Need help to understand the difference of init parent process and setns parent process

[ningziwen]

Hi I'm new to runc and trying to learn runc from reading the docs and source code.

Looking into the method to start a process in runc, I found it creates and starts a parent process first.

If the Init flag is true, it starts a init parent process.
If the Init flag is false, it starts a setns parent process.

There are multiple differences between them but I noticed major difference of setns comparing to init is setns did a execSetns step.

In run command Init is true but in exec command Init is false.

Could anyone help me understand:

• Why is a parent process always needed?
• What are the exact differences between init and setns parent process?
• What does "Init" flag mean here?
• Why is Init true in run command but is false in exec command? What causes the difference?