linux-kernel - Re: [PATCH] x86 trace: Fix page fault tracing bug

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Message-ID: <20140228154708.GI9987@twins.programming.kicks-ass.net>
Date:	Fri, 28 Feb 2014 16:47:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	Seiji Aguchi <seiji.aguchi@....com>
Subject: Re: [PATCH] x86 trace: Fix page fault tracing bug

On Fri, Feb 28, 2014 at 04:33:40PM +0100, Jiri Olsa wrote:

While I like the idea of just pushing up the CR2 read; the below does
the read too late still, exception_enter() also has a tracepoint in.

> @@ -1267,9 +1269,18 @@ dotraplinkage void __kprobes
>  trace_do_page_fault(struct pt_regs *regs, unsigned long error_code)
>  {
>  	enum ctx_state prev_state;
> +	unsigned long address;
>  
>  	prev_state = exception_enter();
> +
> +	/*
> +	 * The tracepoint processing could trigger another page
> +	 * fault (user space callchain reading) and destroy the
> +	 * original cr2 value, so read the faulting address now.
> +	 */
> +	address = read_cr2();
> +
>  	trace_page_fault_entries(regs, error_code);
> -	__do_page_fault(regs, error_code);
> +	__do_page_fault(regs, error_code, address);
>  	exception_exit(prev_state);
>  }
> -- 
> 1.7.11.7
> 
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