linux-kernel - Re: [PATCH v1] arm64: allow building with kcov coverage on ARM64

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Date:	Wed, 13 Apr 2016 18:35:50 +0200
From:	Alexander Potapenko <glider@...gle.com>
To:	James Morse <james.morse@....com>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	Mark Rutland <mark.rutland@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	Will Deacon <will.deacon@....com>,
	Kostya Serebryany <kcc@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	syzkaller <syzkaller@...glegroups.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	marc.zyngier@....com,
	Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH v1] arm64: allow building with kcov coverage on ARM64

Hi James,

On Wed, Apr 13, 2016 at 6:12 PM, James Morse <james.morse@....com> wrote:
> Hi Alex,
>
> On 12/04/16 12:17, Alexander Potapenko wrote:
>> I also wonder if we can, say, land the change to arch/arm64/Kconfig
>> separately from makefile changes that improve the precision or fix
>> certain build configurations.
>
> (I'm not sure what you mean by precision)
>
> It depends which build configurations get broken, for example the first build I
> tried doesn't boot.
>
> I tested the Kconfig change, and added 'KCOV_INSTRUMENT := n' to kvm's two
> Makefiles[0].
>
> defconfig+KCOV boots fine, and I can start a guest, but if I build with
> defconfig+KCOV+STACK_TRACER, I get a kernel that fails to boot. It boils down to
> a loop between ftrace and kcov, I will send a patch.
>
>
> It looks like syzkaller is the only user of this data, and it doesn't appear to
> support arm64:
>> 2016/04/13 15:42:55 failed to create instance: qemu stopped:
>> "kvm" accelerator not found.
>> No accelerator found!
syzcaller is able to work with bare-metal Android devices using ADB,
but those are usually running ancient kernels.
Looks like It's time for me to start cross-compiling and testing an
arm64 kernel on a QEMU, so I'll try that out and update the patch.

> This happens because syzkaller expects to be able to use 'qemu-system-x86_64':
>> [pid  3670] execve("/usr/bin/qemu-system-x86_64", ["qemu-system-x86_64",
>> "-hda", "/foo.img", "-snapshot", "-m", "1024", "-net", "nic", "-net",
>> "user,host=10.0.2.10,hostfwd=tcp:"..., "-nographic", "-enable-kvm", "-numa",
>> "node,nodeid=0,cpus=0-1", "-numa", "node,nodeid=1,cpus=2-3", ...],
>
> qemu-system-x86_64 is installed and works fine, it just doesn't have the
> expected hardware acceleration...
>
> My approximation of your qemu command for arm64 may be useful[1], but I'm not
> able to hack the go source to fix it!
>
>
>
> Thanks,
>
> James
>
>
>
> [0]
> diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
> index 122cff482ac4..7d111f06bbf3 100644
> --- a/arch/arm64/kvm/Makefile
> +++ b/arch/arm64/kvm/Makefile
> @@ -9,6 +9,10 @@ CFLAGS_mmu.o := -I.
>  KVM=../../../virt/kvm
>  ARM=../../../arch/arm/kvm
>
> +# Code built here may run at EL2, in which case __sanitizer_cov_trace_pc() will
> +# not be callable. For now, disable the instrumentation.
> +KCOV_INSTRUMENT := n
> +
>  obj-$(CONFIG_KVM_ARM_HOST) += kvm.o
>  obj-$(CONFIG_KVM_ARM_HOST) += hyp/
>
> diff --git a/arch/arm64/kvm/hyp/Makefile b/arch/arm64/kvm/hyp/Makefile
> index 778d0effa2af..1150f8664c85 100644
> --- a/arch/arm64/kvm/hyp/Makefile
> +++ b/arch/arm64/kvm/hyp/Makefile
> @@ -20,3 +20,4 @@ obj-$(CONFIG_KVM_ARM_HOST) += s2-setup.o
>  GCOV_PROFILE   := n
>  KASAN_SANITIZE := n
>  UBSAN_SANITIZE := n
> +KCOV_INSTRUMENT := n
>
>
> [1]
> qemu-system-aarch64 -enable-kvm
>   -cpu host -m 1024 -M virt -nographic
>   -kernel ./syzkaller/Image
>   -append "console=ttyAMA0,115200 root=/dev/vda"
>   -drive format=raw,file=/foo.img,media=disk,if=none,cache=writeback,id=root
>   -device virtio-blk-device,drive=root
>   -netdev user,host=10.0.2.10,hostfwd=tcp::23505-:22,id=unet
>   -device virtio-net-device,netdev=unet
>
>



-- 
Alexander Potapenko
Software Engineer

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