eventpoll: add epoll_sendevents() helper

Basic helper that copies ready events to the specified userspace
address. The event checking is quick and racy, it's up to the caller
to ensure it retries appropriately in case 0 events are copied.

Signed-off-by: Jens Axboe <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

fs/eventpoll.c

···
 	return 0;
 }
 
+int epoll_sendevents(struct file *file, struct epoll_event __user *events,
+		     int maxevents)
+{
+	struct eventpoll *ep;
+	int ret;
+
+	ret = ep_check_params(file, events, maxevents);
+	if (unlikely(ret))
+		return ret;
+
+	ep = file->private_data;
+	/*
+	 * Racy call, but that's ok - it should get retried based on
+	 * poll readiness anyway.
+	 */
+	if (ep_events_available(ep))
+		return ep_try_send_events(ep, events, maxevents);
+	return 0;
+}
+
 /*
  * Implement the event wait interface for the eventpoll file. It is the kernel
  * part of the user space epoll_wait(2).

include/linux/eventpoll.h

···
 /* Used to release the epoll bits inside the "struct file" */
 void eventpoll_release_file(struct file *file);
 
+/* Copy ready events to userspace */
+int epoll_sendevents(struct file *file, struct epoll_event __user *events,
+		     int maxevents);
+
 /*
  * This is called from inside fs/file_table.c:__fput() to unlink files
  * from the eventpoll interface. We need to have this facility to cleanup