1CGROUP_NAMESPACES(7) Linux Programmer's Manual CGROUP_NAMESPACES(7)
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6 cgroup_namespaces - overview of Linux cgroup namespaces
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9 For an overview of namespaces, see namespaces(7).
10 Cgroup namespaces virtualize the view of a process's cgroups (see
12 cgroups(7)) as seen via /proc/[pid]/cgroup and /proc/[pid]/mountinfo.
13 Each cgroup namespace has its own set of cgroup root directories.
15 These root directories are the base points for the relative locations
16 displayed in the corresponding records in the /proc/[pid]/cgroup file.
17 When a process creates a new cgroup namespace using clone(2) or
18 unshare(2) with the CLONE_NEWCGROUP flag, its current cgroups directo‐
19 ries become the cgroup root directories of the new namespace. (This
20 applies both for the cgroups version 1 hierarchies and the cgroups ver‐
21 sion 2 unified hierarchy.)
22 When reading the cgroup memberships of a "target" process from
24 /proc/[pid]/cgroup, the pathname shown in the third field of each
25 record will be relative to the reading process's root directory for the
26 corresponding cgroup hierarchy. If the cgroup directory of the target
27 process lies outside the root directory of the reading process's cgroup
28 namespace, then the pathname will show ../ entries for each ancestor
29 level in the cgroup hierarchy.
30 The following shell session demonstrates the effect of creating a new
32 cgroup namespace.
33 First, (as superuser) in a shell in the initial cgroup namespace, we
35 create a child cgroup in the freezer hierarchy, and place a process in
36 that cgroup that we will use as part of the demonstration below:
37 # mkdir -p /sys/fs/cgroup/freezer/sub2
39 # sleep 10000 & # Create a process that lives for a while
40 [1] 20124
41 # echo 20124 > /sys/fs/cgroup/freezer/sub2/cgroup.procs
42 We then create another child cgroup in the freezer hierarchy and put
44 the shell into that cgroup:
45 # mkdir -p /sys/fs/cgroup/freezer/sub
47 # echo $$ # Show PID of this shell
48 30655
49 # echo 30655 > /sys/fs/cgroup/freezer/sub/cgroup.procs
50 # cat /proc/self/cgroup | grep freezer
51 7:freezer:/sub
52 Next, we use unshare(1) to create a process running a new shell in new
54 cgroup and mount namespaces:
55 # PS1="sh2# " unshare -Cm bash
57 From the new shell started by unshare(1), we then inspect the
59 /proc/[pid]/cgroup files of, respectively, the new shell, a process
60 that is in the initial cgroup namespace (init, with PID 1), and the
61 process in the sibling cgroup (sub2):
62 sh2# cat /proc/self/cgroup | grep freezer
64 7:freezer:/
65 sh2# cat /proc/1/cgroup | grep freezer
66 7:freezer:/..
67 sh2# cat /proc/20124/cgroup | grep freezer
68 7:freezer:/../sub2
69 From the output of the first command, we see that the freezer cgroup
71 membership of the new shell (which is in the same cgroup as the initial
72 shell) is shown defined relative to the freezer cgroup root directory
73 that was established when the new cgroup namespace was created. (In
74 absolute terms, the new shell is in the /sub freezer cgroup, and the
75 root directory of the freezer cgroup hierarchy in the new cgroup names‐
76 pace is also /sub. Thus, the new shell's cgroup membership is dis‐
77 played as '/'.)
78 However, when we look in /proc/self/mountinfo we see the following
80 anomaly:
81 sh2# cat /proc/self/mountinfo | grep freezer
83 155 145 0:32 /.. /sys/fs/cgroup/freezer ...
84 The fourth field of this line (/..) should show the directory in the
86 cgroup filesystem which forms the root of this mount. Since by the
87 definition of cgroup namespaces, the process's current freezer cgroup
88 directory became its root freezer cgroup directory, we should see '/'
89 in this field. The problem here is that we are seeing a mount entry
90 for the cgroup filesystem corresponding to the initial cgroup namespace
91 (whose cgroup filesystem is indeed rooted at the parent directory of
92 sub). To fix this problem, we must remount the freezer cgroup filesys‐
93 tem from the new shell (i.e., perform the mount from a process that is
94 in the new cgroup namespace), after which we see the expected results:
95 sh2# mount --make-rslave / # Don't propagate mount events
97 # to other namespaces
98 sh2# umount /sys/fs/cgroup/freezer
99 sh2# mount -t cgroup -o freezer freezer /sys/fs/cgroup/freezer
100 sh2# cat /proc/self/mountinfo | grep freezer
101 155 145 0:32 / /sys/fs/cgroup/freezer rw,relatime ...
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104 Namespaces are a Linux-specific feature.
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107 Use of cgroup namespaces requires a kernel that is configured with the
108 CONFIG_CGROUPS option.
109 The virtualization provided by cgroup namespaces serves a number of
111 purposes:
112 * It prevents information leaks whereby cgroup directory paths outside
114 of a container would otherwise be visible to processes in the con‐
115 tainer. Such leakages could, for example, reveal information about
116 the container framework to containerized applications.
117 * It eases tasks such as container migration. The virtualization pro‐
119 vided by cgroup namespaces allows containers to be isolated from
120 knowledge of the pathnames of ancestor cgroups. Without such isola‐
121 tion, the full cgroup pathnames (displayed in /proc/self/cgroups)
122 would need to be replicated on the target system when migrating a
123 container; those pathnames would also need to be unique, so that they
124 don't conflict with other pathnames on the target system.
125 * It allows better confinement of containerized processes, because it
127 is possible to mount the container's cgroup filesystems such that the
128 container processes can't gain access to ancestor cgroup directories.
129 Consider, for example, the following scenario:
130 · We have a cgroup directory, /cg/1, that is owned by user ID 9000.
132 · We have a process, X, also owned by user ID 9000, that is names‐
134 paced under the cgroup /cg/1/2 (i.e., X was placed in a new
135 cgroup namespace via clone(2) or unshare(2) with the CLONE_NEWC‐
136 GROUP flag).
137 In the absence of cgroup namespacing, because the cgroup directory
139 /cg/1 is owned (and writable) by UID 9000 and process X is also owned
140 by user ID 9000, then process X would be able to modify the contents
141 of cgroups files (i.e., change cgroup settings) not only in /cg/1/2
142 but also in the ancestor cgroup directory /cg/1. Namespacing process
143 X under the cgroup directory /cg/1/2, in combination with suitable
144 mount operations for the cgroup filesystem (as shown above), prevents
145 it modifying files in /cg/1, since it cannot even see the contents of
146 that directory (or of further removed cgroup ancestor directories).
147 Combined with correct enforcement of hierarchical limits, this pre‐
148 vents process X from escaping the limits imposed by ancestor cgroups.
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151 unshare(1), clone(2), setns(2), unshare(2), proc(5), cgroups(7), cre‐
152 dentials(7), namespaces(7), user_namespaces(7)
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155 This page is part of release 5.04 of the Linux man-pages project. A
156 description of the project, information about reporting bugs, and the
157 latest version of this page, can be found at
158 https://www.kernel.org/doc/man-pages/.
159Linux 2019-08-02 CGROUP_NAMESPACES(7)