[PATCH v3 1/4] perf/core: check return value of perf_event_read IPI

[David Carrillo-Cisneros <davidcc@google.com>]

The call to smp_call_function_single in perf_event_read() may fail if
an invalid or not online CPU index is passed. Warn user if such bug is
present and return error.

Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
---
 kernel/events/core.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 803481c..35afbfa 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3524,9 +3524,11 @@ static int perf_event_read(struct perf_event *event, bool group)
 			.group = group,
 			.ret = 0,
 		};
-		smp_call_function_single(event->oncpu,
-					 __perf_event_read, &data, 1);
-		ret = data.ret;
+		ret = smp_call_function_single(event->oncpu,
+					       __perf_event_read, &data, 1);
+		/* event must have been read from an online CPU. */
+		WARN_ON_ONCE(ret);
+		ret = ret ? : data.ret;
 	} else if (event->state == PERF_EVENT_STATE_INACTIVE) {
 		struct perf_event_context *ctx = event->ctx;
 		unsigned long flags;
-- 
2.8.0.rc3.226.g39d4020