-
-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.