linux-kernel - Re: [PATCH 2/5] signal x86: Clear RF EFLAGS bit for signal handler

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Date:	Fri, 01 Mar 2013 10:57:50 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jiri Olsa <jolsa@...hat.com>
CC:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Vince Weaver <vincent.weaver@...ne.edu>
Subject: Re: [PATCH 2/5] signal x86: Clear RF EFLAGS bit for signal handler

On 03/01/2013 10:11 AM, Jiri Olsa wrote:
> Clearing RF EFLAGS bit for signal handler. The reason is,
> that this flag is set by debug exception code to prevent
> the recursive exception entry.
> 
> Leaving it set for signal handler might prevent debug
> exception of the signal handler itself.
> 
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index b6fe116..e273571 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -726,6 +726,13 @@ handle_signal(unsigned long sig, siginfo_t *info, struct k_sigaction *ka,
>  	regs->flags &= ~X86_EFLAGS_DF;
>  
>  	/*
> +	 * Clear RF when entering the signal handler, because
> +	 * it might disable possible debug exception from the
> +	 * signal handler.
> +	 */
> +	regs->flags &= ~X86_EFLAGS_RF;
> +
> +	/*
>  	 * Clear TF when entering the signal handler, but
>  	 * notify any tracer that was single-stepping it.
>  	 * The tracer may want to single-step inside the
> 

Makes sense.  However, can you combine all the flags-clearing into one
statement while you are at it?

	-hpa

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