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       cgroup_namespaces - overview of Linux cgroup namespaces


       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

           # 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

           # 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 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  filesystem  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 ...


       Namespaces are a Linux-specific feature.


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

       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,  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 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 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.


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