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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 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, 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)


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