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

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Message-ID: <20130906124935.3b4aa870@gandalf.local.home>
Date:	Fri, 6 Sep 2013 12:49:35 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Seiji Aguchi <seiji.aguchi@....com>
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@....com
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).

-- Steve


> +
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