{
    "mode": "man",
    "parameter": "cgroup_namespaces",
    "section": "7",
    "url": "https://www.chedong.com/phpMan.php/man/cgroup_namespaces/7/json",
    "generated": "2026-08-26T06:46:41Z",
    "sections": {
        "NAME": {
            "content": "cgroupnamespaces - overview of Linux cgroup namespaces\n",
            "subsections": []
        },
        "DESCRIPTION": {
            "content": "For an overview of namespaces, see namespaces(7).\n\nCgroup  namespaces  virtualize  the  view of a process's cgroups (see cgroups(7)) as seen via\n/proc/pid/cgroup and /proc/pid/mountinfo.\n\nEach cgroup namespace has its own set of cgroup root directories.  These root directories are\nthe base points for the relative locations displayed in  the  corresponding  records  in  the\n/proc/pid/cgroup  file.   When a process creates a new cgroup namespace using clone(2) or un‐\nshare(2) with the CLONENEWCGROUP flag, its current cgroups  directories  become  the  cgroup\nroot  directories of the new namespace.  (This applies both for the cgroups version 1 hierar‐\nchies and the cgroups version 2 unified hierarchy.)\n\nWhen reading the cgroup memberships of a \"target\" process from /proc/pid/cgroup, the pathname\nshown in the third field of each record will be relative to the reading process's root direc‐\ntory for the corresponding cgroup hierarchy.  If the cgroup directory of the  target  process\nlies  outside the root directory of the reading process's cgroup namespace, then the pathname\nwill show ../ entries for each ancestor level in the cgroup hierarchy.\n\nThe following shell session demonstrates the effect of creating a new cgroup namespace.\n\nFirst, (as superuser) in a shell in the initial cgroup namespace, we create a child cgroup in\nthe freezer hierarchy, and place a process in that cgroup that we will use  as  part  of  the\ndemonstration below:\n\n# mkdir -p /sys/fs/cgroup/freezer/sub2\n# sleep 10000 &     # Create a process that lives for a while\n[1] 20124\n# echo 20124 > /sys/fs/cgroup/freezer/sub2/cgroup.procs\n\nWe  then  create  another  child  cgroup in the freezer hierarchy and put the shell into that\ncgroup:\n\n# mkdir -p /sys/fs/cgroup/freezer/sub\n# echo $$                      # Show PID of this shell\n30655\n# echo 30655 > /sys/fs/cgroup/freezer/sub/cgroup.procs\n# cat /proc/self/cgroup | grep freezer\n7:freezer:/sub\n\nNext, we use unshare(1) to create a process running a new shell in new cgroup and mount name‐\nspaces:\n\n# PS1=\"sh2# \" unshare -Cm bash\n\nFrom the new shell started by unshare(1), we then inspect the /proc/pid/cgroup files of,  re‐\nspectively,  the new shell, a process that is in the initial cgroup namespace (init, with PID\n1), and the process in the sibling cgroup (sub2):\n\nsh2# cat /proc/self/cgroup | grep freezer\n7:freezer:/\nsh2# cat /proc/1/cgroup | grep freezer\n7:freezer:/..\nsh2# cat /proc/20124/cgroup | grep freezer\n7:freezer:/../sub2\n\nFrom the output of the first command, we see that the freezer cgroup membership  of  the  new\nshell  (which  is  in  the same cgroup as the initial shell) is shown defined relative to the\nfreezer cgroup root directory that was established when the new cgroup namespace was created.\n(In absolute terms, the new shell is in the /sub freezer cgroup, and the  root  directory  of\nthe freezer cgroup hierarchy in the new cgroup namespace is also /sub.  Thus, the new shell's\ncgroup membership is displayed as '/'.)\n\nHowever, when we look in /proc/self/mountinfo we see the following anomaly:\n\nsh2# cat /proc/self/mountinfo | grep freezer\n155 145 0:32 /.. /sys/fs/cgroup/freezer ...\n\nThe fourth field of this line (/..)  should show the directory in the cgroup filesystem which\nforms  the  root  of this mount.  Since by the definition of cgroup namespaces, the process's\ncurrent freezer cgroup directory became its root freezer cgroup directory, we should see  '/'\nin  this field.  The problem here is that we are seeing a mount entry for the cgroup filesys‐\ntem corresponding to the initial cgroup namespace (whose cgroup filesystem is  indeed  rooted\nat  the  parent  directory  of sub).  To fix this problem, we must remount the freezer cgroup\nfilesystem from the new shell (i.e., perform the mount from a process  that  is  in  the  new\ncgroup namespace), after which we see the expected results:\n\nsh2# mount --make-rslave /     # Don't propagate mount events\n# to other namespaces\nsh2# umount /sys/fs/cgroup/freezer\nsh2# mount -t cgroup -o freezer freezer /sys/fs/cgroup/freezer\nsh2# cat /proc/self/mountinfo | grep freezer\n155 145 0:32 / /sys/fs/cgroup/freezer rw,relatime ...\n",
