eventpoll: abstract out parameter sanity checking

Add a helper that checks the validity of the file descriptor and
other parameters passed in to epoll_wait().

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20250219172552.1565603-2-axboe@kernel.dk
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: axboe

fs/eventpoll.c

@@ -2445,6 +2445,27 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
 	return do_epoll_ctl(epfd, op, fd, &epds, false);
 }
 
+static int ep_check_params(struct file *file, struct epoll_event __user *evs,
+			   int maxevents)
+{
+	/* The maximum number of event must be greater than zero */
+	if (maxevents <= 0 || maxevents > EP_MAX_EVENTS)
+		return -EINVAL;
+
+	/* Verify that the area passed by the user is writeable */
+	if (!access_ok(evs, maxevents * sizeof(struct epoll_event)))
+		return -EFAULT;
+
+	/*
+	 * We have to check that the file structure underneath the fd
+	 * the user passed to us _is_ an eventpoll file.
+	 */
+	if (!is_file_epoll(file))
+		return -EINVAL;
+
+	return 0;
+}
+
 /*
  * Implement the event wait interface for the eventpoll file. It is the kernel
  * part of the user space epoll_wait(2).
@@ -2453,26 +2474,16 @@ static int do_epoll_wait(int epfd, struct epoll_event __user *events,
 			 int maxevents, struct timespec64 *to)
 {
 	struct eventpoll *ep;
-
-	/* The maximum number of event must be greater than zero */
-	if (maxevents <= 0 || maxevents > EP_MAX_EVENTS)
-		return -EINVAL;
-
-	/* Verify that the area passed by the user is writeable */
-	if (!access_ok(events, maxevents * sizeof(struct epoll_event)))
-		return -EFAULT;
+	int ret;
 
 	/* Get the "struct file *" for the eventpoll file */
 	CLASS(fd, f)(epfd);
 	if (fd_empty(f))
 		return -EBADF;
 
-	/*
-	 * We have to check that the file structure underneath the fd
-	 * the user passed to us _is_ an eventpoll file.
-	 */
-	if (!is_file_epoll(fd_file(f)))
-		return -EINVAL;
+	ret = ep_check_params(fd_file(f), events, maxevents);
+	if (unlikely(ret))
+		return ret;
 
 	/*
 	 * At this point it is safe to assume that the "private_data" contains