TTY: pty, fix pty counting

tty_operations->remove is normally called like:
queue_release_one_tty
 ->tty_shutdown
   ->tty_driver_remove_tty
     ->tty_operations->remove

However tty_shutdown() is called from queue_release_one_tty() only if
tty_operations->shutdown is NULL. But for pty, it is not.
pty_unix98_shutdown() is used there as ->shutdown.

So tty_operations->remove of pty (i.e. pty_unix98_remove()) is never
called. This results in invalid pty_count. I.e. what can be seen in
/proc/sys/kernel/pty/nr.

I see this was already reported at:
  https://lkml.org/lkml/2009/11/5/370
But it was not fixed since then.

This patch is kind of a hackish way. The problem lies in ->install. We
allocate there another tty (so-called tty->link). So ->install is
called once, but ->remove twice, for both tty and tty->link. The fix
here is to count both tty and tty->link and divide the count by 2 for
user.

And to have ->remove called, let's make tty_driver_remove_tty() global
and call that from pty_unix98_shutdown() (tty_operations->shutdown).

While at it, let's document that when ->shutdown is defined,
tty_shutdown() is not called.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

2 files changed, 5 insertions, 0 deletions

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 44bc0c5617e..5f2ede82b3d 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -421,6 +421,8 @@ extern void tty_driver_flush_buffer(struct tty_struct *tty);
 extern void tty_throttle(struct tty_struct *tty);
 extern void tty_unthrottle(struct tty_struct *tty);
 extern int tty_do_resize(struct tty_struct *tty, struct winsize *ws);
+extern void tty_driver_remove_tty(struct tty_driver *driver,
+				  struct tty_struct *tty);
 extern void tty_shutdown(struct tty_struct *tty);
 extern void tty_free_termios(struct tty_struct *tty);
 extern int is_current_pgrp_orphaned(void);
diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h
index 9deeac85524..ecdaeb98b29 100644
--- a/include/linux/tty_driver.h
+++ b/include/linux/tty_driver.h
@@ -47,6 +47,9 @@
  *
  * 	This routine is called synchronously when a particular tty device
  *	is closed for the last time freeing up the resources.
+ *	Note that tty_shutdown() is not called if ops->shutdown is defined.
+ *	This means one is responsible to take care of calling ops->remove (e.g.
+ *	via tty_driver_remove_tty) and releasing tty->termios.
  *
  *
  * void (*cleanup)(struct tty_struct * tty);