|
NAME | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
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
namespace 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
30655
# sh -c '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:
# 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
7:freezer:/
$ cat /proc/1/cgroup | grep freezer
7:freezer:/..
$ 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 ini‐
tial shell) is shown defined relative to the freezer cgroup root
directory that was established when the new cgroup namespace was cre‐
ated. (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 mem‐
bership is displayed as '/'.)
However, when we look in /proc/self/mountinfo we see the following
anomaly:
# 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 parent directory of sub). We need to remount the freezer
cgroup filesystem inside this cgroup namespace, after which we see
the expected results:
# 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 CONFIG_CGROUPS option.
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 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, 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 prevents process X
from escaping the limits imposed by ancestor cgroups.
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 4.12 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 2017-07-13 CGROUP_NAMESPACES(7)
Pages that refer to this page: nsenter(1), unshare(1), clone(2), cgroups(7), namespaces(7), user_namespaces(7)