Sending Signals in The Multi-Variant Execution System

As mentioned in the previous posts, multi-variant execution must be transparent to the variants. Also as mentioned previously, the variants must call the same system calls with equal or equivalent arguments.

The system calls which obtain the process ID (PID) of a running process (getpid), or its parent (getppid) or its children (fork), return a different value for each variant when executed normally. These IDs may be used later as arguments to other system calls. Different IDs in each variant make equivalence checking on the syscall arguments difficult. In order to make the equivalence checking more efficient, the monitor intercepts the ID returning syscalls before they return the results to the variants and replaces their results as though the syscalls were invoked by one of the variants. For example, the return value of getpid is replaced by the ID of the first variant and it seems that getpid returns the same ID, no matter which variant has called it.

While this method makes the equivalence checking fast and efficient, it can cause confusion in the system calls that use process IDs, such as wait4, waitpid, kill, .... In our system, these system calls are called with the same ID by all the variants. For example, if the variants want to send signals to their parents, they invoke kill with the ID of the first variant's parent. Obviously, if the system call is invoked as is, only the parent of the first variant receives the signal. To avoid this situation, the monitor keeps the real PID of all variants and their parents and children and when any of the system calls that use PIDs is encountered, the PID is compared to the variants real PIDs. If the PID is equal to that of one of the variants, it is replaced in each variant by the real PID of that variant before letting the system call get executed. If the PID is equal to the PID of one of the variants' parents, it is replaced in each variant by the PID of that variants' parent. A similar action is taken if the PID is equal to the PID of one the children. The only difference is that each process can have many children and the PID which replaces the syscall argument must be the PID of one of the children that corresponds to the child whose PID is reported to all the variants and is currently passed as the syscall argument.
After replacing the PID, the monitor lets the variants continue and invoke the system call.