cgroup_namespaces(7) — Linux manual page


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

NAME         top

       cgroup_namespaces - overview of Linux cgroup namespaces

DESCRIPTION         top

       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 ancestor 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
           # echo 30655 > /sys/fs/cgroup/freezer/sub/cgroup.procs
           # cat /proc/self/cgroup | grep freezer

       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
           sh2# cat /proc/1/cgroup | grep freezer
           sh2# cat /proc/20124/cgroup | grep freezer

       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 di‐
       rectory that was established when the new cgroup namespace was cre‐
       ated.  (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 mem‐
       bership is displayed as '/'.)

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

           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 name‐
       space (whose cgroup filesystem is indeed rooted at the parent direc‐
       tory 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         top

       Namespaces are a Linux-specific feature.

NOTES         top

       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

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

       * 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:

           • We have a cgroup directory, /cg/1, that is owned by user ID

           • 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, 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         top

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

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-11-01             CGROUP_NAMESPACES(7)

Pages that refer to this page: nsenter(1)unshare(1)clone2(2)__clone2(2)clone(2)clone3(2)cgroups(7)namespaces(7)user_namespaces(7)