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

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

On 2015/7/17 19:21, Wangnan (F) wrote:
>
>
> 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.
>

Shall we put atomic_long_inc_not_zero() between fdget() and fdput()?

Thank you.