PID Namespace

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Basic Information

The PID (Process IDentifier) namespace is a feature in the Linux kernel that provides process isolation by enabling a group of processes to have their own set of unique PIDs, separate from the PIDs in other namespaces. This is particularly useful in containerization, where process isolation is essential for security and resource management.

When a new PID namespace is created, the first process in that namespace is assigned PID 1. This process becomes the "init" process of the new namespace and is responsible for managing other processes within the namespace. Each subsequent process created within the namespace will have a unique PID within that namespace, and these PIDs will be independent of PIDs in other namespaces.

From the perspective of a process within a PID namespace, it can only see other processes in the same namespace. It is not aware of processes in other namespaces, and it cannot interact with them using traditional process management tools (e.g., kill, wait, etc.). This provides a level of isolation that helps prevent processes from interfering with one another.

How it works:

When a new process is created (e.g., by using the clone() system call), the process can be assigned to a new or existing PID namespace. If a new namespace is created, the process becomes the "init" process of that namespace.
The kernel maintains a mapping between the PIDs in the new namespace and the corresponding PIDs in the parent namespace (i.e., the namespace from which the new namespace was created). This mapping allows the kernel to translate PIDs when necessary, such as when sending signals between processes in different namespaces.
Processes within a PID namespace can only see and interact with other processes in the same namespace. They are not aware of processes in other namespaces, and their PIDs are unique within their namespace.
When a PID namespace is destroyed (e.g., when the "init" process of the namespace exits), all processes within that namespace are terminated. This ensures that all resources associated with the namespace are properly cleaned up.

Lab:

Create Different Namespaces

CLI

sudo unshare -pf --mount-proc /bin/bash

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 are your process in

ls -l /proc/self/ns/pid
lrwxrwxrwx 1 root root 0 Apr  3 18:45 /proc/self/ns/pid -> 'pid:[4026532412]'

PID namespace

Let’s look at processes running in Container ubuntu1:

root@3a1bf12161c9:/# ps
  PID TTY          TIME CMD
    1 ?        00:00:00 bash
   15 ?        00:00:00 ps

Let’s look at processes running in Container ubuntu2:

root@8beb85abe6a5:/# ps
  PID TTY          TIME CMD
    1 ?        00:00:00 bash
   14 ?        00:00:00 ps

Let’s look at the 2 “bash” process in host machine:

$ ps -eaf|grep root | grep bash
root      5413  1697  0 05:54 pts/28   00:00:00 bash
root      5516  1697  0 05:54 pts/31   00:00:00 bash

bash process in Container1 and Container2 have the same PID 1 since they have their own process namespace. The same bash process shows up in host machine as a different pid.