2.1 Calling Mechanism of Operating SystemConsidering the stability and security of operating system, Linux kernel module adopts LSM (Linux Security Modules) mechanism to realize accessing and controlling between user mode and kernel mode through system call, and to drive hardware to satisfy the requests from user mode via system kernel [2,3]. As the number of single-function system calls is large, most programmers prefer to call API rather than choose system call in the developing process. API is the secondary packaging of system call, and one API can corresponds to the realization of several system calls, which helps API to obtain more powerful functions and to provide more convenient services for applications on operating system. Fig. 62.1 shows the relation among applications, APIs and kernels.