[PATCH] perf: arm_spe: Make wakeup range check overflow safe

[Leo Yan <leo.yan@arm.com>]

The current code checks whether the wakeup point is in the current
writable range by comparing it with handle->head + handle->size.

The perf AUX head is a monotonically increasing index, so that addition
can overflow when head is close to ULONG_MAX. In that case, a wakeup
point which is still inside the free space range can be missed.

Use unsigned subtraction to compare the distance from head to wakeup
against the handle->size. This can dismiss the issue when addition
overflow.

This is unlikely to happen in practice, but the change makes the
watermark check logically correct.

Fixes: d5d9696b0380 ("drivers/perf: Add support for ARMv8.2 Statistical Profiling Extension")
Signed-off-by: Leo Yan <leo.yan@arm.com>
---
 drivers/perf/arm_spe_pmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
index dbd0da111639..b64cf2313a20 100644
--- a/drivers/perf/arm_spe_pmu.c
+++ b/drivers/perf/arm_spe_pmu.c
@@ -577,7 +577,7 @@ static u64 __arm_spe_pmu_next_off(struct perf_output_handle *handle)
 	 * the page boundary following it. Keep the tail boundary if
 	 * that's lower.
 	 */
-	if (handle->wakeup < (handle->head + handle->size) && head <= wakeup)
+	if ((handle->wakeup - handle->head) < handle->size && head <= wakeup)
 		limit = min(limit, round_up(wakeup, PAGE_SIZE));
 
 	if (limit > head)
-- 
2.34.1