n_tty: wake up poll(POLLRDNORM) on receiving data
authorTATSUKAWA KOSUKE (立川 江介) <tatsu-ab1@nec.com>
Wed, 26 Jan 2022 23:35:02 +0000 (23:35 +0000)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Mon, 31 Jan 2022 13:31:28 +0000 (14:31 +0100)
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

index 8933ef1f83c0e5a499975accbbb8bf92343c5dc3..5e988e5146531a82c9af01ec94654d4d6828a515 100644 (file)
@@ -1329,7 +1329,7 @@ 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;
                }
        }
@@ -1561,7 +1561,7 @@ 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);
        }
 }