linux-kernel - RE: [PATCH v2] Introduce page fault tracepoint

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Date:	Fri, 6 Sep 2013 16:53:03 +0000
From:	Seiji Aguchi <seiji.aguchi@....com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
	"mingo@...e.hu" <mingo@...e.hu>, "bp@...en8.de" <bp@...en8.de>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"fdeslaur@...il.com" <fdeslaur@...il.com>,
	"raphael.beamonte@...il.com" <raphael.beamonte@...il.com>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Tomoki Sekiyama <tomoki.sekiyama@....com>
Subject: RE: [PATCH v2] Introduce page fault tracepoint



> -----Original Message-----
> From: Steven Rostedt [mailto:rostedt@...dmis.org]
> Sent: Friday, September 06, 2013 12:50 PM
> To: Seiji Aguchi
> Cc: linux-kernel@...r.kernel.org; x86@...nel.org; hpa@...or.com; mingo@...e.hu; bp@...en8.de; tglx@...utronix.de;
> fdeslaur@...il.com; raphael.beamonte@...il.com; dle-develop@...ts.sourceforge.net; Tomoki Sekiyama
> Subject: Re: [PATCH v2] Introduce page fault tracepoint
> 
> On Fri, 23 Aug 2013 11:37:43 -0400
> Seiji Aguchi <seiji.aguchi@....com> wrote:
> 
> 
> > +#if !defined(_TRACE_PAGE_FAULT_H) || defined(TRACE_HEADER_MULTI_READ)
> > +#define _TRACE_PAGE_FAULT_H
> > +
> > +#include <linux/tracepoint.h>
> > +
> > +extern void trace_irq_vector_regfunc(void);
> > +extern void trace_irq_vector_unregfunc(void);
> > +
> > +DECLARE_EVENT_CLASS(x86_exceptions,
> > +
> > +	TP_PROTO(unsigned long address, struct pt_regs *regs,
> > +		 unsigned long error_code),
> > +
> > +	TP_ARGS(address, regs, error_code),
> > +
> > +	TP_STRUCT__entry(
> > +		__field(		unsigned long, address	)
> > +		__field(		struct pt_regs *, regs	)
> > +		__field(		unsigned long, error_code )
> > +	),
> > +
> > +	TP_fast_assign(
> > +		__entry->address = address;
> > +		__entry->regs = regs;
> > +		__entry->error_code = error_code;
> > +	),
> > +
> > +	TP_printk("address=0x%lx ip=0x%lx error_code=0x%lx",
> > +		  __entry->address, __entry->regs->ip, __entry->error_code) );
> 
> This is sure to crash the kernel.
> 
> You just saved the address of a pointer to some task's stack in the
> ring buffer. And then on output (which can happen a long time from when
> it was recorded), you are dereferencing that same address!
> 
> That __entry->regs->ip *will* crash the kernel!
> 
> What you want is to save ip in the fast_assign:
> 
> 	__entry->ip = regs->ip
> 
> And then print that. Never dereference a pointer directly from the ring
> buffer unless it's a constant value (like a global string).

I see..
Thank you for reviewing.

I will fix it.

Seiji
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