            "subsections": []
        },
        "STANDARDS": {
            "content": "Linux.\n",
            "subsections": []
        },
        "NOTES": {
            "content": "Use of cgroup namespaces requires a kernel that is configured with the CONFIGCGROUPS option.\n\nThe virtualization provided by cgroup namespaces serves a number of purposes:\n\n•  It  prevents information leaks whereby cgroup directory paths outside of a container would\notherwise be visible to processes in the container.  Such leakages could, for example, re‐\nveal information about the container framework to containerized applications.\n\n•  It eases tasks such as container migration.  The virtualization provided by  cgroup  name‐\nspaces  allows  containers  to  be  isolated  from  knowledge of the pathnames of ancestor\ncgroups.   Without   such   isolation,   the   full   cgroup   pathnames   (displayed   in\n/proc/self/cgroups) would need to be replicated on the target system when migrating a con‐\ntainer;  those  pathnames  would  also need to be unique, so that they don't conflict with\nother pathnames on the target system.\n\n•  It allows better confinement of containerized processes, because it is possible  to  mount\nthe  container's cgroup filesystems such that the container processes can't gain access to\nancestor cgroup directories.  Consider, for example, the following scenario:\n\n•  We have a cgroup directory, /cg/1, that is owned by user ID 9000.\n\n•  We have a process, X, also owned by user ID 9000, that is namespaced under  the  cgroup\n/cg/1/2  (i.e.,  X was placed in a new cgroup namespace via clone(2) or unshare(2) with\nthe CLONENEWCGROUP flag).\n\nIn the absence of cgroup namespacing, because the cgroup directory  /cg/1  is  owned  (and\nwritable) by UID 9000 and process X is also owned by user ID 9000, process X would be able\nto modify the contents of cgroups files (i.e., change cgroup settings) not only in /cg/1/2\nbut  also  in the ancestor cgroup directory /cg/1.  Namespacing process X under the cgroup\ndirectory /cg/1/2, in combination with suitable mount operations for the cgroup filesystem\n(as shown above), prevents it modifying files in /cg/1, since it cannot even see the  con‐\ntents  of  that  directory  (or of further removed cgroup ancestor directories).  Combined\nwith correct enforcement of hierarchical limits, this prevents process X from escaping the\nlimits imposed by ancestor cgroups.\n",
            "subsections": []
        },
        "SEE ALSO": {
            "content": "unshare(1),  clone(2),  setns(2),  unshare(2),  proc(5),  cgroups(7),  credentials(7),  name‐\nspaces(7), usernamespaces(7)\n\nLinux man-pages 6.7                          2023-10-31                         cgroupnamespaces(7)",
            "subsections": []
        }
    },
    "summary": "cgroupnamespaces - overview of Linux cgroup namespaces",
    "flags": [],
    "examples": [],
    "see_also": [
        {
            "name": "unshare",
            "section": "1",
            "url": "https://www.chedong.com/phpMan.php/man/unshare/1/json"
        },
        {
            "name": "clone",
            "section": "2",
            "url": "https://www.chedong.com/phpMan.php/man/clone/2/json"
        },
        {
            "name": "setns",
            "section": "2",
            "url": "https://www.chedong.com/phpMan.php/man/setns/2/json"
        },
        {
            "name": "unshare",
            "section": "2",
            "url": "https://www.chedong.com/phpMan.php/man/unshare/2/json"
        },
        {
            "name": "proc",
            "section": "5",
            "url": "https://www.chedong.com/phpMan.php/man/proc/5/json"
        },
        {
            "name": "cgroups",
            "section": "7",
            "url": "https://www.chedong.com/phpMan.php/man/cgroups/7/json"
        },
        {
            "name": "credentials",
            "section": "7",
            "url": "https://www.chedong.com/phpMan.php/man/credentials/7/json"
        },
        {
            "name": "spaces",
            "section": "7",
            "url": "https://www.chedong.com/phpMan.php/man/spaces/7/json"
        },
        {
            "name": "usernamespaces",
            "section": "7",
            "url": "https://www.chedong.com/phpMan.php/man/usernamespaces/7/json"
        }
    ]
}