[PATCH] n_tty_read: check for hanging tty while waiting for input

[Aristeu Rozanski]

If the console has a canonical reader and the respective tty hangs up,
it'll waste a wake up and will never release the last ldisc reference so
the hangup process can finish:

n_tty_read():
(..)
          add_wait_queue(&tty->read_wait, &wait);
          while (nr) {
(..)
                  if (!input_available_p(tty, 0)) {
                          if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
                                  up_read(&tty->termios_rwsem);
                                  tty_flush_to_ldisc(tty);
                                  down_read(&tty->termios_rwsem);
                                  if (!input_available_p(tty, 0)) {
                                          retval = -EIO;
                                          break;
                                  }
                          } else {
->                                if (tty_hung_up_p(file))
                                          break;
this won't work because file->f_op never gets set to &hung_up_tty_fops:
__tty_hangup():

        spin_lock(&tty_files_lock);
        /* This breaks for file handles being sent over AF_UNIX sockets ? */
        list_for_each_entry(priv, &tty->tty_files, list) {
                filp = priv->file;
                if (filp->f_op->write == redirected_tty_write)
                        cons_filp = filp;
->              if (filp->f_op->write != tty_write)
->                      continue;
                closecount++;
                __tty_fasync(-1, filp, 0);      /* can't block */
->              filp->f_op = &hung_up_tty_fops;
        }
        spin_unlock(&tty_files_lock);

        refs = tty_signal_session_leader(tty, exit_session);
        /* Account for the p->signal references we killed */
        while (refs--)
                tty_kref_put(tty);

        /*
         * it drops BTM and thus races with reopen
         * we protect the race by TTY_HUPPING
         */
->      tty_ldisc_hangup(tty);

So while the canonical read waits for input, it'll sleep, be awaken by
tty_ldisc_hangup() and then immediately going back to sleep without
dropping the reference to the ldisc gained on tty_read(). This isn't
noticiable in a non canonical read due that it'll eventually timeout.

The proposed patch checks for TTY_HUPPING flag in order to leave if
there's no input.

This is easily reproduced by opening /dev/console (my test case was a
virtual machine with serial console), setting as canonical and waiting
on a read(). Then, in another session, killing agetty that is running on
ttyS0 which will issue a hangup.

	[  240.439045] INFO: task (agetty):1323 blocked for more than 120 seconds.
	[  240.439569]       Not tainted 3.13.0-rc3+ #11
	[  240.439972] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
	[  240.440596] (agetty)        D ffff88007fd94440     0  1323      1 0x00000080
	[  240.441253]  ffff88007bca1c50 0000000000000086 ffff88007989b0c0 0000000000014440
	[  240.441857]  ffff88007bca1fd8 0000000000014440 ffff88007989b0c0 ffff88007989b0c0
	[  240.442561]  ffff88007ad46c30 7fffffffffffffff 0000000000000001 ffff88007ad46c28
	[  240.443296] Call Trace:
	[  240.443506]  [<ffffffff815c8c99>] schedule+0x29/0x70
	[  240.443883]  [<ffffffff815c7f59>] schedule_timeout+0x209/0x2d0
	[  240.444395]  [<ffffffff810974b5>] ? check_preempt_curr+0x85/0xa0
	[  240.444850]  [<ffffffff810974e9>] ? ttwu_do_wakeup+0x19/0xd0
	[  240.445343]  [<ffffffff8109764d>] ? ttwu_do_activate.constprop.80+0x5d/0x70
	[  240.445868]  [<ffffffff810995eb>] ? try_to_wake_up+0xeb/0x2b0
	[  240.446363]  [<ffffffff815cbdaa>] ldsem_down_write+0xda/0x227
	[  240.446797]  [<ffffffff81099822>] ? default_wake_function+0x12/0x20
	[  240.447359]  [<ffffffff815cc43d>] tty_ldisc_lock_pair_timeout+0x7d/0x100
	[  240.447861]  [<ffffffff8136e519>] tty_ldisc_hangup+0xc9/0x220
	[  240.448355]  [<ffffffff81365463>] __tty_hangup+0x363/0x4b0
	[  240.448768]  [<ffffffff81367cc5>] tty_ioctl+0x865/0xbb0
	[  240.449219]  [<ffffffff811bb52a>] ? do_filp_open+0x3a/0x90
	[  240.449634]  [<ffffffff811bd900>] do_vfs_ioctl+0x2e0/0x4c0
	[  240.450066]  [<ffffffff8124ea76>] ? file_has_perm+0x86/0xa0
	[  240.450543]  [<ffffffff811bdb61>] SyS_ioctl+0x81/0xa0
	[  240.450921]  [<ffffffff815d4b69>] system_call_fastpath+0x16/0x1b

Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Jiri Slaby <jslaby@xxxxxxx>
Cc: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Aristeu Rozanski <aris@xxxxxxxxxx>

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 0f74945..4fb909d 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2189,6 +2189,8 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 			} else {
 				if (tty_hung_up_p(file))
 					break;
+				if (test_bit(TTY_HUPPING, &tty->flags))
+					break;
 				if (!timeout)
 					break;
 				if (file->f_flags & O_NONBLOCK) {
--
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