Questions and Answers of Operating System

Do an Internet search to find two real-life examples of virtual appliances.
Give a reason why an individual at home might be interested in virtualization.
Why do you think virtualization took so long to become popular? After all, the key paper was written in 1974 and IBM mainframes had the necessary hardware and software throughout the 1970s and beyond.
What is the difference between full virtualization and paravirtualization? Which do you think is harder to do? Explain your answer.
Can the USENET newsgroup system or the SETI@home project be considered distributed systems? (SETI@home uses several million idle personal computers to analyze radio telescope data to search for
When the TSL instruction is used for multiprocessor synchronization, the cache block containing the mutex will get shuttled back and forth between the CPU holding the lock and the CPU requesting it
Suppose that the TSL instruction was not available for synchronizing a multiprocessor. Instead, another instruction, SWP, was provided that atomically swapped the contents of a register with a word
In this problem you are to compute how much of a bus load a spin lock puts on the bus. Imagine that each instruction executed by a CPU takes 5 nsec. After an instruction has completed, any bus cycles
Affinity scheduling reduces cache misses. Does it also reduce TLB misses? What about page faults?
For each of the topologies of Fig. 8-16, what is the diameter of the interconnection network? Count all hops (host-router and router-router) equally for this problem.
Consider the double-torus topology of Fig. 8-16(d) but expanded to size k × k. What is the diameter of the network?
The bisection bandwidth of an interconnection network is often used as a measure of its capacity. It is computed by removing a minimal number of links that splits the network into two equal-size
Consider a multicomputer in which the network interface is in user mode, so only three copies are needed from source RAM to destination RAM. Assume that moving a 32-bit word to or from the network
What happens if three CPUs in a multiprocessor attempt to access exactly the same word of memory at exactly the same instant?
When transferring data from RAM to a network interface, pinning a page can be used, but suppose that system calls to pin and unpin pages each take 1 μ sec. Copying takes 5 bytes/nsec using DMA but
When a procedure is scooped up from one machine and placed on another to be called by RPC, some problems can occur. In the text, we pointed out four of these: pointers, unknown array sizes, unknown
In a DSM system, when a page fault occurs, the needed page has to be located. List two possible ways to find the page.
Consider the processor allocation of Fig. 8-24. Suppose that process H is moved from node 2 to node 3. What is the total weight of the external traffic now?
Some multicomputers allow running processes to be migrated from one node to another. Is it sufficient to stop a process, freeze its memory image, and just ship that off to a different node? Name two
Why is there a limit to cable length on an Ethernet network?
In Fig. 8-27, the third and fourth layers are labeled Middleware and Application on all four machines. In what sense are they all the same across platforms, and in what sense are they different?
Figure 8-30 lists six different types of service. For each of the following applications, which service type is most appropriate? (a) Video on demand over the Internet. (b) Downloading a Web page.
DNS names have a hierarchical structure, such as sales.general-widget.com. or cs.uni.edu One way to maintain the DNS database would be as one centralized database, but that is not done because it
In the discussion of how URLs are processed by a browser, it was stated that connections are made to port 80. Why?
If a CPU issues one memory request every instruction and the computer runs at 200 MIPS, about how many CPUs will it take to saturate a 400-MHz bus? Assume that a memory reference requires one bus
Migrating virtual machines may be easier than migrating processes, but migration can still be difficult. What problems can arise when migrating a virtual machine? Discuss.
When a browser fetches a Web page, it first makes a TCP connection to get the text on the page (in the HTML language). Then it closes the connection and examines the page. If there are figures or
When session semantics are used, it is always true that changes to a file are immediately visible to the process making the change and never visible to processes on other machines. However, it is an
When multiple processes need access to data, in what way is object-based access better than shared memory?
When a Linda in operation is done to locate a tuple, searching the entire tuple space linearly is very inefficient. Design a way to organize the tuple space that will speed up searches on all in
How is signal handling done in the model of Fig. 8-7?
When a system call is made in the model of Fig. 8-8, a problem has to be solved immediately after the trap that does not occur in the model of Fig. 8-7. What is the nature of this problem and how
Rewrite the enter region code of Fig. 2-22 using the pure read to reduce thrashing induced by the TSL instruction.
