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

Peter Zijlstra
2015-07-17T11:41:01Z

On Fri, Jul 17, 2015 at 07:34:22PM +0800, Wangnan (F) wrote:
> 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?

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

You pretty much _have_ to do that.