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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Kosuke Tatsukawa <tatsu-ab1@nec.com>
Subject: [PATCH 5.10 089/116] n_tty: wake up poll(POLLRDNORM) on receiving data
Date:   Mon, 14 Feb 2022 10:26:28 +0100
Message-Id: <20220214092501.842412778@linuxfoundation.org>
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From: TATSUKAWA KOSUKE (立川 江介) <tatsu-ab1@nec.com>

commit c816b2e65b0e86b95011418cad334f0524fc33b8 upstream.

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 57087d515441 ("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: 57087d515441 ("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.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/n_tty.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1372,7 +1372,7 @@ handle_newline:
 			put_tty_queue(c, ldata);
 			smp_store_release(&ldata->canon_head, ldata->read_head);
 			kill_fasync(&tty->fasync, SIGIO, POLL_IN);
-			wake_up_interruptible_poll(&tty->read_wait, EPOLLIN);
+			wake_up_interruptible_poll(&tty->read_wait, EPOLLIN | EPOLLRDNORM);
 			return 0;
 		}
 	}
@@ -1653,7 +1653,7 @@ static void __receive_buf(struct tty_str
 
 	if (read_cnt(ldata)) {
 		kill_fasync(&tty->fasync, SIGIO, POLL_IN);
-		wake_up_interruptible_poll(&tty->read_wait, EPOLLIN);
+		wake_up_interruptible_poll(&tty->read_wait, EPOLLIN | EPOLLRDNORM);
 	}
 }