Subject: Re: [PATCH v2 net-next] bpf: fix bpf_perf_event_read() helper

From: Alexei Starovoitov <[email-masked]>
Date: Thu, 22 Oct 2015 08:53:00 -0700

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.