Fsconfig is to call SYS_FSCONFIG syscall.

GetUsernsFD returns a userns file descriptor.

IDMapMount calls mount_setattr syscall with a given userns fd.

ProcSyncExit is to notify parent that the child is going to exit.

ProcSyncReady is to notify parent that child is running with pdeathsig and parent can start to write uid/gid maps.

ProcIDMap holds Container ID to Host ID mappings used for User Namespaces in Linux.

ProcSyncType is used for synchronisation between parent and child process during setup user namespace mappings.