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
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
# mkdir -p /sys/fs/cgroup/freezer/sub2 # sleep 10000 & # Create a process that lives for a while  20124 # echo 20124 > /sys/fs/cgroup/freezer/sub2/cgroup.procs
We then create another child cgroup in the
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
/proc/[pid]/cgroup files of, respectively, the new
shell, a process that is in the initial cgroup namespace (
with PID 1), and the process in the sibling cgroup (
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
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
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 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
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
We have a process,
X, also owned by user ID 9000, that
is namespaced under the cgroup
placed in a new cgroup namespace via clone(2) or
unshare(2) with the CLONE_NEWCGROUP
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
/cg/1. Namespacing process
X under the
/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
X from escaping the limits imposed by ancestor
unshare(1), clone(2), setns(2), unshare(2), proc(5), cgroups(7), credentials(7), namespaces(7), user_namespaces(7)
This page is part of release 5.10 of the Linux
project. A description of the project, information about reporting bugs,
and the latest version of this page, can be found at