Fault - Injector using UNIX ptrace Interface

The goal of this internal report is to summarize the experiences with the fault injector based on the Unix ptrace function. It should be explained how to use the fault injector for special experiments and how to interpret the results. In the first chapter (" ptrace(2) Interface ") an overview on the facilities of the Unix ptrace interface is given to see what faults/errors may be simulated by this interface. The limitations of this function are shown. The currently implemented faults which can be injected by the fault injector are enlisted in the second part of this paper (" Fault Model "). It is described how to implement other fault injecting functions to the fault injector program. The next chapter (" Measurements using the Fault Injector ") gives hints to the user how to interpret the results of the fault injector used in experiments. Many effects may influence the measurements. Some of them are discussed in this part of the paper. The syntax of the input and output of the fault injector is presented in the section " Command Language and Output Format ". Some examples are given to ease the use of this program. The fault injector was used to evaluate fault coverage and fault latency of some methods of fault tolerance. The experiences of this experiments and a summary of the advantages and disadvantages are given in the last section of this paper (" Experience "). Normally no Unix process has access to the internal state of another process. The processes are separated from each other by using different physical address ranges protected by the MMU and time shared use of the CPU. Direct access to hardware registers is prohibited for every process. The ptrace(2) interface of the Unix operating system was originally designed to allow debug-ging of single processes. A debugger must be able to read and modify the local and global variables and the program counter of his debuggee. The data flow between the debugger and the UNIX-OS Fault Injector Program Hardware Volkmar Sieh Fault Injector 30.11.93 3 tested program is checked by the kernel. So changes in the state of the debugged process cannot affect other running processes. The normal multiuser and multi-tasking facilities of the UNIX operating system are still provided. Access to the internal state of a process is granted only if the following conditions are met: 1. The tested process must agree …