The syscalls pledge and unveil were created on OpenBSD to easily limit what a process can do. The pledge syscall specifies what syscalls that process can access

The syscalls pledge and unveil were created on OpenBSD to easily limit what a process can do. The pledge syscall specifies what syscalls that process can access while the unveil syscall specifies what directories it can access. After calling each in a certain way it is no longer possible to call any syscall not allowed by pledge or access any directory not allowed by unveil. The idea is that every process should call each early when it starts running and, after that, if the process is ever compromised, it will not be able to do much harm since the files and syscalls it can interact with are limited. For example, a browser should never access /etc/passwd or call the exec syscall. So, a browser, when run, should as early as possible call pledge and unveil to prevent itself from accessing /etc/passwd or calling exec if it ever becomes compromised.