An Operating Systems Vade Mecum

226 File Structures Chapter 6machine 1152 3 46 7 81disk Adisk Bmachine 22 (5) 3 (9) 4 (13)196 7 8 10 11 12 14 15 162 3 410 11 12disk Cdisk Dmachine 311323414 15 16disk Edisk FFigure 6.17 Network File System with three machinesconvenient. Of course, the installation managers can decide to mount disks in a muchless symmetric fashion.Mounting disks remotely exacerbates the security problem that we touched on earlier.NFS defines the access rights of a subject making a remote access to be identical tothe rights for local access. That is, a single subject can own files on various machines,and those files may be accessed as if they were local. Therefore, even if accounts are notidentical on all machines, user identifiers must be unique across machines. It would bedisastrous for two individuals, having accounts on machine 1 and machine 2 respectively,to have the same user identifier: They would each have rights over the other’s files. Anyonewho has superuser privilege on one machine automatically has it on all machines.For this reason, NFS makes an exception to its rule and treats the superuser as an ordinarysubject with respect to remote files.