devpts: Make each mount of devpts an independent filesystem.

The /dev/ptmx device node is changed to lookup the directory entry "pts"
in the same directory as the /dev/ptmx device node was opened in.  If
there is a "pts" entry and that entry is a devpts filesystem /dev/ptmx
uses that filesystem.  Otherwise the open of /dev/ptmx fails.

The DEVPTS_MULTIPLE_INSTANCES configuration option is removed, so that
userspace can now safely depend on each mount of devpts creating a new
instance of the filesystem.

Each mount of devpts is now a separate and equal filesystem.

Reserved ttys are now available to all instances of devpts where the
mounter is in the initial mount namespace.

A new vfs helper path_pts is introduced that finds a directory entry
named "pts" in the directory of the passed in path, and changes the
passed in path to point to it.  The helper path_pts uses a function
path_parent_directory that was factored out of follow_dotdot.

In the implementation of devpts:
 - devpts_mnt is killed as it is no longer meaningful if all mounts of
   devpts are equal.
 - pts_sb_from_inode is replaced by just inode->i_sb as all cached
   inodes in the tty layer are now from the devpts filesystem.
 - devpts_add_ref is rolled into the new function devpts_ptmx.  And the
   unnecessary inode hold is removed.
 - devpts_del_ref is renamed devpts_release and reduced to just a
   deacrivate_super.
 - The newinstance mount option continues to be accepted but is now
   ignored.

In devpts_fs.h definitions for when !CONFIG_UNIX98_PTYS are removed as
they are never used.

Documentation/filesystems/devices.txt is updated to describe the current
situation.

This has been verified to work properly on openwrt-15.05, centos5,
centos6, centos7, debian-6.0.2, debian-7.9, debian-8.2, ubuntu-14.04.3,
ubuntu-15.10, fedora23, magia-5, mint-17.3, opensuse-42.1,
slackware-14.1, gentoo-20151225 (13.0?), archlinux-2015-12-01.  With the
caveat that on centos6 and on slackware-14.1 that there wind up being
two instances of the devpts filesystem mounted on /dev/pts, the lower
copy does not end up getting used.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Greg KH <greg@kroah.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Peter Anvin <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Serge Hallyn <serge.hallyn@ubuntu.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Cc: Jann Horn <jann@thejh.net>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Florian Weimer <fw@deneb.enyo.de>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/tty/Kconfig

@@ -120,17 +120,6 @@ config UNIX98_PTYS
 	  All modern Linux systems use the Unix98 ptys.  Say Y unless
 	  you're on an embedded system and want to conserve memory.
 
-config DEVPTS_MULTIPLE_INSTANCES
-	bool "Support multiple instances of devpts"
-	depends on UNIX98_PTYS
-	default n
-	---help---
-	  Enable support for multiple instances of devpts filesystem.
-	  If you want to have isolated PTY namespaces (eg: in containers),
-	  say Y here.  Otherwise, say N. If enabled, each mount of devpts
-	  filesystem with the '-o newinstance' option will create an
-	  independent PTY namespace.
-
 config LEGACY_PTYS
 	bool "Legacy (BSD) PTY support"
 	default y

drivers/tty/pty.c

@@ -668,7 +668,7 @@ static void pty_unix98_remove(struct tty_driver *driver, struct tty_struct *tty)
 	else
 		fsi = tty->link->driver_data;
 	devpts_kill_index(fsi, tty->index);
-	devpts_put_ref(fsi);
+	devpts_release(fsi);
 }
 
 static const struct tty_operations ptm_unix98_ops = {
@@ -733,10 +733,11 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 	if (retval)
 		return retval;
 
-	fsi = devpts_get_ref(inode, filp);
-	retval = -ENODEV;
-	if (!fsi)
+	fsi = devpts_acquire(filp);
+	if (IS_ERR(fsi)) {
+		retval = PTR_ERR(fsi);
 		goto out_free_file;
+	}
 
 	/* find a device that is not in use. */
 	mutex_lock(&devpts_mutex);
@@ -745,7 +746,7 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 
 	retval = index;
 	if (index < 0)
-		goto out_put_ref;
+		goto out_put_fsi;
 
 
 	mutex_lock(&tty_mutex);
@@ -789,8 +790,8 @@ err_release:
 	return retval;
 out:
 	devpts_kill_index(fsi, index);
-out_put_ref:
-	devpts_put_ref(fsi);
+out_put_fsi:
+	devpts_release(fsi);
 out_free_file:
 	tty_free_file(filp);
 	return retval;