CGROUP_NAMESPACES(7) Linux Programmer's Manual CGROUP_NAMESPACES(7)
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 /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, it enters a new cgroup names-
pace in which 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 viewing /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) we create a 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
# sh -c 'echo 30655 > /sys/fs/cgroup/freezer/sub/cgroup.procs'
# cat /proc/self/cgroup | grep freezer
Next, we use unshare(1) to create a process running a new shell in new
cgroup and mount namespaces:
# unshare -Cm bash
We then inspect the /proc/[pid]/cgroup files of, respectively, the new
shell process started by the unshare(1) command, a process that is in
the original cgroup namespace (init, with PID 1), and a process in a
sibling cgroup (sub2):
$ cat /proc/self/cgroup | grep freezer
$ cat /proc/1/cgroup | grep freezer
$ cat /proc/20124/cgroup | grep freezer
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 names-
pace is also /sub. Thus, the new shell's cgroup membership is dis-
played as '/'.)
However, when we look in /proc/self/mountinfo we see the following
# 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 our initial shell process's
cgroup namespace (whose cgroup filesystem is indeed rooted in the par-
ent directory of sub). We need to remount the freezer cgroup filesys-
tem inside this cgroup namespace, after which we see the expected
# mount --make-rslave / # Don't propagate mount events
# to other namespaces
# umount /sys/fs/cgroup/freezer
# mount -t cgroup -o freezer freezer /sys/fs/cgroup/freezer
# cat /proc/self/mountinfo | grep freezer
155 145 0:32 / /sys/fs/cgroup/freezer rw,relatime ...
Use of cgroup namespaces requires a kernel that is configured with the
Namespaces are a Linux-specific feature.
Among the purposes served by the virtualization provided by cgroup
namespaces are the following:
* It prevents information leaks whereby cgroup directory paths outside
of a container would otherwise be visible to processes in the con-
tainer. Such leakages could, for example, reveal information about
the container framework to containerized applications.
* It eases tasks such as container migration. The virtualization pro-
vided by cgroup namespaces allows containers to be isolated from
knowledge of the pathnames of ancestor cgroups. Without such isola-
tion, 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:
o We have a cgroup directory, /cg/1, that is owned by user ID 9000.
o We have a process, X, also owned by user ID 9000, that is names-
paced 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_NEWC-
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, then 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 pre-
vents process X from escaping the limits imposed by ancestor cgroups.
unshare(1), clone(2), setns(2), unshare(2), proc(5), cgroups(7), cre-
dentials(7), namespaces(7), user_namespaces(7)
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