mirror of
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
synced 2025-10-31 08:44:41 +00:00
784c4d8b1b
Changelog [v2]: - Add note indicating strict isolation is not possible unless all mounts of devpts use the 'newinstance' mount option. Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> Signed-off-by: Alan Cox <alan@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
132 lines
5 KiB
Text
132 lines
5 KiB
Text
|
|
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.
|