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

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Message-ID: <20130310171518.GA1724@krava.brq.redhat.com>
Date:	Sun, 10 Mar 2013 18:15:18 +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 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
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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