The poll man page says POLLRDNORM is equivalent to POLLIN when used as
an event.
$ man poll
<snip>
              POLLRDNORM
                     Equivalent to POLLIN.

However, in n_tty driver, POLLRDNORM does not return until timeout even
if there is terminal input, whereas POLLIN returns.

The following test program works until kernel-3.17, but the test stops
in poll() after commit 57087d51 ("tty: Fix spurious poll() wakeups").

[Steps to run test program]
  $ cc -o test-pollrdnorm test-pollrdnorm.c
  $ ./test-pollrdnorm
  foo          <-- Type in something from the terminal followed by [RET].
                   The string should be echoed back.

  ------------------------< test-pollrdnorm.c >------------------------
  #include <stdio.h>
  #include <errno.h>
  #include <poll.h>
  #include <unistd.h>

  void main(void)
  {
	int		n;
	unsigned char	buf[8];
	struct pollfd	fds[1] = {{ 0, POLLRDNORM, 0 }};

	n = poll(fds, 1, -1);
	if (n < 0)
		perror("poll");
	n = read(0, buf, 8);
	if (n < 0)
		perror("read");
	if (n > 0)
		write(1, buf, n);
  }
  ------------------------------------------------------------------------

The attached patch fixes this problem.  Many calls to
wake_up_interruptible_poll() in the kernel source code already specify
"POLLIN | POLLRDNORM".

Fixes: 57087d51 ("tty: Fix spurious poll() wakeups")
Cc: stable@vger.kernel.org
Signed-off-by: Kosuke Tatsukawa <tatsu-ab1@nec.com>
Link: https://lore.kernel.org/r/TYCPR01MB81901C0F932203D30E452B3EA5209@TYCPR01MB8190.jpnprd01.prod.outlook.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>