cgroup_namespaces(7) Miscellaneous Information Manual cgroup_namespaces(7)
NAME
cgroup_namespaces - overview of Linux cgroup namespaces
DESCRIPTION
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 un-
share(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 corre-
sponding 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 $$ # 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 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 name-
space 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
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 de-
finition of cgroup namespaces, the process's current freezer cgroup di-
rectory 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 filesys-
tem 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
Linux.
NOTES
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 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 direc-
tories. 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 name-
spaced 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-
GROUP 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 con-
tents of cgroups files (i.e., change cgroup settings) not only in
/cg/1/2 but also in the ancestor cgroup directory /cg/1. Name-
spacing 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 an-
cestor directories). Combined with correct enforcement of hierar-
chical limits, this prevents process X from escaping the limits im-
posed by ancestor cgroups.
SEE ALSO
unshare(1), clone(2), setns(2), unshare(2), proc(5), cgroups(7), cre-
dentials(7), namespaces(7), user_namespaces(7)
Linux man-pages 6.7 2023-10-31 cgroup_namespaces(7)
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