[PATCH 2/2 Trusty SRU v2] tty: Fix pty master poll() after slave closes v2

tim.gardner at canonical.com tim.gardner at canonical.com
Tue Sep 1 16:06:33 UTC 2015


From: Francesco Ruggeri <fruggeri at aristanetworks.com>

BugLink: http://bugs.launchpad.net/bugs/1397976

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 at arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from commit c4dc304677e8d566572c4738d95c48be150c6606)
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
---
 drivers/tty/n_tty.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index a6d5445..3480a67 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2478,12 +2478,17 @@ 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))) {
-- 
1.9.1





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