epoll: remember the event if epoll_wait returns -EFAULT

If epoll_wait returns -EFAULT, the event that was being returned when the
fault was encountered will be forgotten.  This is not a big deal since
EFAULT will happen only if a buggy userspace program passes in a bad
address, in which case what happens later usually doesn't matter.
However, it is easy to remember the event for later, and this patch makes
a simple change to do that.
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Acked-by: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -1054,8 +1054,10 @@ static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head,
 		 */
 		if (revents) {
 			if (__put_user(revents, &uevent->events) ||
-			    __put_user(epi->event.data, &uevent->data))
+			    __put_user(epi->event.data, &uevent->data)) {
+				list_add(&epi->rdllink, head);
 				return eventcnt ? eventcnt : -EFAULT;
+			}
 			eventcnt++;
 			uevent++;
 			if (epi->event.events & EPOLLONESHOT)