計算機科學-hw操作系統(tǒng)概念課后習題答案-ch14

Chapter:Chapter14

MultipleChoice

1.IntheUNIXoperatingsystem,adomainisassociatedwiththe.

Ax

—

7user

B\

)

zprocess

c\

!

/procedure

D\

!

/task

A

Section:14.3.2

Difficulty:Easy

z

InMULTICS,theprotectiondomainsareorganizedina.

A\

l

Bzstarstructure

linearstructure

c\

!

D/ringstructure

directorystructure

Ans:C

Section:14.3.3

Difficulty:Easy

Inanaccessmatrix,therightallowsaprocesstochangetheentriesinarow.

A\

l

zowner

B\

7copy

cX

I

/control,

D\

l

zswitch

Ans:C

Section:14.4

Difficulty:Medium

4A.Theimplementationofanaccesstableconsistsofsetsoforderedtriples,

\

l

Bzglobaltable

x

)

czaccesslistforobjects

X

I

D/lock-keymechanism

\

1

/capabilitylist

Ans:A

Section:14.5.1

Difficulty:Easy

AIncapabilitylists,eachobjecthasatodenoteitstype,

gate

B\

)

ztag

c\

!

/key

D\

1

7lock

Ans:B

Section:14.5.3

Difficulty:Medium

6.Whichofthefollowingimplementationsoftheaccessmatrixisacompromisebetweentwo

otherimplementationslistedbelow?

A)accesslist

B)capabilitylist

C)globaltable

D)lock-key

Ans:D

Section:14.5

Difficulty:Medium

7.Inthereacquisitionschemeforimplementingtherevocationofcapabilities,.

A)akeyisdefinedwhenthecapabilityiscreated

B)thecapabilitiespointindirectly,notdirectly,totheobjects

C)alistofpointersismaintainedwitheachobjectthatpointtoallcapabilitiesassociatedwith

thatobject

D)capabilitiesareperiodicallydeletedfromeachdomain

Ans:D

Section:14.7

Difficulty:Medium

8.Whichofthefollowingisanadvantageofcompiler-basedenforcementofaccesscontrol?

A)Protectionschemesareprogrammedasopposedtosimplydeclared.

B)Protectionrequirementsaredependantofthefacilitiesprovidedbyaparticularoperating

system.

C)Themeansfbrenforcementneedstobeprovidedbythedesignerofthesubsystem.

D)Accessprivilegesarecloselyrelatedtothelinguisticconceptofadatatype.

Ans:D

Section:14.9.1

Difficulty:Difficult

9.Whichofthefollowingisatruestatementregardingtherelativemeritsbetweenaccess

rightsenforcementbasedsolelyonakernelasopposedtoenforcementprovidedlargelybya

compiler?

A)Enforcementbythecompilerprovidesagreaterdegreeofsecurity.

B)Enforcementbythekernelislessflexiblethanenforcementbytheprogramminglanguage

foruser-definedpolicy.

C)Kernel-basedenforcementhastheadvantagethatstaticaccessenforcementcanbeverified

off-lineatcompiletime.

D)Thefixedoverheadofkernelcallscannotoftenbeavoidedinacompiler-based

enforcement.

Ans:B

Section:14.9

Difficulty:Difficult

10.WhichofthefollowingistrueoftheJavaprogramminglanguageinrelationtoprotection?

A)Whenaclassisloaded,theJVMassignstheclasstoaprotectiondomainthatgivesthe

permissionsofthatclass.

B)Itdoesnotsupportthedynamicloadingofuntrustedclassesoveranetwork.

C)ItdoesnotsupporttheexecutionofmutuallydistrustingclasseswithinthesameJVM.

D)Methodsinthecallingsequencearenotresponsibleforrequeststoaccessaprotected

resource.

Ans:A

Section:14.14.9.2

Difficulty:Medium

Essay

11.Explainthemeaningofthetermobjectasitrelatestoprotectioninacomputersystem.

Whatarethetwogeneraltypesofobjectsinasystem?

Ans:Acomputersystemisacollectionofprocessesandobjects.Eachobjecthasaunique

namethatdifferentiatesitfromallotherobjectsinthesystem,andeachcanbeaccessedonly

throughwell-definedandmeaningfuloperations.Objectsareessentiallyabstractdatatypesand

includehardwareobjects(suchastheCPU,memorysegments,printer,anddisks)andsoftware

objects(suchasfiles,programs,andsemaphores).

Section:14.3

Difficulty:Medium

12.Aprocessissaidtooperatewithinaprotectiondomainwhichspecifiestheresourcesthat

theprocessmayaccess.Listthewaysthatadomaincanberealized.