Multicore CPUs are beginning to appear in conventional desktop machines and laptop computers. Desktops with tens or hundreds of cores are not far off. One possible way to harness this power is to
Are critical regions on code sections really necessary in an SMP operating system to avoid race conditions or will mutexes on data structures do the job as well?
In the Amoeba scheme for protecting capabilities, a user can ask the server to produce a new capability with fewer rights, which can then be given to a friend. What happens if the friend asks the
If process-to-process messages were allowed in Fig. 9-11, what rules would apply to them? For process B in particular, to which processes could it send messages and which not?
Consider the steganographic system of Fig. 9-14. Each pixel can be represented in a color space by a point in the three-dimensional system with axes for the R, G, and B values. Using this space,
Break the following monoalphabetic cipher. The plaintext, consisting of letters only, is a well-known excerpt from a poem by Lewis Carroll. kfd ktbd fzm eubd kfd pzyiom mztx ku kzyg ur bzha kfthcm ur
Consider a secret-key cipher that has a 26 × 26 matrix with the columns headed by ABC ... Z and the rows also named ABC ... Z. Plaintext is encrypted two characters at a time. The first character is
Consider the following way to encrypt a file. The encryption algorithm uses two n-byte arrays, A and B. The first n bytes are read from the file into A. Then A[0] is copied to B[i], A[1] is copied to
Secret-key cryptography is more efficient than public-key cryptography, but requires the sender and receiver to agree on a key in advance. Suppose that the sender and receiver have never met, but
Suppose that two strangers A and B want to communicate with each other using secret key cryptography, but do not share a key. Suppose both of them trust a third party C whose public key is well
As Internet cafes become more widespread, people are going to want ways of going to one anywhere in the world and conducting business there. Describe a way to produce signed documents from one using
Natural-language text in ASCII can be compressed by at least 50% using various compression algorithms. Using this knowledge, what is the steganographic carrying capacity for ASCII text (in bytes) of
Suppose that a tightly knit group of political dissidents living in a repressive country are using steganography to send out messages to the world about conditions in their country. The government is
Not having the computer echo the password is safer than having it echo an asterisk for each character typed, since the latter discloses the password length to anyone nearby who can see the screen.
After getting your degree, you apply for a job as director of a large university computer center that has just put its ancient mainframe system out to pasture and switched over to a large LAN server
The Morris-Thompson protection scheme with n-bit random numbers (salt) was designed to make it difficult for an intruder to discover a large number of passwords by encrypting common strings in
Suppose the password file of a system is available to a cracker. How much extra time does the cracker need to crack all passwords if the system is using the Morris-Thompson protection scheme with
What is a covert channel? What is the basic requirement for a covert channel to exist?
Name three characteristics that a good biometric indicator must have in order to be useful as a login authenticator.
Authentication mechanisms are divided into three categories: Something the user knows, something the user has, and something the user is. Imagine an authentication system that uses a combination of
A computer science department has a large collection of UNIX machines on its local network. Users on any machine can issue a command of the form rexec machine4 who and have the command executed on
Is there any feasible way to use the MMU hardware to prevent the kind of overflow attack shown in Fig. 9-21? Explain why or why not.
Describe how stack canaries work and how they can be circumvented by the attackers.
The TOCTOU attack exploits race condition between the attacker and the victim. One way to prevent race conditions is make file system accesses transactions. Explain how this approach might work and
Name a C compiler feature that could eliminate a large number of security holes. Why is it not more widely implemented?
When a file is removed, its blocks are generally put back on the free list, but they are not erased. Do you think it would be a good idea to have the operating system erase each block before
How can a parasitic virus (a) ensure that it will be executed before its host program, and (b) pass control back to its host after doing whatever it does?
Some operating systems require that disk partitions must start at the beginning of a track. How does this make life easier for a boot-sector virus?
Change the program of Fig. 9-28 so that it finds all the C programs instead of all the executable files.
The virus of Fig. 9-33(c) has both a compressor and a decompressor. The decompressor is needed to expand and run the compressed executable program. What is the compressor for?
Name one disadvantage of a polymorphic encrypting virus from the point of view of the virus writer.
Often one sees the following instructions for recovering from a virus attack: 1. Boot the infected system. 2. Back up all files to an external medium. 3. Run fdisk (or a similar program) to format
Are companion viruses (viruses that do not modify any existing files) possible in UNIX? If so, how? If not, why not?
