cgroup_namespaces(7) - Linux manual page (original) (raw)

cgroupnamespaces(7) Miscellaneous Information Manual_cgroupnamespaces_(7)

NAME top

   cgroup_namespaces - overview of Linux cgroup namespaces

DESCRIPTION top

   For an overview of namespaces, see [namespaces(7)](../man7/namespaces.7.html).

   Cgroup namespaces virtualize the view of a process's cgroups (see
   [cgroups(7)](../man7/cgroups.7.html)) 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)](../man2/clone.2.html) or [unshare(2)](../man2/unshare.2.html) 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 <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow></mrow><annotation encoding="application/x-tex"></annotation></semantics></math></span><span class="katex-html" aria-hidden="true"></span></span>** # 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)](../man1/unshare.1.html) 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)](../man1/unshare.1.html), 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 top

   Linux.

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)](../man2/clone.2.html) or [unshare(2)](../man2/unshare.2.html) 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.

COLOPHON top

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Linux man-pages 6.10 2024-05-02 cgroupnamespaces(7)

Pages that refer to this page:nsenter(1), unshare(1), clone(2), lttng-ust(3), cgroups(7), namespaces(7), user_namespaces(7)

cgroup_namespaces(7) - Linux manual page (original) (raw)

NAME top

DESCRIPTION top

STANDARDS top

NOTES top

SEE ALSO top

COLOPHON top