linux-kernel - [PATCH] n_tty: wake up poll(POLLRDNORM) on receiving data

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Date:   Mon, 24 Jan 2022 08:17:22 +0000
From:   TATSUKAWA KOSUKE(立川　江介) 
        <tatsu-ab1@....com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [PATCH] n_tty: wake up poll(POLLRDNORM) on receiving data

Event POLLRDNORM should be equivalent to POLLIN when used as event in
poll().  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.

Signed-off-by: Kosuke Tatsukawa <tatsu-ab1@....com>
Fixes: commit 57087d515441 ("tty: Fix spurious poll() wakeups")
Cc: stable@...r.kernel.org
---
 drivers/tty/n_tty.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 5be6d02..b2b98fe 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1369,7 +1369,7 @@ static void n_tty_receive_char_special(struct tty_struct *tty, unsigned char c)
 			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;
 		}
 	}
@@ -1589,7 +1589,7 @@ static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,
 
 	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);
 	}
 }