cgroup_namespaces

CGROUP_NAMESPACES(7)        Linux Programmer's 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 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 ...

CONFORMING TO
       Namespaces are a Linux-specific feature.

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.

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

COLOPHON
       This page is part of release 5.12 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                              2020-11-01               CGROUP_NAMESPACES(7)