tty: Fix pty master poll() after slave closes v2

Commit f95499c3030f ("n_tty: Don't wait for buffer work in read() loop")
introduces a race window where a pty master can be signalled that the pty
slave was closed before all the data that the slave wrote is delivered.
Commit f8747d4a466a ("tty: Fix pty master read() after slave closes") fixed the
problem in case of n_tty_read, but the problem still exists for n_tty_poll.
This can be seen by running 'for ((i=0; i<100;i++));do ./test.py ;done'
where test.py is:

import os, select, pty

(pid, pty_fd) = pty.fork()

if pid == 0:
   os.write(1, 'This string should be received by parent')
else:
   poller = select.epoll()
   poller.register( pty_fd, select.EPOLLIN )
   ready = poller.poll( 1 * 1000 )
   for fd, events in ready:
      if not events & select.EPOLLIN:
         print 'missed POLLIN event'
      else:
         print os.read(fd, 100)
   poller.close()

The string from the slave is missed several times.
This patch takes the same approach as the fix for read and special cases
this condition for poll.
Tested on 3.16.

Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 89c4cee253e34419006b670089cc54b0257dde52..2e900a98c3e34e9d732deee15cb009db2372d3c9 100644 (file)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2413,12 +2413,17 @@ static unsigned int n_tty_poll(struct tty_struct *tty, struct file *file,
 
        poll_wait(file, &tty->read_wait, wait);
        poll_wait(file, &tty->write_wait, wait);
+       if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
+               mask |= POLLHUP;
        if (input_available_p(tty, 1))
                mask |= POLLIN | POLLRDNORM;
+       else if (mask & POLLHUP) {
+               tty_flush_to_ldisc(tty);
+               if (input_available_p(tty, 1))
+                       mask |= POLLIN | POLLRDNORM;
+       }
        if (tty->packet && tty->link->ctrl_status)
                mask |= POLLPRI | POLLIN | POLLRDNORM;
-       if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
-               mask |= POLLHUP;
        if (tty_hung_up_p(file))
                mask |= POLLHUP;
        if (!(mask & (POLLHUP | POLLIN | POLLRDNORM))) {