NAME | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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, it enters a new cgroup
       namespace in which 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 viewing /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) we create a 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
           # sh -c '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:

           # unshare -Cm bash

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

           $ cat /proc/self/cgroup | grep freezer
           7:freezer:/
           $ cat /proc/1/cgroup | grep freezer
           7:freezer:/..
           $ 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 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
       anomaly:

           # 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 our initial shell
       process's cgroup namespace (whose cgroup filesystem is indeed rooted
       in the parent directory of sub).  We need to remount the freezer
       cgroup filesystem inside this cgroup namespace, after which we see
       the expected results:

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

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

CONFORMING TO         top

       Namespaces are a Linux-specific feature.

NOTES         top

       Among the purposes served by the virtualization provided by cgroup
       namespaces are the following:

       * 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
         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:

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

           · 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         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 4.13 of the Linux man-pages project.  A
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       latest version of this page, can be found at
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Linux                            2017-09-15             CGROUP_NAMESPACES(7)

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