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

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Date:	Mon, 01 Dec 2014 18:37:31 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Steve Capper <steve.capper@...aro.org>
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: Re: [PATCH v3 0/5] ARM64: Add kernel probes(Kprobes) support

(2014/11/29 1:01), Steve Capper wrote:
> 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).

Agreed. On x86, prev_kprobe has old_flags and saved_flags, this
at least must have saved_irqflag and save/restore it in
save/restore_previous_kprobe().

What about adding this?

 struct prev_kprobe {
 	struct kprobe *kp;
 	unsigned int status;
+	unsigned long saved_irqflag;
 };

and

 static void __kprobes save_previous_kprobe(struct kprobe_ctlblk *kcb)
 {
 	kcb->prev_kprobe.kp = kprobe_running();
 	kcb->prev_kprobe.status = kcb->kprobe_status;
+	kcb->prev_kprobe.saved_irqflag = kcb->saved_irqflag;
 }

 static void __kprobes restore_previous_kprobe(struct kprobe_ctlblk *kcb)
 {
 	__this_cpu_write(current_kprobe, kcb->prev_kprobe.kp);
 	kcb->kprobe_status = kcb->prev_kprobe.status;
+	kcb->saved_irqflag = kcb->prev_kprobe.saved_irqflag;
 }



> 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.

Ah, I see.

Thank you,

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


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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