netdev - Re: [PATCH v2 net-next] bpf: fix bpf_perf_event

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Message-ID: <20151022135745.GM3604@twins.programming.kicks-ass.net>
Date:	Thu, 22 Oct 2015 15:57:45 +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 v2 net-next] bpf: fix bpf_perf_event_read() helper

On Wed, Oct 21, 2015 at 03:58:03PM -0700, Alexei Starovoitov wrote:
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 64754bfecd70..0b6333265872 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3258,7 +3258,7 @@ static inline u64 perf_event_count(struct perf_event *event)
>  u64 perf_event_read_local(struct perf_event *event)
>  {
>  	unsigned long flags;
> -	u64 val;
> +	u64 val = -EINVAL;

No, you cannot do this, -EINVAL is a valid count value. You simply must
not call this function on !local events, ever.
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