# cgroup_namespaces

## 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 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 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 namespaces:
    
               # **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** ;
               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 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 ...
    


## 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, 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.
    


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