Document usage of multiple-instances of devpts

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.

To preserve backward compatibility, this support for multiple instances is

enabled only if:

	- 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.

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.