Techniques for extending operating system subsystems are provided. The techniques involve using a subsystem of an operating system having a set of insertion points that identify points in the program logic of the subsystem where logic extensions may be added. The subsystem may receive a library that has functions comprising program logic for extending the subsystem and a list that provides a correspondence between the insertion points and the functions. When the subsystem receives a request to perform an action that has a corresponding insertion point for which the list provides an associated function, the subsystem may perform the function. The function may be indicated to be performed at various points in the program logic of the subsystem, including before or after the requested action.