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

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Message-ID: <20130310183140.GB1724@krava.brq.redhat.com>
Date:	Sun, 10 Mar 2013 19:31:40 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.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 Sun, Mar 10, 2013 at 06:15:18PM +0100, Jiri Olsa wrote:
> On Sun, Mar 03, 2013 at 11:43:21AM +0100, Jiri Olsa wrote:
> > On Fri, Mar 01, 2013 at 10:57:50AM -0800, H. Peter Anvin wrote:
> > > 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?
> 
> Is there any other reason for this besides having just signal

s/signal/single/ ;-)

jirka
> instruction doing this update?
> 
> It looks like gcc is smart enough to do that anyway:
> 
> ...
> andq   $0xfffffffffffffaff,(%rdi)
> ...
> 
> 
> I need to send V2 anyway, since the patchset no longer
> applies into latest tip.
> 
> thanks,
> jirka
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