[PATCH] perf/core: Fix overflow on perf_callchain_entry

[Namhyung Kim]

The commit 97c79a38cd45 add a check whether per-event max stack is
greater than the global max.  But it missed to do it for the first
event.  So if the event had a stack depth greater than the global max,
it could overflow the callchain entry list.

Reported-by: Fengguang Wu <fengguang.wu@xxxxxxxxx>
Fixes: 97c79a38cd45 ("perf core: Per event callchain limit")
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 kernel/events/callchain.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 1b2be63c8528..e449e23802eb 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -119,19 +119,22 @@ int get_callchain_buffers(int event_max_stack)
 		goto exit;
 	}
 
+	/*
+	 * If requesting per event more than the global cap,
+	 * return a different error to help userspace figure this out.
+	 *
+	 * And also do it here so that we have &callchain_mutex held.
+	 */
+	if (event_max_stack > sysctl_perf_event_max_stack) {
+		err = -EOVERFLOW;
+		goto exit;
+	}
+
 	if (count > 1) {
 		/* If the allocation failed, give up */
 		if (!callchain_cpus_entries)
 			err = -ENOMEM;
-		/*
-		 * If requesting per event more than the global cap,
-		 * return a different error to help userspace figure
-		 * this out.
-		 *
-		 * And also do it here so that we have &callchain_mutex held.
-		 */
-		if (event_max_stack > sysctl_perf_event_max_stack)
-			err = -EOVERFLOW;
+
 		goto exit;
 	}
 
-- 
2.15.0