linux-kernel - Re: [PATCH v15 00/10] arm64: Add kernel probes (kprobes) support

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Message-ID: <20160715075001.GA13360@e104818-lin.cambridge.arm.com>
Date:	Fri, 15 Jul 2016 08:50:02 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	William Cohen <wcohen@...hat.com>
Cc:	David Long <dave.long@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Petr Mladek <pmladek@...e.com>,
	Zi Shen Lim <zlim.lnx@...il.com>,
	Will Deacon <will.deacon@....com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	yalin wang <yalin.wang2010@...il.com>,
	Li Bin <huawei.libin@...wei.com>,
	John Blackwood <john.blackwood@...r.com>,
	Pratyush Anand <panand@...hat.com>,
	Daniel Thompson <daniel.thompson@...aro.org>,
	Huang Shijie <shijie.huang@....com>,
	Dave P Martin <Dave.Martin@....com>,
	Jisheng Zhang <jszhang@...vell.com>,
	Vladimir Murzin <Vladimir.Murzin@....com>,
	Steve Capper <steve.capper@...aro.org>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Marc Zyngier <marc.zyngier@....com>,
	Yang Shi <yang.shi@...aro.org>,
	Mark Brown <broonie@...nel.org>,
	Sandeepa Prabhu <sandeepa.s.prabhu@...il.com>,
	Alex Bennée <alex.bennee@...aro.org>,
	Adam Buchbinder <adam.buchbinder@...il.com>,
	linux-arm-kernel@...ts.infradead.org,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Robin Murphy <robin.murphy@....com>,
	Jens Wiklander <jens.wiklander@...aro.org>,
	Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH v15 00/10] arm64: Add kernel probes (kprobes) support

On Thu, Jul 14, 2016 at 01:09:08PM -0400, William Cohen wrote:
> On 07/14/2016 12:22 PM, Catalin Marinas wrote:
> > On Fri, Jul 08, 2016 at 12:35:44PM -0400, David Long wrote:
> >> David A. Long (3):
> >>   arm64: Add HAVE_REGS_AND_STACK_ACCESS_API feature
> >>   arm64: Add more test functions to insn.c
> >>   arm64: add conditional instruction simulation support
> >>
> >> Pratyush Anand (2):
> >>   arm64: Blacklist non-kprobe-able symbol
> >>   arm64: Treat all entry code as non-kprobe-able
> >>
> >> Sandeepa Prabhu (4):
> >>   arm64: Kprobes with single stepping support
> >>   arm64: kprobes instruction simulation support
> >>   arm64: Add kernel return probes support (kretprobes)
> >>   kprobes: Add arm64 case in kprobe example module
> >>
> >> William Cohen (1):
> >>   arm64: Add trampoline code for kretprobes
> > 
> > I applied these patches on top of the arm64 for-next/core branch an
> > tried to run the resulting kernel in a guest (on a Juno platform using
> > both kvmtool and qemu) with KPROBES_SANITY_TEST enabled. Unfortunately,
> > the kernel fails to boot with lots of "Unexpected kernel single-step
> > exception at EL1".
> > 
> > Did you manage to run Kprobes in a guest before?
> 
> I ran the systemtap testsuite several times on a physical machine
> running a kernel with the kprobe v15 patches without problem.
> Shouldn't the guest machine behave in the same manner as a host
> machine for single stepping and exception handling?  If the guest
> machine is failing, wouldn't that suggest there is a problem with the
> KVM handling of single stepping for guests?

It didn't fail for me on the host either. What's strange is that on some
occasions even the guest managed to get to a prompt. I'll do more tests
today on different CPU configurations, just to rule out potential
hardware issues. If not hardware related, it's possible that the
interaction with KVM doesn't work as expected, maybe the
saving/restoring of the guest debug state loses information.

-- 
Catalin