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>

1 files changed, 15 insertions, 130 deletions

diff --git a/Documentation/filesystems/devpts.txt b/Documentation/filesystems/devpts.txt
index 30d2fcb32f72..9f94fe276dea 100644
--- a/Documentation/filesystems/devpts.txt
+++ b/Documentation/filesystems/devpts.txt
@@ -1,141 +1,26 @@
+Each mount of the devpts filesystem is now distinct such that ptys
+and their indicies allocated in one mount are independent from ptys
+and their indicies in all other mounts.
 
-To support containers, we now allow multiple instances of devpts filesystem,
-such that indices of ptys allocated in one instance are independent of indices
-allocated in other instances of devpts.
+All mounts of the devpts filesystem now create a /dev/pts/ptmx node
+with permissions 0000.
 
-To preserve backward compatibility, this support for multiple instances is
-enabled only if:
+To retain backwards compatibility the a ptmx device node (aka any node
+created with "mknod name c 5 2") when opened will look for an instance
+of devpts under the name "pts" in the same directory as the ptmx device
+node.
 
-        - CONFIG_DEVPTS_MULTIPLE_INSTANCES=y, and
-        - '-o newinstance' mount option is specified while mounting devpts
-
-IOW, devpts now supports both single-instance and multi-instance semantics.
-
-If CONFIG_DEVPTS_MULTIPLE_INSTANCES=n, there is no change in behavior and
-this referred to as the "legacy" mode. In this mode, the new mount options
-(-o newinstance and -o ptmxmode) will be ignored with a 'bogus option' message
-on console.
-
-If CONFIG_DEVPTS_MULTIPLE_INSTANCES=y and devpts is mounted without the
-'newinstance' option (as in current start-up scripts) the new mount binds
-to the initial kernel mount of devpts. This mode is referred to as the
-'single-instance' mode and the current, single-instance semantics are
-preserved, i.e PTYs are common across the system.
-
-The only difference between this single-instance mode and the legacy mode
-is the presence of new, '/dev/pts/ptmx' node with permissions 0000, which
-can safely be ignored.
-
-If CONFIG_DEVPTS_MULTIPLE_INSTANCES=y and 'newinstance' option is specified,
-the mount is considered to be in the multi-instance mode and a new instance
-of the devpts fs is created. Any ptys created in this instance are independent
-of ptys in other instances of devpts. Like in the single-instance mode, the
-/dev/pts/ptmx node is present. To effectively use the multi-instance mode,
-open of /dev/ptmx must be a redirected to '/dev/pts/ptmx' using a symlink or
-bind-mount.
-
-Eg: A container startup script could do the following:
-
-        $ chmod 0666 /dev/pts/ptmx
-        $ rm /dev/ptmx
-        $ ln -s pts/ptmx /dev/ptmx
-        $ ns_exec -cm /bin/bash
-
-        # We are now in new container
-
-        $ umount /dev/pts
-        $ mount -t devpts -o newinstance lxcpts /dev/pts
-        $ sshd -p 1234
-
-where 'ns_exec -cm /bin/bash' calls clone() with CLONE_NEWNS flag and execs
-/bin/bash in the child process.  A pty created by the sshd is not visible in
-the original mount of /dev/pts.
+As an option instead of placing a /dev/ptmx device node at /dev/ptmx
+it is possible to place a symlink to /dev/pts/ptmx at /dev/ptmx or
+to bind mount /dev/ptx/ptmx to /dev/ptmx.  If you opt for using
+the devpts filesystem in this manner devpts should be mounted with
+the ptmxmode=0666, or chmod 0666 /dev/pts/ptmx should be called.
 
 Total count of pty pairs in all instances is limited by sysctls:
 kernel.pty.max = 4096           - global limit
-kernel.pty.reserve = 1024       - reserve for initial instance
+kernel.pty.reserve = 1024       - reserved for filesystems mounted from the initial mount namespace
 kernel.pty.nr                   - current count of ptys
 
 Per-instance limit could be set by adding mount option "max=<count>".
 This feature was added in kernel 3.4 together with sysctl kernel.pty.reserve.
 In kernels older than 3.4 sysctl kernel.pty.max works as per-instance limit.
-
-User-space changes
-------------------
-
-In multi-instance mode (i.e '-o newinstance' mount option is specified at least
-once), following user-space issues should be noted.
-
-1. If -o newinstance mount option is never used, /dev/pts/ptmx can be ignored
-   and no change is needed to system-startup scripts.
-
-2. To effectively use multi-instance mode (i.e -o newinstance is specified)
-   administrators or startup scripts should "redirect" open of /dev/ptmx to
-   /dev/pts/ptmx using either a bind mount or symlink.
-
-        $ mount -t devpts -o newinstance devpts /dev/pts
-
-   followed by either
-
-        $ rm /dev/ptmx
-        $ ln -s pts/ptmx /dev/ptmx
-        $ chmod 666 /dev/pts/ptmx
-   or
-        $ mount -o bind /dev/pts/ptmx /dev/ptmx
-
-3. The '/dev/ptmx -> pts/ptmx' symlink is the preferred method since it
-   enables better error-reporting and treats both single-instance and
-   multi-instance mounts similarly.
-
-   But this method requires that system-startup scripts set the mode of
-   /dev/pts/ptmx correctly (default mode is 0000). The scripts can set the
-   mode by, either
-
-        - adding ptmxmode mount option to devpts entry in /etc/fstab, or
-        - using 'chmod 0666 /dev/pts/ptmx'
-
-4. If multi-instance mode mount is needed for containers, but the system
-   startup scripts have not yet been updated, container-startup scripts
-   should bind mount /dev/ptmx to /dev/pts/ptmx to avoid breaking single-
-   instance mounts.
-
-   Or, in general, container-startup scripts should use:
-
-        mount -t devpts -o newinstance -o ptmxmode=0666 devpts /dev/pts
-        if [ ! -L /dev/ptmx ]; then
-                mount -o bind /dev/pts/ptmx /dev/ptmx
-        fi
-
-   When all devpts mounts are multi-instance, /dev/ptmx can permanently be
-   a symlink to pts/ptmx and the bind mount can be ignored.
-
-5. A multi-instance mount that is not accompanied by the /dev/ptmx to
-   /dev/pts/ptmx redirection would result in an unusable/unreachable pty.
-
-        mount -t devpts -o newinstance lxcpts /dev/pts
-
-   immediately followed by:
-
-        open("/dev/ptmx")
-
-    would create a pty, say /dev/pts/7, in the initial kernel mount.
-    But /dev/pts/7 would be invisible in the new mount.
-
-6. The permissions for /dev/pts/ptmx node should be specified when mounting
-   /dev/pts, using the '-o ptmxmode=%o' mount option (default is 0000).
-
-        mount -t devpts -o newinstance -o ptmxmode=0644 devpts /dev/pts
-
-   The permissions can be later be changed as usual with 'chmod'.
-
-        chmod 666 /dev/pts/ptmx
-
-7. A mount of devpts without the 'newinstance' option results in binding to
-   initial kernel mount.  This behavior while preserving legacy semantics,
-   does not provide strict isolation in a container environment. i.e by
-   mounting devpts without the 'newinstance' option, a container could
-   get visibility into the 'host' or root container's devpts.
-   
-   To workaround this and have strict isolation, all mounts of devpts,
-   including the mount in the root container, should use the newinstance
-   option.