Documentation/filesystems/devpts.txt

   1
   2 To support containers, we now allow multiple instances of devpts filesystem,
   3 such that indices of ptys allocated in one instance are independent of indices
   4 allocated in other instances of devpts.
   5
   6 To preserve backward compatibility, this support for multiple instances is
   7 enabled only if:
   8
   9         - CONFIG_DEVPTS_MULTIPLE_INSTANCES=y, and
  10         - '-o newinstance' mount option is specified while mounting devpts
  11
  12 IOW, devpts now supports both single-instance and multi-instance semantics.
  13
  14 If CONFIG_DEVPTS_MULTIPLE_INSTANCES=n, there is no change in behavior and
  15 this 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
  17 on console.
  18
  19 If CONFIG_DEVPTS_MULTIPLE_INSTANCES=y and devpts is mounted without the
  20 'newinstance' option (as in current start-up scripts) the new mount binds
  21 to the initial kernel mount of devpts. This mode is referred to as the
  22 'single-instance' mode and the current, single-instance semantics are
  23 preserved, i.e PTYs are common across the system.
  24
  25 The only difference between this single-instance mode and the legacy mode
  26 is the presence of new, '/dev/pts/ptmx' node with permissions 0000, which
  27 can safely be ignored.
  28
  29 If CONFIG_DEVPTS_MULTIPLE_INSTANCES=y and 'newinstance' option is specified,
  30 the mount is considered to be in the multi-instance mode and a new instance
  31 of the devpts fs is created. Any ptys created in this instance are independent
  32 of 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,
  34 open of /dev/ptmx must be a redirected to '/dev/pts/ptmx' using a symlink or
  35 bind-mount.
  36
  37 Eg: 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
  50 where '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
  52 the original mount of /dev/pts.
  53
  54 User-space changes
  55 ------------------
  56
  57 In multi-instance mode (i.e '-o newinstance' mount option is specified at least
  58 once), following user-space issues should be noted.
  59
  60 1. If -o newinstance mount option is never used, /dev/pts/ptmx can be ignored
  61    and no change is needed to system-startup scripts.
  62
  63 2. To effectively use multi-instance mode (i.e -o newinstance is specified)
  64    administrators or startup scripts should "redirect" open of /dev/ptmx to
  65    /dev/pts/ptmx using either a bind mount or symlink.
  66
  67         $ mount -t devpts -o newinstance devpts /dev/pts
  68
  69    followed by either
  70
  71         $ rm /dev/ptmx
  72         $ ln -s pts/ptmx /dev/ptmx
  73         $ chmod 666 /dev/pts/ptmx
  74    or
  75         $ mount -o bind /dev/pts/ptmx /dev/ptmx
  76
  77 3. The '/dev/ptmx -> pts/ptmx' symlink is the preferred method since it
  78    enables better error-reporting and treats both single-instance and
  79    multi-instance mounts similarly.
  80
  81    But this method requires that system-startup scripts set the mode of
  82    /dev/pts/ptmx correctly (default mode is 0000). The scripts can set the
  83    mode by, either
  84
  85         - adding ptmxmode mount option to devpts entry in /etc/fstab, or
  86         - using 'chmod 0666 /dev/pts/ptmx'
  87
  88 4. If multi-instance mode mount is needed for containers, but the system
  89    startup scripts have not yet been updated, container-startup scripts
  90    should bind mount /dev/ptmx to /dev/pts/ptmx to avoid breaking single-
  91    instance mounts.
  92
  93    Or, in general, container-startup scripts should use:
  94
  95         mount -t devpts -o newinstance -o ptmxmode=0666 devpts /dev/pts
  96         if [ ! -L /dev/ptmx ]; then
  97                 mount -o bind /dev/pts/ptmx /dev/ptmx
  98         fi
  99
 100    When all devpts mounts are multi-instance, /dev/ptmx can permanently be
 101    a symlink to pts/ptmx and the bind mount can be ignored.
 102
 103 5. A multi-instance mount that is not accompanied by the /dev/ptmx to
 104    /dev/pts/ptmx redirection would result in an unusable/unreachable pty.
 105
 106         mount -t devpts -o newinstance lxcpts /dev/pts
 107
 108    immediately followed by:
 109
 110         open("/dev/ptmx")
 111
 112     would create a pty, say /dev/pts/7, in the initial kernel mount.
 113     But /dev/pts/7 would be invisible in the new mount.
 114
 115 6. The permissions for /dev/pts/ptmx node should be specified when mounting
 116    /dev/pts, using the '-o ptmxmode=%o' mount option (default is 0000).
 117
 118         mount -t devpts -o newinstance -o ptmxmode=0644 devpts /dev/pts
 119
 120    The permissions can be later be changed as usual with 'chmod'.
 121
 122         chmod 666 /dev/pts/ptmx
 123
 124 7. A mount of devpts without the 'newinstance' option results in binding to
 125    initial kernel mount.  This behavior while preserving legacy semantics,
 126    does not provide strict isolation in a container environment. i.e by
 127    mounting devpts without the 'newinstance' option, a container could
 128    get visibility into the 'host' or root container's devpts.
 129
 130    To workaround this and have strict isolation, all mounts of devpts,
 131    including the mount in the root container, should use the newinstance
 132    option.