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</details>
## 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.
1. 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**.
2. 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.
3.**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.
4. 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.
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](https://stackoverflow.com/questions/44666700/unshare-pid-bin-bash-fork-cannot-allocate-memory)
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**.
Note that the root use from the initial (default) PID namespace can see all the processes, even the ones in new PID names paces, thats why we can see all the PID namespaces.
When you enter inside a PID namespace from the default namespace, you will still be able to see all the processes. And the process from that PID ns will be able to see the new bash on the PID ns.
Also, you can only **enter in another process PID namespace if you are root**. And you **cannot****enter** in other namespace **without a descriptor** pointing to it (like `/proc/self/ns/pid`)
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