[PATCH 1/2] arm64: hw_breakpoint: Allow stepping if a kernel mode overflow handler exists

[Pratyush Anand]

Currently we allow to single step only for the perf user. However, we
have a kernel sample test (samples/hw_breakpoint/data_breakpoint.c)
which implements its own overflow handler. Therefore, additionally
allow single stepping if there exists a overflow handler in kernel mode.

We still have issues with test, which causes kernel to go into an
infinite loop of overflow_handler being called, and that reveals a
corner case bug with perf breakpoint implementation as well. See
the next patch, which talks more about it and attempts to resolve it.

Signed-off-by: Pratyush Anand <panand at redhat.com>
---
 arch/arm64/kernel/hw_breakpoint.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
index 749f81779420..46dbbf94f72d 100644
--- a/arch/arm64/kernel/hw_breakpoint.c
+++ b/arch/arm64/kernel/hw_breakpoint.c
@@ -661,7 +661,8 @@ static int breakpoint_handler(unsigned long unused, unsigned int esr,
 		perf_bp_event(bp, regs);
 
 		/* Do we need to handle the stepping? */
-		if (is_default_overflow_handler(bp))
+		if (is_default_overflow_handler(bp) ||
+				(!user_mode(regs) && bp->overflow_handler))
 			step = 1;
 unlock:
 		rcu_read_unlock();
@@ -789,7 +790,8 @@ static int watchpoint_handler(unsigned long addr, unsigned int esr,
 		perf_bp_event(wp, regs);
 
 		/* Do we need to handle the stepping? */
-		if (is_default_overflow_handler(wp))
+		if (is_default_overflow_handler(wp) ||
+				(!user_mode(regs) && wp->overflow_handler))
 			step = 1;
 	}
 	if (min_dist > 0 && min_dist != -1) {
@@ -800,7 +802,8 @@ static int watchpoint_handler(unsigned long addr, unsigned int esr,
 		perf_bp_event(wp, regs);
 
 		/* Do we need to handle the stepping? */
-		if (is_default_overflow_handler(wp))
+		if (is_default_overflow_handler(wp) ||
+				(!user_mode(regs) && wp->overflow_handler))
 			step = 1;
 	}
 	rcu_read_unlock();
-- 
2.9.3