linux-kernel - Re: [PATCH v2] arm64: allow building with kcov coverage on ARM64

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Date:	Tue, 14 Jun 2016 18:55:44 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Alexander Potapenko <glider@...gle.com>
Cc:	dvyukov@...gle.com, catalin.marinas@....com,
	quentin.casasnovas@...cle.com, will.deacon@....com,
	ard.biesheuvel@...aro.org, marc.zyngier@....com,
	christoffer.dall@...aro.org, kcc@...gle.com,
	akpm@...ux-foundation.org, syzkaller@...glegroups.com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2] arm64: allow building with kcov coverage on ARM64

On Tue, Jun 14, 2016 at 06:57:21PM +0200, Alexander Potapenko wrote:
> Add ARCH_HAS_KCOV to ARM64 config. To avoid crashes, disable
> instrumentation of the following files:
> 
> arch/arm64/boot/*
> arch/arm64/kvm/hyp/*
> 
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> ---
> v2: - disable instrumentation of arch/arm64/{boot,kvm/hyp}
>     - enable instrumentation of arch/arm64/lib/delay.c
> ---
>  arch/arm64/Kconfig          | 1 +
>  arch/arm64/boot/Makefile    | 4 ++++
>  arch/arm64/kvm/hyp/Makefile | 4 ++++
>  3 files changed, 9 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 5a0a691..eb0b0a0 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -7,6 +7,7 @@ config ARM64
>  	select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
>  	select ARCH_HAS_ELF_RANDOMIZE
>  	select ARCH_HAS_GCOV_PROFILE_ALL
> +	select ARCH_HAS_KCOV
>  	select ARCH_HAS_SG_CHAIN
>  	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
>  	select ARCH_USE_CMPXCHG_LOCKREF
> diff --git a/arch/arm64/boot/Makefile b/arch/arm64/boot/Makefile
> index 305c552..74cec89 100644
> --- a/arch/arm64/boot/Makefile
> +++ b/arch/arm64/boot/Makefile
> @@ -14,6 +14,10 @@
>  # Based on the ia64 boot/Makefile.
>  #
>  
> +# Avoid potential boot-time problems with kcov instrumentation. We are mostly
> +# interested in syscall coverage, so boot code is not interesting anyway.
> +KCOV_INSTRUMENT		:= n

We have no code under our boot directory, so I don't think the changes
to arch/arm64/boot are necessary.

> +
>  targets := Image Image.gz
>  
>  $(obj)/Image: vmlinux FORCE
> diff --git a/arch/arm64/kvm/hyp/Makefile b/arch/arm64/kvm/hyp/Makefile
> index 778d0ef..0c85feb 100644
> --- a/arch/arm64/kvm/hyp/Makefile
> +++ b/arch/arm64/kvm/hyp/Makefile
> @@ -17,6 +17,10 @@ obj-$(CONFIG_KVM_ARM_HOST) += tlb.o
>  obj-$(CONFIG_KVM_ARM_HOST) += hyp-entry.o
>  obj-$(CONFIG_KVM_ARM_HOST) += s2-setup.o
>  
> +# KVM code is run at a different exception code with a different map, so
> +# compiler instrumentation that inserts callbacks or checks into the code may
> +# cause crashes. Just disable it.
>  GCOV_PROFILE	:= n
>  KASAN_SANITIZE	:= n
>  UBSAN_SANITIZE	:= n
> +KCOV_INSTRUMENT	:= n

This looks sane to me.

With VHE this code _may_ run in the same memory map as the kernel, but
it's not something we can determine at compile time.

Otherwise, I believe that the rest of the C code under arch/arm64 runs
in the usual kernel memory map (including the special case of kaslr.c),
and the EFI stub code has already been covered, so I'm not immediately
aware of anything else that needs to be special-cased.

I built and booted (via EFI) a kernel with this feature enabled (also
with the boot/Makefile change removed). I haven't tested the feature
itself as such, as I'm not sure how to do that.

FWIW, with the boot/Makefile change removed, feel free to add:

Acked-by: Mark Rutland <mark.rutland@....com>

Thanks,
Mark.