x86/signals: Clear RF EFLAGS bit for signal handler

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.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Tested-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
Originally-Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: Stephane Eranian <eranian@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1367421944-19082-3-git-send-email-jolsa@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/signal.c

@@ -664,6 +664,12 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 		 * Clear the direction flag as per the ABI for function entry.
 		 */
 		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.