man > cgroup_namespaces(7)

cgroup_namespaces(7)              Miscellaneous Information 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 un‐
       share(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 hierar‐
       chies 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 direc‐
       tory 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
           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 name‐
       spaces:

           # PS1="sh2# " unshare -Cm bash

       From the new shell started by unshare(1), we then inspect the /proc/pid/cgroup files of,  re‐
       spectively,  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 initial 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 filesys‐
       tem corresponding to the initial cgroup namespace (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 ...

STANDARDS
       Linux.

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 container.  Such leakages could, for example, re‐
          veal information about the container framework to containerized applications.

       •  It eases tasks such as container migration.  The virtualization provided by  cgroup  name‐
          spaces  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 con‐
          tainer;  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, 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  con‐
          tents  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),  name‐
       spaces(7), user_namespaces(7)

Linux man-pages 6.7                          2023-10-31                         cgroup_namespaces(7)