CGroup Namespace

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Basic Information

A cgroup namespace is a Linux kernel feature that provides isolation of cgroup hierarchies for processes running within a namespace. Cgroups, short for control groups, are a kernel feature that allows organizing processes into hierarchical groups to manage and enforce limits on system resources like CPU, memory, and I/O.

While cgroup namespaces are not a separate namespace type like the others we discussed earlier (PID, mount, network, etc.), they are related to the concept of namespace isolation. Cgroup namespaces virtualize the view of the cgroup hierarchy, so that processes running within a cgroup namespace have a different view of the hierarchy compared to processes running in the host or other namespaces.

How it works:

When a new cgroup namespace is created, it starts with a view of the cgroup hierarchy based on the cgroup of the creating process. This means that processes running in the new cgroup namespace will only see a subset of the entire cgroup hierarchy, limited to the cgroup subtree rooted at the creating process's cgroup.
Processes within a cgroup namespace will see their own cgroup as the root of the hierarchy. This means that, from the perspective of processes inside the namespace, their own cgroup appears as the root, and they cannot see or access cgroups outside of their own subtree.
Cgroup namespaces do not directly provide isolation of resources; they only provide isolation of the cgroup hierarchy view. Resource control and isolation are still enforced by the cgroup subsystems (e.g., cpu, memory, etc.) themselves.

For more information about CGroups check:

{% content-ref url="../cgroups.md" %} cgroups.md {% endcontent-ref %}

Lab:

Create different Namespaces

CLI

sudo unshare -C [--mount-proc] /bin/bash

By mounting a new instance of the /proc filesystem if you use the param --mount-proc, you ensure that the new mount namespace has an accurate and isolated view of the process information specific to that namespace.

Error: bash: fork: Cannot allocate memory

If you run the previous line without -f you will get that error.
The error is caused by the PID 1 process exits in the new namespace.

After bash start to run, bash will fork several new sub-processes to do somethings. If you run unshare without -f, bash will have the same pid as the current "unshare" process. The current "unshare" process call the unshare systemcall, create a new pid namespace, but the current "unshare" process is not in the new pid namespace. It is the desired behavior of linux kernel: process A creates a new namespace, the process A itself won't be put into the new namespace, only the sub-processes of process A will be put into the new namespace. So when you run:

unshare -p /bin/bash

The unshare process will exec /bin/bash, and /bin/bash forks several sub-processes, the first sub-process of bash will become PID 1 of the new namespace, and the subprocess will exit after it completes its job. So the PID 1 of the new namespace exits.

The PID 1 process has a special function: it should become all the orphan processes' parent process. If PID 1 process in the root namespace exits, kernel will panic. If PID 1 process in a sub namespace exits, linux kernel will call the disable_pid_allocation function, which will clean the PIDNS_HASH_ADDING flag in that namespace. When linux kernel create a new process, kernel will call alloc_pid function to allocate a PID in a namespace, and if the PIDNS_HASH_ADDING flag is not set, alloc_pid function will return a -ENOMEM error. That's why you got the "Cannot allocate memory" error.

You can resolve this issue by use the '-f' option:

unshare -fp /bin/bash

If you run unshare with '-f' option, unshare will fork a new process after it create the new pid namespace. And run /bin/bash in the new process. The new process will be the pid 1 of the new pid namespace. Then bash will also fork several sub-processes to do some jobs. As bash itself is the pid 1 of the new pid namespace, its sub-processes can exit without any problem.

Copied from https://stackoverflow.com/questions/44666700/unshare-pid-bin-bash-fork-cannot-allocate-memory

Docker

docker run -ti --name ubuntu1 -v /usr:/ubuntu1 ubuntu bash

Check which namespace is your process in

ls -l /proc/self/ns/cgroup
lrwxrwxrwx 1 root root 0 Apr  4 21:19 /proc/self/ns/cgroup -> 'cgroup:[4026531835]'

Find all CGroup namespaces

{% code overflow="wrap" %}

sudo find /proc -maxdepth 3 -type l -name cgroup -exec readlink {} \; 2>/dev/null | sort -u
# Find the processes with an specific namespace
sudo find /proc -maxdepth 3 -type l -name cgroup -exec ls -l  {} \; 2>/dev/null | grep <ns-number>

{% endcode %}

Enter inside an CGroup namespace

nsenter -C TARGET_PID --pid /bin/bash

Also, you can only enter in another process namespace if you are root. And you cannot enter in other namespace without a descriptor pointing to it (like /proc/self/ns/cgroup).