Ans:Adomainmayberealizedwhereeachuser,process,orproceduremaybeadomain.In

thefirstcase,thesetofobjectsthatcanbeaccesseddependsontheidentityoftheuser.Inthe

secondcase,thesetofobjectsthatcanbeaccesseddependsupontheidentityoftheprocess.

Finally,thethirdcasespecifiesthatthesetofobjectsthatcanbeaccesseddependsonthelocal

variablesdefinedwiththeprocedure.

Section:14.3.1

Difficulty:Medium

13.Whatisanaccessmatrixandhowcanitbeimplemented?

Ans:Anaccessmatrixisanabstractmodelofprotectionwheretherowsrepresentdomainsand

thecolumnsrepresentobjects.Eachentryinthematrixconsistsofasetofaccessrights.Access

matricesaretypicallyimplementedusingaglobaltable,anaccesslistforobjects,acapabilitylist

fordomains,oralock-keymechanism.

Section:14.4

Difficulty:Difficult

14.Whatwasthemaindisadvantagetothestructureusedtoorganizeprotectiondomainsinthe

MULTICSsystem?

Ans:Theringstructurehadthedisadvantageinthatitdidnotallowtheenforcementofa

need-to-knowprinciple.Forexample,ifanobjectneededtobeaccessibleinonedomain,butnot

inanother,thenthedomainthatrequiredtheprivilegedinformationneededtobelocatedsuch

thatitwasinaringclosertothecenterthantheotherdomain.Thisalsoforcedeveryobjectin

theouterdomaintobeaccessiblebytheinnerdomainwhichisnotnecessarilydesired.

Section:14.3.3

Difficulty:Medium

15.Whyisaglobaltableimplementationofanaccessmatrixnottypicallyimplemented?

Ans:Theglobaltableimplementationsuffersfromacoupleofdrawbacksthatkeepitfrom

beingapopularimplementationtype.Thefirstdrawbackisthatthetableisusuallylargeand

cannotbestoredinmainmemory.Ifthetablecannotbestoredinmainmemory,extraI/Omust

beusedtoaccessthistable.Inaddition,aglobaltablemakesitdifficulttotakeadvantageof

specialgroupingsofobjectsordomains.

Section:14.5.1

Difficulty:Medium

16.Howdoesthelock-keymechanismforimplementationofanaccessmatrixwork?

Ans:Inalock-keymechanism,eachobjectisgivenalistofuniquebitpatterns,calledlocks.

Similarly,eachdomainhasalistofuniquebitpatterns,calledkeys.Aprocessinadomaincan

onlyaccessanobjectifthatdomainhasthematchingkeyforthelock.Usersarenotallowedto

examineormodifythelistofkeys(orlocks)directly.

Section:14.5.4

Difficulty:Medium

17.Whatisaconfinementproblem?

Ans:Aconfinementproblemistheproblemofguaranteeingthatnoinformationinitiallyheld

inanobjectcanmigrateoutsideofitsexecutionenvironment.Althoughcopyandownerrights

provideamechanismtolimitthepropagationofaccessrights,theydonotprovideappropriate

toolsforpreventingthepropagation(ordisclosure)ofinformation.Theconfinementproblem

isingeneralunsolvable.

Section:14.4

Difficulty:Medium

18.WhatisrightsamplificationwithrespecttotheHydraprotectionsystem?

Ans:Rightsamplificationallowscertificationofaprocedureastrustworthytoactonaformal

parameterofaspecifiedtypeonbehalfofanyprocessthatholdsarighttoexecutetheprocedure.

Therightsheldbythetrustworthyprocedureareindependentof,andmayexceed,therightsheld

bythecallingprocess.

Section:14.8.1

Difficulty:Medium

19.DescribethetwokindsofcapabilitiesinCAP.

Ans:Datacapabilitiesonlyprovidethestandardread,write,andexecuteoperationsofthe

individualstoragesegmentsassociatedwiththeobject.Datacapabilitiesareinterpretedbythe

microcodeintheCAPmachine.Softwarecapabilitiesareprotected,butnotinterpretedbythe

CAPmicrocode.Thesecapabilitiesareinterpretedbyaprotectedprocedurewhichmaybe

writtenbyanapplicationprogrammeraspartofasubsystem.

Section:

Difficulty:

20.ExplainhowJavaprovidesprotectionthroughtypesafety.

Ans:Java'sload-timeandrun-timechecksenforcetypesafetyofJavaclasses.Typesafety

ensuresthatclassescannottreatintegersasp