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 dis-
played 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 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 ab-
solute 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 anom-
aly:
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 def-
inition 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 pur-
poses:
• 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:
• 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 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.
SEE ALSO
unshare(1), clone(2), setns(2), unshare(2), proc(5), cgroups(7), creden-
tials(7), namespaces(7), user_namespaces(7)
Linux man-pages 6.9.1 2024-05-02 cgroup_namespaces(7)
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