netdev - Re: [PATCH v3 net-next] bpf: fix bpf_perf_event

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Date:	Fri, 23 Oct 2015 14:03:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>, Wang Nan <wangnan0@...wei.com>,
	He Kuang <hekuang@...wei.com>, Kaixu Xia <xiakaixu@...wei.com>,
	Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 net-next] bpf: fix bpf_perf_event_read() helper

On Thu, Oct 22, 2015 at 05:10:14PM -0700, Alexei Starovoitov wrote:
> +++ b/kernel/trace/bpf_trace.c
> @@ -199,6 +199,11 @@ static u64 bpf_perf_event_read(u64 r1, u64 index, u64 r3, u64 r4, u64 r5)
>  	if (!event)
>  		return -ENOENT;
>  
> +	/* make sure event is local and doesn't have pmu::count */
> +	if (event->oncpu != smp_processor_id() ||
> +	    event->pmu->count)
> +		return -EINVAL;
> +
>  	/*
>  	 * we don't know if the function is run successfully by the
>  	 * return value. It can be judged in other places, such as

I might want to go turn that into a helper function to keep !perf code
from poking around in the event itself, but its ok for now I suppose.

> @@ -207,7 +212,7 @@ static u64 bpf_perf_event_read(u64 r1, u64 index, u64 r3, u64 r4, u64 r5)
>  	return perf_event_read_local(event);
>  }

So the bpf_perf_event_read() returns the count value, does this not also
mean that returning -EINVAL here is also 'wrong'?

I mean, sure an actual count value that high is unlikely, but its still
a broken interface.
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