powerpc, hw_breakpoint: Enable hw-breakpoints while handling intervening signals

A signal delivered between a hw_breakpoint_handler() and the
single_step_dabr_instruction() will not have the breakpoint active
while the signal handler is running -- the signal delivery will
set up a new MSR value which will not have MSR_SE set, so we
won't get the signal step interrupt until and unless the signal
handler returns (which it may never do).

To fix this, we restore the breakpoint when delivering a signal --
we clear the MSR_SE bit and set the DABR again.  If the signal
handler returns, the DABR interrupt will occur again when the
instruction that we were originally trying to single-step gets
re-executed.

[Paul Mackerras <paulus@samba.org> pointed out the need to do this.]

Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

2 files changed, 21 insertions, 0 deletions

diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
index 7a2ad5e..7bd01a5 100644
--- a/arch/powerpc/kernel/hw_breakpoint.c
+++ b/arch/powerpc/kernel/hw_breakpoint.c
@@ -175,6 +175,24 @@ int arch_validate_hwbkpt_settings(struct perf_event *bp)
 }
 
 /*
+ * Restores the breakpoint on the debug registers.
+ * Invoke this function if it is known that the execution context is
+ * about to change to cause loss of MSR_SE settings.
+ */
+void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs)
+{
+	struct arch_hw_breakpoint *info;
+
+	if (likely(!tsk->thread.last_hit_ubp))
+		return;
+
+	info = counter_arch_bp(tsk->thread.last_hit_ubp);
+	regs->msr &= ~MSR_SE;
+	set_dabr(info->address | info->type | DABR_TRANSLATION);
+	tsk->thread.last_hit_ubp = NULL;
+}
+
+/*
  * Handle debug exception notifications.
  */
 int __kprobes hw_breakpoint_handler(struct die_args *args)
diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
index a0afb55..7109f5b 100644
--- a/arch/powerpc/kernel/signal.c
+++ b/arch/powerpc/kernel/signal.c
@@ -11,6 +11,7 @@
 
 #include <linux/tracehook.h>
 #include <linux/signal.h>
+#include <asm/hw_breakpoint.h>
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
 
@@ -149,6 +150,8 @@ static int do_signal_pending(sigset_t *oldset, struct pt_regs *regs)
 	if (current->thread.dabr)
 		set_dabr(current->thread.dabr);
 #endif
+	/* Re-enable the breakpoints for the signal stack */
+	thread_change_pc(current, regs);
 
 	if (is32) {
         	if (ka.sa.sa_flags & SA_SIGINFO)