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