[PATCH] perf: correctly handle failed perf_get_aux_event() (was: Re: [perf] perf_event_open() sometimes returning 0)

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Jan 02, 2020 at 02:22:47PM -0500, Vince Weaver wrote:
> On Thu, 2 Jan 2020, Vince Weaver wrote:
> 
> > 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.

Yikes.

> 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)

Looks like that was introduced in commit:

  ab43762ef010967e ("perf: Allow normal events to output AUX data")
  
I guess we should return -EINVAL in this case.

Vince, does the below (untested) patch work for you?

Thanks,
Mark.

---->8----
From c79f31b42aaf85f3a924af9218794b3bc8b79892 Mon Sep 17 00:00:00 2001
From: Mark Rutland <mark.rutland@arm.com>
Date: Mon, 6 Jan 2020 11:51:06 +0000
Subject: [PATCH] perf: correctly handle failed perf_get_aux_event()

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@maine.edu>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
---
 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