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| -rw-r--r-- | Documentation/filesystems/devpts.txt | 132 |
1 files changed, 132 insertions, 0 deletions
diff --git a/Documentation/filesystems/devpts.txt b/Documentation/filesystems/devpts.txt new file mode 100644 index 000000000000..68dffd87f9b7 --- /dev/null +++ b/Documentation/filesystems/devpts.txt @@ -0,0 +1,132 @@ + +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. |