -ffixed-x1 -ffixed-x2		\
>  		   -fcall-saved-x10 -fcall-saved-x11 -fcall-saved-x12	\
>  		   -fcall-saved-x13 -fcall-saved-x14 -fcall-saved-x15	\
>  		   -fcall-saved-x18
> +CC_INSTRUMENT_atomic_ll_sc.o := n

Does this mean we can lose the "notrace" definition of __LL_SC_INLINE
when generating the out-of-line atomics?

Will