From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1757273AbbJVPxD (ORCPT <rfc822;w@1wt.eu>);
	Thu, 22 Oct 2015 11:53:03 -0400
Received: from mail-pa0-f52.google.com ([209.85.220.52]:36641 "EHLO
	mail-pa0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752234AbbJVPxA (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 22 Oct 2015 11:53:00 -0400
Subject: Re: [PATCH v2 net-next] bpf: fix bpf_perf_event_read() helper
To: Peter Zijlstra <peterz@infradead.org>
References: <1445468283-4592-1-git-send-email-ast@kernel.org>
 <20151022135745.GM3604@twins.programming.kicks-ass.net>
Cc: "David S. Miller" <davem@davemloft.net>, Ingo Molnar <mingo@kernel.org>,
        Wang Nan <wangnan0@huawei.com>, He Kuang <hekuang@huawei.com>,
        Kaixu Xia <xiakaixu@huawei.com>,
        Daniel Borkmann <daniel@iogearbox.net>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
From: Alexei Starovoitov <ast@plumgrid.com>
Message-ID: <5629065C.6020103@plumgrid.com>
Date: Thu, 22 Oct 2015 08:53:00 -0700
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:38.0)
 Gecko/20100101 Thunderbird/38.3.0
MIME-Version: 1.0
In-Reply-To: <20151022135745.GM3604@twins.programming.kicks-ass.net>
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 10/22/15 6:57 AM, Peter Zijlstra wrote:
> 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.

agree. Will keep perf_event_read_local() as-is and do all safety
checks on bpf side.