+6. Namespace
+
+6-1. Basics
+
+cgroup namespace provides a mechanism to virtualize the view of the
+"/proc/$PID/cgroup" file and cgroup mounts. The CLONE_NEWCGROUP clone
+flag can be used with clone(2) and unshare(2) to create a new cgroup
+namespace. The process running inside the cgroup namespace will have
+its "/proc/$PID/cgroup" output restricted to cgroupns root. The
+cgroupns root is the cgroup of the process at the time of creation of
+the cgroup namespace.
+
+Without cgroup namespace, the "/proc/$PID/cgroup" file shows the
+complete path of the cgroup of a process. In a container setup where
+a set of cgroups and namespaces are intended to isolate processes the
+"/proc/$PID/cgroup" file may leak potential system level information
+to the isolated processes. For Example:
+
+ # cat /proc/self/cgroup
+ 0::/batchjobs/container_id1
+
+The path '/batchjobs/container_id1' can be considered as system-data
+and undesirable to expose to the isolated processes. cgroup namespace
+can be used to restrict visibility of this path. For example, before
+creating a cgroup namespace, one would see:
+
+ # ls -l /proc/self/ns/cgroup
+ lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835]
+ # cat /proc/self/cgroup
+ 0::/batchjobs/container_id1
+
+After unsharing a new namespace, the view changes.
+
+ # ls -l /proc/self/ns/cgroup
+ lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183]
+ # cat /proc/self/cgroup
+ 0::/
+
+When some thread from a multi-threaded process unshares its cgroup
+namespace, the new cgroupns gets applied to the entire process (all
+the threads). This is natural for the v2 hierarchy; however, for the
+legacy hierarchies, this may be unexpected.
+
+A cgroup namespace is alive as long as there are processes inside or
+mounts pinning it. When the last usage goes away, the cgroup
+namespace is destroyed. The cgroupns root and the actual cgroups
+remain.
+
+
+6-2. The Root and Views
+
+The 'cgroupns root' for a cgroup namespace is the cgroup in which the
+process calling unshare(2) is running. For example, if a process in
+/batchjobs/container_id1 cgroup calls unshare, cgroup
+/batchjobs/container_id1 becomes the cgroupns root. For the
+init_cgroup_ns, this is the real root ('/') cgroup.
+
+The cgroupns root cgroup does not change even if the namespace creator
+process later moves to a different cgroup.
+
+ # ~/unshare -c # unshare cgroupns in some cgroup
+ # cat /proc/self/cgroup
+ 0::/
+ # mkdir sub_cgrp_1
+ # echo 0 > sub_cgrp_1/cgroup.procs
+ # cat /proc/self/cgroup
+ 0::/sub_cgrp_1
+
+Each process gets its namespace-specific view of "/proc/$PID/cgroup"
+
+Processes running inside the cgroup namespace will be able to see
+cgroup paths (in /proc/self/cgroup) only inside their root cgroup.
+From within an unshared cgroupns:
+
+ # sleep 100000 &
+ [1] 7353
+ # echo 7353 > sub_cgrp_1/cgroup.procs
+ # cat /proc/7353/cgroup
+ 0::/sub_cgrp_1
+
+From the initial cgroup namespace, the real cgroup path will be
+visible:
+
+ $ cat /proc/7353/cgroup
+ 0::/batchjobs/container_id1/sub_cgrp_1
+
+From a sibling cgroup namespace (that is, a namespace rooted at a
+different cgroup), the cgroup path relative to its own cgroup
+namespace root will be shown. For instance, if PID 7353's cgroup
+namespace root is at '/batchjobs/container_id2', then it will see
+
+ # cat /proc/7353/cgroup
+ 0::/../container_id2/sub_cgrp_1
+
+Note that the relative path always starts with '/' to indicate that
+its relative to the cgroup namespace root of the caller.
+
+
+6-3. Migration and setns(2)
+
+Processes inside a cgroup namespace can move into and out of the
+namespace root if they have proper access to external cgroups. For
+example, from inside a namespace with cgroupns root at
+/batchjobs/container_id1, and assuming that the global hierarchy is
+still accessible inside cgroupns:
+
+ # cat /proc/7353/cgroup
+ 0::/sub_cgrp_1
+ # echo 7353 > batchjobs/container_id2/cgroup.procs
+ # cat /proc/7353/cgroup
+ 0::/../container_id2
+
+Note that this kind of setup is not encouraged. A task inside cgroup
+namespace should only be exposed to its own cgroupns hierarchy.
+
+setns(2) to another cgroup namespace is allowed when:
+
+(a) the process has CAP_SYS_ADMIN against its current user namespace
+(b) the process has CAP_SYS_ADMIN against the target cgroup
+ namespace's userns
+
+No implicit cgroup changes happen with attaching to another cgroup
+namespace. It is expected that the someone moves the attaching
+process under the target cgroup namespace root.
+
+
+6-4. Interaction with Other Namespaces
+
+Namespace specific cgroup hierarchy can be mounted by a process
+running inside a non-init cgroup namespace.
+
+ # mount -t cgroup2 none $MOUNT_POINT
+
+This will mount the unified cgroup hierarchy with cgroupns root as the
+filesystem root. The process needs CAP_SYS_ADMIN against its user and
+mount namespaces.
+
+The virtualization of /proc/self/cgroup file combined with restricting
+the view of cgroup hierarchy by namespace-private cgroupfs mount
+provides a properly isolated cgroup view inside the container.
+
+
projects / cascardo / linux.git / blobdiff