[PATCH] perf: correctly handle failed perf_get_aux_event()

[Mark Rutland]

Vince reports a worrying issue:

| so I was tracking down some odd behavior in the perf_fuzzer which turns
| out to be because perf_even_open() sometimes returns 0 (indicating a file
| descriptor of 0) even though as far as I can tell stdin is still open.

... and further the cause:

| error is triggered if aux_sample_size has non-zero value.
|
| seems to be this line in kernel/events/core.c:
|
| if (perf_need_aux_event(event) && !perf_get_aux_event(event, group_leader))
|                goto err_locked;
|
| (note, err is never set)

This seems to be a thinko in commit:

  ab43762ef010967e ("perf: Allow normal events to output AUX data")

... and we should probably return -EINVAL here, as this should only
happen when the new event is mis-configured or does not have a
compatible aux_event group leader.

Fixes: ab43762ef010967e ("perf: Allow normal events to output AUX data")
Reported-by: Vince Weaver <vincent.weaver@xxxxxxxxx>
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
---
 kernel/events/core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index a1f8bde19b56..2173c23c25b4 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -11465,8 +11465,10 @@ SYSCALL_DEFINE5(perf_event_open,
 		}
 	}
 
-	if (perf_need_aux_event(event) && !perf_get_aux_event(event, group_leader))
+	if (perf_need_aux_event(event) && !perf_get_aux_event(event, group_leader)) {
+		err = -EINVAL;
 		goto err_locked;
+	}
 
 	/*
 	 * Must be under the same ctx::mutex as perf_install_in_context(),
-- 
2.11.0