cgroup_namespaces(7) — Linux manual page

NAME | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

CGROUP_NAMESPACES(7)    Linux Programmer's Manual   CGROUP_NAMESPACES(7)

NAME         top

       cgroup_namespaces - overview of Linux cgroup namespaces

DESCRIPTION         top

       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         top

       Namespaces are a Linux-specific feature.

NOTES         top

       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         top

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

COLOPHON         top

       This page is part of release 5.11 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)

Pages that refer to this page: nsenter(1)unshare(1)clone(2)cgroups(7)namespaces(7)user_namespaces(7)