CGROUP_NAMESPACES(7)					     Linux Programmer's Manual					      CGROUP_NAMESPACES(7)

NAME

       cgroup_namespaces - overview of Linux cgroup namespaces

DESCRIPTION

       For an overview of namespaces, see namespaces(7).

       Cgroup namespaces virtualize the view of a process's cgroups (see cgroups(7)) as seen via /proc/[pid]/cgroup and /proc/[pid]/mountinfo.

       Each  cgroup  namespace	has its own set of cgroup root directories.  These root directories are the base points for the relative locations
       displayed in the corresponding records in the /proc/[pid]/cgroup file.  When a process creates a new cgroup  namespace  using  clone(2)	or
       unshare(2)  with  the CLONE_NEWCGROUP flag, its current cgroups directories become the cgroup root directories of the new namespace.  (This
       applies both for the cgroups version 1 hierarchies and the cgroups version 2 unified hierarchy.)

       When reading the cgroup memberships of a "target" process from /proc/[pid]/cgroup, the pathname shown in the third  field  of  each  record
       will  be  relative  to  the reading process's root directory for the corresponding cgroup hierarchy.  If the cgroup directory of the target
       process lies outside the root directory of the reading process's cgroup namespace, then the pathname will show ../ entries for each  ances-
       tor level in the cgroup hierarchy.

       The following shell session demonstrates the effect of creating a new cgroup namespace.

       First, (as superuser) in a shell in the initial cgroup namespace, we create a child cgroup in the freezer hierarchy, and place a process in
       that cgroup that we will use as part of the demonstration below:

	   # mkdir -p /sys/fs/cgroup/freezer/sub2
	   # sleep 10000 &     # Create a process that lives for a while
	   [1] 20124
	   # echo 20124 > /sys/fs/cgroup/freezer/sub2/cgroup.procs

       We then create another child cgroup in the freezer hierarchy and put the shell into that cgroup:

	   # mkdir -p /sys/fs/cgroup/freezer/sub
	   # echo $$			  # Show PID of this shell
	   30655
	   # echo 30655 > /sys/fs/cgroup/freezer/sub/cgroup.procs
	   # cat /proc/self/cgroup | grep freezer
	   7:freezer:/sub

       Next, we use unshare(1) to create a process running a new shell in new cgroup and mount namespaces:

	   # PS1="sh2# " unshare -Cm bash

       From the new shell started by unshare(1), we then inspect the /proc/[pid]/cgroup files of, respectively, the new shell, a process  that	is
       in the initial cgroup namespace (init, with PID 1), and the process in the sibling cgroup (sub2):

	   sh2# cat /proc/self/cgroup | grep freezer
	   7:freezer:/
	   sh2# cat /proc/1/cgroup | grep freezer
	   7:freezer:/..
	   sh2# cat /proc/20124/cgroup | grep freezer
	   7:freezer:/../sub2

       From  the  output of the first command, we see that the freezer cgroup membership of the new shell (which is in the same cgroup as the ini-
       tial shell) is shown defined relative to the freezer cgroup root directory that was established when the new cgroup namespace was  created.
       (In  absolute  terms, the new shell is in the /sub freezer cgroup, and the root directory of the freezer cgroup hierarchy in the new cgroup
       namespace is also /sub.	Thus, the new shell's cgroup membership is displayed as '/'.)

       However, when we look in /proc/self/mountinfo we see the following anomaly:

	   sh2# cat /proc/self/mountinfo | grep freezer
	   155 145 0:32 /.. /sys/fs/cgroup/freezer ...

       The fourth field of this line (/..)  should show the directory in the cgroup filesystem which forms the root of this mount.  Since  by  the
       definition of cgroup namespaces, the process's current freezer cgroup directory became its root freezer cgroup directory, we should see '/'
       in this field.  The problem here is that we are seeing a mount entry for the cgroup filesystem corresponding to the initial  cgroup  names-
       pace  (whose  cgroup  filesystem is indeed rooted at the parent directory of sub).  To fix this problem, we must remount the freezer cgroup
       filesystem from the new shell (i.e., perform the mount from a process that is in the new cgroup namespace), after which we see the expected
       results:

	   sh2# mount --make-rslave /	  # Don't propagate mount events
					  # to other namespaces
	   sh2# umount /sys/fs/cgroup/freezer
	   sh2# mount -t cgroup -o freezer freezer /sys/fs/cgroup/freezer
	   sh2# cat /proc/self/mountinfo | grep freezer
	   155 145 0:32 / /sys/fs/cgroup/freezer rw,relatime ...

CONFORMING TO

       Namespaces are a Linux-specific feature.

NOTES

       Use of cgroup namespaces requires a kernel that is configured with the CONFIG_CGROUPS option.

       The virtualization provided by cgroup namespaces serves a number of purposes:

       * It  prevents  information leaks whereby cgroup directory paths outside of a container would otherwise be visible to processes in the con-
	 tainer.  Such leakages could, for example, reveal information about the container framework to containerized applications.

       * It eases tasks such as container migration.  The virtualization provided by cgroup namespaces	allows	containers  to	be  isolated  from
	 knowledge  of	the  pathnames	of  ancestor cgroups.  Without such isolation, the full cgroup pathnames (displayed in /proc/self/cgroups)
	 would need to be replicated on the target system when migrating a container; those pathnames would also need to be unique, so	that  they
	 don't conflict with other pathnames on the target system.

       * It allows better confinement of containerized processes, because it is possible to mount the container's cgroup filesystems such that the
	 container processes can't gain access to ancestor cgroup directories.	Consider, for example, the following scenario:

	   o We have a cgroup directory, /cg/1, that is owned by user ID 9000.

	   o We have a process, X, also owned by user ID 9000, that is namespaced under the cgroup /cg/1/2 (i.e., X was placed	in  a  new  cgroup
	     namespace via clone(2) or unshare(2) with the CLONE_NEWCGROUP flag).

	 In  the absence of cgroup namespacing, because the cgroup directory /cg/1 is owned (and writable) by UID 9000 and process X is also owned
	 by user ID 9000, then process X would be able to modify the contents of cgroups files (i.e., change cgroup settings) not only in  /cg/1/2
	 but  also in the ancestor cgroup directory /cg/1.  Namespacing process X under the cgroup directory /cg/1/2, in combination with suitable
	 mount operations for the cgroup filesystem (as shown above), prevents it modifying files in /cg/1, since it cannot even see the  contents
	 of  that  directory  (or of further removed cgroup ancestor directories).  Combined with correct enforcement of hierarchical limits, this
	 prevents process X from escaping the limits imposed by ancestor cgroups.

SEE ALSO

       unshare(1), clone(2), setns(2), unshare(2), proc(5), cgroups(7), credentials(7), namespaces(7), user_namespaces(7)

Linux								    2019-03-06						      CGROUP_NAMESPACES(7)