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 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:
# 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.
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 theCLONE_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 4.16 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/.
Copyright (c) 2016 by Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END |