The time namespace in Linux allows for per-namespace offsets to the system monotonic and boot-time clocks. It is commonly used in Linux containers to change the date/time within a container and adjust clocks after restoring from a checkpoint or snapshot.
Lab:
Create different Namespaces
CLI
sudounshare-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
When unshare is executed without the -f option, an error is encountered due to the way Linux handles new PID (Process ID) namespaces. The key details and the solution are outlined below:
Problem Explanation:
The Linux kernel allows a process to create new namespaces using the unshare system call. However, the process that initiates the creation of a new PID namespace (referred to as the "unshare" process) does not enter the new namespace; only its child processes do.
Running %unshare -p /bin/bash% starts /bin/bash in the same process as unshare. Consequently, /bin/bash and its child processes are in the original PID namespace.
The first child process of /bin/bash in the new namespace becomes PID 1. When this process exits, it triggers the cleanup of the namespace if there are no other processes, as PID 1 has the special role of adopting orphan processes. The Linux kernel will then disable PID allocation in that namespace.
Consequence:
The exit of PID 1 in a new namespace leads to the cleaning of the PIDNS_HASH_ADDING flag. This results in the alloc_pid function failing to allocate a new PID when creating a new process, producing the "Cannot allocate memory" error.
Solution:
The issue can be resolved by using the -f option with unshare. This option makes unshare fork a new process after creating the new PID namespace.
Executing %unshare -fp /bin/bash% ensures that the unshare command itself becomes PID 1 in the new namespace. /bin/bash and its child processes are then safely contained within this new namespace, preventing the premature exit of PID 1 and allowing normal PID allocation.
By ensuring that unshare runs with the -f flag, the new PID namespace is correctly maintained, allowing /bin/bash and its sub-processes to operate without encountering the memory allocation error.
sudofind/proc-maxdepth3-typel-nametime-execreadlink{} \; 2>/dev/null|sort-u# Find the processes with an specific namespacesudofind/proc-maxdepth3-typel-nametime-execls-l{} \; 2>/dev/null|grep<ns-number>
Enter inside a Time namespace
nsenter-TTARGET_PID--pid/bin/bash
Also, you can only enter in another process namespace if you are root. And you cannotenter in other namespace without a descriptor pointing to it (like /proc/self/ns/net).
