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

[Vince Weaver]

On Mon, 6 Jan 2020, Mark Rutland wrote:

> On Thu, Jan 02, 2020 at 02:22:47PM -0500, Vince Weaver wrote:
> > On Thu, 2 Jan 2020, Vince Weaver wrote:
> > 
> Vince, does the below (untested) patch work for you?


yes, this patch fixes things for me.

Tested-by: Vince Weaver <vincent.weaver@xxxxxxxxx>



> ---->8----
> From c79f31b42aaf85f3a924af9218794b3bc8b79892 Mon Sep 17 00:00:00 2001
> From: Mark Rutland <mark.rutland@xxxxxxx>
> 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@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
> 
>