Root/Documentation/filesystems/devpts.txt

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
54User-space changes
55------------------
56
57In multi-instance mode (i.e '-o newinstance' mount option is specified at least
58once), following user-space issues should be noted.
59
601. 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
632. 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
773. 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
884. 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
1035. 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
1156. 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
1247. 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.
133

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