1
  2To support containers, we now allow multiple instances of devpts filesystem,
  3such that indices of ptys allocated in one instance are independent of indices
  4allocated in other instances of devpts.
  5
  6To preserve backward compatibility, this support for multiple instances is
  7enabled only if:
  8
  9	- CONFIG_DEVPTS_MULTIPLE_INSTANCES=y, and
 10	- '-o newinstance' mount option is specified while mounting devpts
 11
 12IOW, devpts now supports both single-instance and multi-instance semantics.
 13
 14If CONFIG_DEVPTS_MULTIPLE_INSTANCES=n, there is no change in behavior and
 15this referred to as the "legacy" mode. In this mode, the new mount options
 16(-o newinstance and -o ptmxmode) will be ignored with a 'bogus option' message
 17on console.
 18
 19If CONFIG_DEVPTS_MULTIPLE_INSTANCES=y and devpts is mounted without the
 20'newinstance' option (as in current start-up scripts) the new mount binds
 21to the initial kernel mount of devpts. This mode is referred to as the
 22'single-instance' mode and the current, single-instance semantics are
 23preserved, i.e PTYs are common across the system.
 24
 25The only difference between this single-instance mode and the legacy mode
 26is the presence of new, '/dev/pts/ptmx' node with permissions 0000, which
 27can safely be ignored.
 28
 29If CONFIG_DEVPTS_MULTIPLE_INSTANCES=y and 'newinstance' option is specified,
 30the mount is considered to be in the multi-instance mode and a new instance
 31of the devpts fs is created. Any ptys created in this instance are independent
 32of ptys in other instances of devpts. Like in the single-instance mode, the
 33/dev/pts/ptmx node is present. To effectively use the multi-instance mode,
 34open of /dev/ptmx must be a redirected to '/dev/pts/ptmx' using a symlink or
 35bind-mount.
 36
 37Eg: A container startup script could do the following:
 38
 39	$ chmod 0666 /dev/pts/ptmx
 40	$ rm /dev/ptmx
 41	$ ln -s pts/ptmx /dev/ptmx
 42	$ ns_exec -cm /bin/bash
 43
 44	# We are now in new container
 45
 46	$ umount /dev/pts
 47	$ mount -t devpts -o newinstance lxcpts /dev/pts
 48	$ sshd -p 1234
 49
 50where 'ns_exec -cm /bin/bash' calls clone() with CLONE_NEWNS flag and execs
 51/bin/bash in the child process.  A pty created by the sshd is not visible in
 52the original mount of /dev/pts.
 53
 54Total count of pty pairs in all instances is limited by sysctls:
 55kernel.pty.max = 4096		- global limit
 56kernel.pty.reserve = 1024	- reserve for initial instance
 57kernel.pty.nr			- current count of ptys
 58
 59Per-instance limit could be set by adding mount option "max=<count>".
 60This feature was added in kernel 3.4 together with sysctl kernel.pty.reserve.
 61In kernels older than 3.4 sysctl kernel.pty.max works as per-instance limit.
 62
 63User-space changes
 64------------------
 65
 66In multi-instance mode (i.e '-o newinstance' mount option is specified at least
 67once), following user-space issues should be noted.
 68
 691. If -o newinstance mount option is never used, /dev/pts/ptmx can be ignored
 70   and no change is needed to system-startup scripts.
 71
 722. To effectively use multi-instance mode (i.e -o newinstance is specified)
 73   administrators or startup scripts should "redirect" open of /dev/ptmx to
 74   /dev/pts/ptmx using either a bind mount or symlink.
 75
 76	$ mount -t devpts -o newinstance devpts /dev/pts
 77
 78   followed by either
 79
 80	$ rm /dev/ptmx
 81	$ ln -s pts/ptmx /dev/ptmx
 82	$ chmod 666 /dev/pts/ptmx
 83   or
 84	$ mount -o bind /dev/pts/ptmx /dev/ptmx
 85
 863. The '/dev/ptmx -> pts/ptmx' symlink is the preferred method since it
 87   enables better error-reporting and treats both single-instance and
 88   multi-instance mounts similarly.
 89
 90   But this method requires that system-startup scripts set the mode of
 91   /dev/pts/ptmx correctly (default mode is 0000). The scripts can set the
 92   mode by, either
 93
 94   	- adding ptmxmode mount option to devpts entry in /etc/fstab, or
 95	- using 'chmod 0666 /dev/pts/ptmx'
 96
 974. If multi-instance mode mount is needed for containers, but the system
 98   startup scripts have not yet been updated, container-startup scripts
 99   should bind mount /dev/ptmx to /dev/pts/ptmx to avoid breaking single-
100   instance mounts.
101
102   Or, in general, container-startup scripts should use:
103
104	mount -t devpts -o newinstance -o ptmxmode=0666 devpts /dev/pts
105	if [ ! -L /dev/ptmx ]; then
106		mount -o bind /dev/pts/ptmx /dev/ptmx
107	fi
108
109   When all devpts mounts are multi-instance, /dev/ptmx can permanently be
110   a symlink to pts/ptmx and the bind mount can be ignored.
111
1125. A multi-instance mount that is not accompanied by the /dev/ptmx to
113   /dev/pts/ptmx redirection would result in an unusable/unreachable pty.
114
115	mount -t devpts -o newinstance lxcpts /dev/pts
116
117   immediately followed by:
118
119	open("/dev/ptmx")
120
121    would create a pty, say /dev/pts/7, in the initial kernel mount.
122    But /dev/pts/7 would be invisible in the new mount.
123
1246. The permissions for /dev/pts/ptmx node should be specified when mounting
125   /dev/pts, using the '-o ptmxmode=%o' mount option (default is 0000).
126
127	mount -t devpts -o newinstance -o ptmxmode=0644 devpts /dev/pts
128
129   The permissions can be later be changed as usual with 'chmod'.
130
131	chmod 666 /dev/pts/ptmx
132
1337. A mount of devpts without the 'newinstance' option results in binding to
134   initial kernel mount.  This behavior while preserving legacy semantics,
135   does not provide strict isolation in a container environment. i.e by
136   mounting devpts without the 'newinstance' option, a container could
137   get visibility into the 'host' or root container's devpts.
138   
139   To workaround this and have strict isolation, all mounts of devpts,
140   including the mount in the root container, should use the newinstance
141   option.