ACCESS MATRIX IMPLEMENTATION AND COMPARISON By: Rushabh Dharwadkar Roll no: TE COMP

ACCESS MATRIX In an Access matrix, there are columns which represent different system resources and rows representing different protection domains. Entries within the matrix indicate what access that domain has to that resource. 2

ACCESS MATRIX 3

4 IMPLEMENTATION OF ACCESS MATRIX There four ways listed below in which Access Matrix can be implemented effectively. Global Table Access Lists for Objects Capability Lists for Domains Lock-Key Mechanism

GLOBAL TABLE This is the simplest implementation of the access matrix. Contains a set of ordered triples Whenever an operation M is executed on an object Oj within domain Di the global table is searched for a triple where M Є R k If this triple is found, the operation is allowed to continue or else an exception is raised. 5

GLOBAL TABLE  Drawbacks Table size is quite large so it cannot be kept in main memory so additional I/O is needed Virtual Memory techniques are used for managing this table Difficult to take advantage of special groupings I.e. If at all everyone can read a particular object, this object must have a separate entry in every domain. 6

ACCESS LISTS FOR OBJECTS When an operation M on an object O j is attempted in domain D i we search the access list for object O j, looking for an entry where M Є R k lookin if the entry is found, we allow the operation,if it is not we check the default set, we allow the access. Otherwise the access is denied, and an exception condition occurs 7

8 CAPABILITY LISTS FOR DOMAINS Capability can be secure pointer to object OS controls who can acquire references requires that pointers be opaque natural model for type-safe languages

CAPABILITY LISTS FOR DOMAINS Here we associate each row with each domain They are a kind of secure pointers An object is often represented by its physical name or address, called a Capability To execute an operation an operation M on object Oj the process executes the operation M, specifying the capability (or pointer) for Oj as a parameter Simple possession of the capability means that the access is allowed 9

CAPABILITY LISTS FOR DOMAINS Capabilities are usually distinguished from each other in two ways.. Each object has a tag to denote whether it’s a capability or accessible data Tags not directly accessible by application programs Hardware/firmware support used to enforce this restriction More than 1 bits used as a distinguishing attribute Hardware can distinguish integers, characters and so on and other values 10

CAPABILITY LISTS FOR DOMAINS Address space associated with a program can b split into 2 parts Part 1 Is accessible to the program and contains its normal instructions Part 2 Contains the capability list and is accessible only by the OS ……. Segmented memory space is useful in this approach 11

LOCK-KEY MECHANISM Compromise between access lists and capability lists Each object has a list of unique bit patterns called locks Each domain has a list of unique bit patterns called keys List of keys for domain must b managed by the OS Users are not allowed to examine or modify keys 12

COMPARISON 1. Global table Simple, but the table can b large and often cannot take advantage of special groupings of objects or domains Access lists correspond directly to the needs of users Determining set access rights for each domain is difficult Every access to the object must be checked, requiring a search of access list in large systems with long access lists, this is time consuming 13

COMPARISON 2. Capability Lists Do not correspond directly to the needs of users Useful for localizing information for a given process Process attempting access must present a capability for that access Protection system needs to verify if the capability is valid Revocation of capabilities inefficient 14

COMPARISON 3. Lock-key mechanism Compromise between access lists and capability lists Effective and flexible depending on the length of keys Keys passed freely from domain to domain Access privileges effectively revoked by changing some of the locks associated with the objects 15

COMPARISON Most systems use a combination of access lists and capabilities When a process tries to access an object, the access list is searched If access is denied an exception occurs Or else a capability is created and attached to the process Capability is used to demonstrate swiftly that access is allowed After last access capability is destroyed 16

Questions… 1. Explain access matrix as model of protection 2. How can a access matrix be implemented? 3. List and explain drawbacks of lock-key mechanism? 4. What are the disadvantages of capability lists? 17

Reference… Operating Systems Concepts 8 th edition (Galvin) Page no

Thank u 19