Re: [RFC PATCH 3/6] bpf: Save the pointer to struct perf_event to map

["Wangnan (F)" <wangnan0@huawei.com>]

On 2015/7/17 19:06, Peter Zijlstra wrote:
> On Fri, Jul 17, 2015 at 06:43:33PM +0800, kaixu xia wrote:
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index e965cfa..c4e34b7 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -8582,6 +8582,28 @@ void perf_event_delayed_put(struct task_struct *task)
>>   		WARN_ON_ONCE(task->perf_event_ctxp[ctxn]);
>>   }
>>   
>> +struct perf_event *perf_event_get(unsigned int fd)
>> +{
>> +	struct perf_event *event;
>> +	struct fd f;
>> +
>> +	f = fdget(fd);
>> +
>> +	if (!f.file)
>> +		return ERR_PTR(-EBADF);
>> +
>> +	if (f.file->f_op != &perf_fops) {
>> +		fdput(f);
>> +		return ERR_PTR(-EINVAL);
>> +	}
>> +
>> +	event = f.file->private_data;
>> +
>> +	fdput(f);
>> +
>> +	return event;
>> +}
> And what is stopping userspace from closing those FDs while you're using
> them?

Please check replace_map_with_perf_event(). Users can close the FDs, but 
the perf
event structure will still valid because we increase its reference 
count. It won't be
close until the map is released. We have test that case.

Thank you.