[PATCH] perf: Correctly handle failed perf_get_aux_event()

[Ingo Molnar]

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Tested-by: Vince Weaver <vincent.weaver@xxxxxxxxx>
(cherry picked from commit da9ec3d3dd0f1240a48920be063448a2242dbd90)
---
 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 00a014670ed0..291fe3e2165f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -11184,8 +11184,10 @@ SYSCALL_DEFINE5(perf_event_open,
 		}
 	}
 
-	if (event->attr.aux_output && !perf_get_aux_event(event, group_leader))
+	if (event->attr.aux_output && !perf_get_aux_event(event, group_leader)) {
+		err = -EINVAL;
 		goto err_locked;
+	}
 
 	/*
 	 * Must be under the same ctx::mutex as perf_install_in_context(),