Self-extracting archives, which contain one or more compressed files packaged with an extraction program, are frequently used to deliver programs or program updates. Discuss the security implications
Suppose that a system has 5000 objects and 100 domains at some time. 1% of the objects are accessible (some combination of r, w and x) in all domains, 10% are accessible in two domains, and the
Section 9.10.1 describes a set of firewall rules that limit outside access to only three services. Describe another set of rules that you can add to this firewall to further restrict access to these
On some machines, the SHR instruction used in Fig. 9-38(b) fills the unused bits with zeros; on others the sign bit is extended to the right. For the correctness of Fig. 9-38(b), does it matter which
Describe three features that make Java a better programming language than C to write secure programs.
Describe three features that make Java a better programming language than C to write secure programs. Discuss.
Assume that your system is using JDK 1.2. Show the rules (similar to those in Figure 9-40) you will use to allow an applet from www.appletsRus.com to run on your machine. This applet may download
Explain which implementation of the protection matrix is more suitable for the following operations: (a) Granting read access to a file for all users. (b) Revoking write access to a file from all
Two different protection mechanisms that we have discussed are capabilities and access- control lists. For each of the following protection problems, tell which of these mechanisms can be used. (a)
Represent the ownerships and permissions shown in this UNIX directory listing as a protection matrix. (Note: asw is a member of two groups: users and devel; gmw is a member only of users.) Treat each
Express the permissions shown in the directory listing of the previous problem as access control lists.
Explain how writing UNIX in C made it easier to port it to new machines.
About how long does it take a traditional UNIX system to fork off a child process under the following conditions: text size = 100 KB, data size = 20 KB, stack size = 10 KB, task structure = 1 KB,
As multimegabyte programs became more common, the time spent executing the fork system call and copying the data and stack segments of the calling process grew proportionally. When fork is executed
A non-real-time Linux process has priority levels from 100 to 139. What is the default static priority and how is the nice value used to change this?
To what hardware concept is a signal closely related? Give two examples of how signals are used.
Why do you think the designers of Linux made it impossible for a process to send a signal to another process that is not in its process group?
A system call is usually implemented using a software interrupt (trap) instruction. Could an ordinary procedure call be used as well on the Pentium hardware? If so, under what conditions and how? If
In general, do you think daemons have higher or lower priority than interactive processes? Why?
When a new process is forked off, it must be assigned a unique integer as its PID. Is it sufficient to have a counter in the kernel that is incremented on each process creation, with the counter used
The POSIX interface defines a set of library procedures. Explain why POSIX standardizes library procedures instead of the system-call interface.
In every process' entry in the task structure, the PID of the parent is stored. Why?
The copy-on-write mechanism is used as an optimization in the fork system call, so that a copy of a page is created only when one of the processes (parent or child) tries to write on the page.
Two tasks A and B need to perform the same amount of work. However, task A has higher priority, and needs to be given more CPU time. Expain how will this be achieved in each of the Linux schedulers
Some UNIX systems are tickless, meaning they do not have periodic clock interrupts. Why is this done? Also, does ticklessness make sense on a computer (such as an embedded system) running only one
When booting Linux (or most other operating systems for that matter), the bootstrap loader in sector 0 of the disk first loads a boot program which then loads the operating system. Why is this extra
A certain editor has 100 KB of program text, 30 KB of initialized data, and 50 KB of BSS. The initial stack is 10 KB. Suppose that three copies of this editor are started simultaneously. How much
Why are open-file-descriptor tables necessary in Linux?
In Linux, the data and stack segments are paged and swapped to a scratch copy kept on a special paging disk or partition, but the text segment uses the executable binary file instead. Why?
Describe a way to use mmap and signals to construct an interprocess-communication mechanism.
Linux depends on gcc compiler to be ported to new architectures. Describe one advantage and one disadvantage of this dependency.
After the system call of the previous problem has been executed, the call munmap(65536, 8192) is carried out. Does it succeed? If so, which bytes of the file remain mapped? If not, why does it fail?
Can a page fault ever lead to the faulting process being terminated? If so, give an example. If not, why not?
Is it possible that with the buddy system of memory management it ever occurs that two adjacent blocks of free memory of the same size coexist without being merged into one block? If so, explain how.

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