Time Namespace

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Basic Information

The time namespace allows for per-namespace offsets to the system monotonic and boot-time clocks. The time namespace is suited for Linux containers usage for allowing the date/time to be changed within a container and for adjusting clocks within a container following restoration from a checkpoint/snapshot.

Lab:

Create different Namespaces

CLI

sudo unshare -T [--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/time
lrwxrwxrwx 1 root root 0 Apr  4 21:16 /proc/self/ns/time -> 'time:[4026531834]'

Find all Time namespaces

{% code overflow="wrap" %}

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

{% endcode %}

Enter inside a Time namespace

nsenter -T 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/net).