Concepts with Symbian OS (779878), страница 41
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Authentication is the verification of user identity. As Chapter 14points out, verification of identity can happen in a number of ways;appropriate verification uniquely and correctly identifies users to theoperating system.Most protection mechanisms verify that file access is permissible byrecording several pieces of information with each file. These piecesusually include:• ownership : the user ID associated with the process that created thefile; this assumes that the operating system identifies users and canrelay that information to files• permission specification : if the owner of a file is recorded, thenownership permission is also recorded; other types of access can berecorded: access for non-owners or for members of the group that theowner belongs to• access control : when permissions are too broad to properly securea file, access control lists (ACLs) can be used to specify users (ratherthan groups) and specific permissions can be associated with certainusers• capabilities : when access control lists are too broad or require toomuch detail, users and applications are assigned a specific type ofcapability (for example, ‘file read’ and ‘file write’).
The capability isalways matched against the type of access requested. Capabilitiesare usually more detailed than permissions or access control. Forexample, ‘file delete’ might be a capability that is usually not assignedas a permission.There are many variations on these security mechanisms. Linux usespermissions for file security. In a Linux system, there are three typesof user: owners, members of the owner’s groups, and others. There areseveral types of file access in Linux.
The most useful access types are‘read’, ‘write’ and ‘execute’. The permissions can be used like bits in aSECURITY193number and are assigned to each of the three types of user. For example,a file may have the following listing:-rwx-----x 1 jipping 11001344 Jul 18 15:49 Presentation.pptThe first entry shows the permissions: owners (the rwx part) have allaccess rights, group members (the --- part) have no access rights and‘others’ (the --x part) have only execute permissions.Security on Symbian OSSmartphone security is an interesting variation on general computersecurity.
There are several aspects of smartphones that make security achallenge. Symbian OS has made several design choices that differentiateit from general-purpose desktop systems and other smartphone platforms.Consider the environment for smartphones. They are single-userdevices and require no user identification. A phone user can executeapplications, dial the phone and access networks all without identification.
In this environment, using permissions-based security is challenging,because the lack of identification means only one set of permissions ispossible – the same set for everyone.2Instead of user permissions, security often takes advantage of othertypes of information. In Symbian OS v9 and later, applications aregiven a set of capabilities when they are installed. (The process thatgrants capabilities to an application is covered in Chapter 14.) Thecapability set for an application is matched against the access that theapplication requests.
If the access is in the capability set, then accessis granted; otherwise, it is refused. Capability matching requires someoverhead – matching occurs at every system call that involves access toa resource – but the overhead of matching file ownership with a file’sowner is gone.
The tradeoff works well for Symbian OS.There are some other forms of file security on Symbian OS. There areareas of the Symbian OS storage medium that applications cannot accesswithout special capability. This special capability is only provided to theapplication that installs software onto the system. The effect of this is thatinstalled applications are protected from non-system access (meaning that2This does not mean that applications are barred from requesting user identification.This discussion only addresses system identification.194FILE SYSTEMS AND STORAGEnon-system malicious programs, such as viruses, cannot infect installedapplications).For Symbian OS, the use of capabilities has worked as well as fileownership for protecting access to files.8.5 SummaryThis chapter has examined how operating systems implement file systemsand files to store data. We began by examining the basic concepts of files,directories and partitioning.
We looked at attributes of files, the name andstructure of files and directories, and the operations that can be performedon them. We then looked at how file systems are implemented and gaveseveral examples – from FAT file systems used in Microsoft Windows tothe file system used in Unix. We gave an overview of some issues withusing file systems on mobile phones and finished the chapter by lookingat file-system security.Exercises1.Explain the difference between a text file and a binary file on aSymbian OS device.2.Consider a file that is 2000 bytes long stored on a medium with512-byte blocks. Assuming that the FCB is in memory, how manystorage I/O operations are required to read the entire file in each ofthe following systems?a. a contiguous storage schemeb.
a linked storage schemec. a FAT implementation schemed. a UFS implementation scheme.3.Consider a UFS design where there are 12 direct pointers, anindirect pointer, a double-indirect point and a triple-indirect pointeras described in Section 8.2. What is the largest possible file that canbe supported with this design?4.Why is it better to store items from the boot sector – root directoryFCB, operating system boot code, etc – on a storage medium ratherthan in the boot sector?EXERCISES1955.Consider a file system that uses linked allocation for file blocks.What benefits are gained by making these blocks adjacent whenthe allocation scheme remains as linked allocation?6.Is it beneficial to locate free space blocks next to each other?7.When defragmentation is attempted on a storage medium, thereare often areas of the medium that cannot be moved. Characterizethese areas and describe why they cannot be moved.8.Consider the situation when a smartphone shares RAM betweenoperating system memory and a file system.a.
What file-system implementation is likely to be used? Explain.b. How can a file system adapt to dynamically changing storagesizes (as operating system needs change, the file space growsand shrinks)?9.Why could Microsoft Windows 98, which used a FAT16 file system,only support partitions with a maximum size of 2 GB?10.If VFS and UFS were to be implemented for Symbian OS, woulda flash memory card that uses this combination have more or lessspace than a card formatted with a FAT16 file system?11.If a new file system were to be invented for the next version ofSymbian OS, what improvements could be made over the FAT16file system?12.Is fragmentation a problem on the storage medium used by SymbianOS? Why or why not?9Input and OutputA computer can do all the computing in the world, but it would be auseless device without input and output.
I think there are many peoplelike this. Sometimes people are thinkers; others like to talk (output) alot without listening (input); still others hear you (input) but do nottalk (output) to you. The most pleasant people to be with are typicallythose who listen, consider what you are saying and reply. Thus it iswith computers. While there are few computers that take input whileproducing absolutely no output and there are no computers that produceonly output (even computer-driven clocks need to be set with input),the most useful computers are those with general input and outputcapabilities.Operating systems must balance the needs for general computing withthe needs to process input and generate output.
It is very easy to get thesetasks out of balance; proper techniques are required to do all of them atthe same time. Management and control of input and output can be adifficult thing.This chapter discusses what is required to bring a balance to input andoutput. We have discussed related topics in other chapters, but here webring the pieces together. We first give an overview of I/O components.We then review I/O hardware and give examples of the concepts neededto manage I/O. Then we look at software issues connected to I/O.We discuss I/O in Symbian OS and conclude with some Symbian OSexamples.198INPUT AND OUTPUT9.1 I/O ComponentsAn operating system must manage input and output just like it managesthe other resources in a computer system.
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