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From:   Alexander Shishkin <alexander.shishkin@linux.intel.com>
To:     Mark Rutland <mark.rutland@arm.com>,
        Vince Weaver <vincent.weaver@maine.edu>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        alexander.shishkin@linux.intel.com
Subject: Re: [PATCH] perf: correctly handle failed perf_get_aux_event() (was: Re: [perf] perf_event_open() sometimes returning 0)
In-Reply-To: <20200106120338.GC9630@lakrids.cambridge.arm.com>
References: <alpine.DEB.2.21.2001021349390.11372@macbook-air> <alpine.DEB.2.21.2001021418590.11372@macbook-air> <20200106120338.GC9630@lakrids.cambridge.arm.com>
Date:   Tue, 07 Jan 2020 09:39:51 +0200
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Mark Rutland <mark.rutland@arm.com> writes:

> 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.

That's right. Thanks for looking into this!

> 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;
> +	}
>  

FWIW,
Acked-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>

Thanks,
--
Alex