[PATCH 10/10] Document usage of multiple-instances of devpts
Serge E. Hallyn
serue at us.ibm.com
Wed Sep 24 13:36:50 PDT 2008
Quoting sukadev at us.ibm.com (sukadev at us.ibm.com):
> >From 6d2fe9386880157f871667077db3180e9f0083a1 Mon Sep 17 00:00:00 2001
> From: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
> Date: Tue, 9 Sep 2008 10:43:50 -0700
> Subject: [PATCH 10/10] Document usage of multiple-instances of devpts
> Signed-off-by: Sukadev Bhattiprolu <sukadev at us.ibm.com>
> Documentation/filesystems/devpts.txt | 122 ++++++++++++++++++++++++++++++++++
> 1 files changed, 122 insertions(+), 0 deletions(-)
> create mode 100644 Documentation/filesystems/devpts.txt
> diff --git a/Documentation/filesystems/devpts.txt b/Documentation/filesystems/devpts.txt
> new file mode 100644
> index 0000000..93d9d01
> --- /dev/null
> +++ b/Documentation/filesystems/devpts.txt
> @@ -0,0 +1,122 @@
> +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
> +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
That is, of course, crucial, this being the documentation :)
> + 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
More information about the Containers