What immediate action occurs in PAM if a module returns success and no previous module failed?

When a module in PAM (Pluggable Authentication Modules) returns success and no prior module has failed, the immediate action taken is to return success to the application. This means that if the authentication process is successfully verified by a module and there are no failures from any previous module, PAM will conclude the authentication process and relay the success outcome back to the application requesting authentication.

This behavior is fundamental as PAM is designed to allow for flexible management of authentication processes. Each module can independently determine success or failure based on a variety of authentication methods. When a success is reported, it signifies that the authentication has met the necessary criteria, and thereby, the process culminates in conveying that outcome to the application without executing additional modules. This efficiency is part of what makes PAM a robust framework for authentication in Linux systems.