[PATCH 08/11] ARM: hw_breakpoint: disallow per-cpu breakpoints without overflow handler

[will.deacon@arm.com (Will Deacon)]

Single-stepping a breakpoint requires us to disable it temporarily so that
we don't get stuck in a recursive debug trap. With per-cpu breakpoints this
presents a problem where an interrupt can be taken before the single-step has
completed and a new task is eventually scheduled. This new task will not
hit the breakpoint because it will have been disabled during the previous
handling code.

This patch disallows per-cpu breakpoints on ARM when an overflow handler
is not present. A similar effect can be created by placing breakpoints on
a shell and then running applications there.

Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 arch/arm/kernel/hw_breakpoint.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
index 36cd768..eef1b1e 100644
--- a/arch/arm/kernel/hw_breakpoint.c
+++ b/arch/arm/kernel/hw_breakpoint.c
@@ -622,10 +622,12 @@ int arch_validate_hwbkpt_settings(struct perf_event *bp)
 	 * Currently we rely on an overflow handler to take
 	 * care of single-stepping the breakpoint when it fires.
 	 * In the case of userspace breakpoints on a core with V7 debug,
-	 * we can use the mismatch feature as a poor-man's hardware single-step.
+	 * we can use the mismatch feature as a poor-man's hardware
+	 * single-step, but this only works for per-task breakpoints.
 	 */
 	if (WARN_ONCE(!bp->overflow_handler &&
-		(arch_check_bp_in_kernelspace(bp) || !core_has_mismatch_brps()),
+		(arch_check_bp_in_kernelspace(bp) || !core_has_mismatch_brps()
+		 || !bp->hw.bp_target),
 			"overflow handler required but none found")) {
 		ret = -EINVAL;
 	}
-- 
1.7.0.4