An Operating Systems Vade Mecum

Access control 199This arrangement is awkward for several reasons:If each capability list has an entry for all files, many entries will indicate that noaccess is allowed. This waste of space can be eliminated by listing only files overwhich the subject has privileges.The set of files accessible by one user may be very large. It can be expensive forthe file manager to check that a particular file is listed in a capability list. In particular,privileged personnel responsible for maintaining the computer installation arelikely to have very long capability lists.When a new user is granted access to the computer, an initial capability list mustbe generated for that new subject. It is not at all clear which files should be on theinitial list.2.3 Access listsThe other obvious way to partition the access matrix is by columns. Each piececorresponds to a particular object and lists all subjects and their privileges over thatobject. Such a piece is called an access list. The access lists corresponding to our accessmatrix are shown in Figure 6.6. The access list for a file might as well be stored with thefred/mailnewsletters/love.texteditorprog.textfredRWDREmuraliARERramonARRWEcheriARRDElottaAREmottlREmardukRWDRWDRWDRWEDRWDkealohaARERWDFigure 6.6 Access lists from Figure 6.4