linux-kernel - Re: Re: [PATCH v3 0/5] ARM64: Add kernel probes(Kprobes) support

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Date:	Fri, 28 Nov 2014 16:01:15 +0000
From:	Steve Capper <steve.capper@...aro.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	David Long <dave.long@...aro.org>,
	"Jon Medhurst (Tixy)" <tixy@...aro.org>,
	Russell King <linux@....linux.org.uk>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	William Cohen <wcohen@...hat.com>,
	David Miller <davem@...emloft.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: Re: [PATCH v3 0/5] ARM64: Add kernel probes(Kprobes) support

On 27 November 2014 at 06:07, Masami Hiramatsu
<masami.hiramatsu.pt@...achi.com> wrote:
> (2014/11/27 3:59), Steve Capper wrote:
>> The crash is extremely easy to reproduce.
>>
>> I've not observed any missed events on a kprobe on an arm64 system
>> that's still alive.
>> My (limited!) understanding is that this suggests there could be a
>> problem with how missed events from a recursive call to memcpy are
>> being handled.
>
> I think so too. BTW, could you bisect that? :)
>

I can't bisect, but the following functions look suspicious to me
(again I'm new to kprobes...):
kprobes_save_local_irqflag
kprobes_restore_local_irqflag

I think these are breaking somehow when nested (i.e. from a recursive probe).

That would explain why the state of play of the interrupts is in an
unexpected state in the crash I reported:
"The point of failure in the panic was:
fs/buffer.c:1257

static inline void check_irqs_on(void)
{
#ifdef irqs_disabled
        BUG_ON(irqs_disabled());
#endif
}
"

This is all new to me so I'm still at the head-scratching stage.

David,
Does the above make sense to you? Have you managed to reproduce the crash I get?

Cheers,
--
Steve
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