21 Due Thursday March 6 1 Introduction The nefarious Dr Evil has planted a slew of binary bombs on our public unix machines A binary bomb is a program that consists of a sequence of phases Each phase expects you to type a particular string on stdin ID: 74905
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Step2:DefuseYourBombYourjobforthislabistodefuseyourbomb.Youmustdotheassignmentononeofthepublicunixmachines.Infact,thereisarumorthatDr.Evilreallyisevil,andthebombwillalwaysblowupifrunelsewhere.Thereareseveralothertamper-proongdevicesbuiltintothebombaswell,orsowehear.Youcanusemanytoolstohelpyoudefuseyourbomb.Pleaselookatthehintssectionforsometipsandideas.Thebestwayistouseyourfavoritedebuggertostepthroughthedisassembledbinary.Eachtimeyourbombexplodesitnotiesthebomblabserver,andyoulose1/2point(uptoamaxof20points)inthenalscoreforthelab.Sothereareconsequencestoexplodingthebomb.Youmustbecareful!Therstfourphasesareworth10pointseach.Phases5and6arealittlemoredifcult,sotheyareworth15pointseach.Sothemaximumscoreyoucangetis70points.Althoughphasesgetprogressivelyhardertodefuse,theexpertiseyougainasyoumovefromphasetophaseshouldoffsetthisdifculty.However,thelastphasewillchallengeeventhebeststudents,sopleasedon'twaituntilthelastminutetostart.Thebombignoresblankinputlines.Ifyourunyourbombwithacommandlineargument,forexample,�linux./bombpsol.txtthenitwillreadtheinputlinesfrompsol.txtuntilitreachesEOF(endofle),andthenswitchovertostdin.Inamomentofweakness,Dr.Eviladdedthisfeaturesoyoudon'thavetokeepretypingthesolutionstophasesyouhavealreadydefused.Toavoidaccidentallydetonatingthebomb,youwillneedtolearnhowtosingle-stepthroughtheassemblycodeandhowtosetbreakpoints.Youwillalsoneedtolearnhowtoinspectboththeregistersandthememorystates.Oneoftheniceside-effectsofdoingthelabisthatyouwillgetverygoodatusingadebugger.Thisisacrucialskillthatwillpaybigdividendstherestofyourcareer.LogisticsThisisanindividualproject.Allhandinsareelectronic.Claricationsandcorrectionswillbepostedonthecoursemessageboard.HandinThereisnoexplicithandin.Thebombwillnotifyyourinstructorautomaticallyaboutyourprogressasyouworkonit.Youcankeeptrackofhowyouaredoingbylookingattheclassscoreboardat:http://butterfinger.cs.utexas.edu:15213/scoreboardThiswebpageisupdatedcontinuouslytoshowtheprogressforeachbomb.2 objdump-dUsethistodisassembleallofthecodeinthebomb.Youcanalsojustlookatindividualfunctions.Readingtheassemblercodecantellyouhowthebombworks.Althoughobjdump-dgivesyoualotofinformation,itdoesn'ttellyouthewholestory.Callstosystem-levelfunctionsaredisplayedinacrypticform.Forexample,acalltosscanfmightappearas:8048c36:e899fcffffcall80488d4&#x_ini;&#xt+0x;ᨀTodeterminethatthecallwastosscanf,youwouldneedtodisassemblewithingdb.stringsThisutilitywilldisplaytheprintablestringsinyourbomb.Lookingforaparticulartool?Howaboutdocumentation?Don'tforget,thecommandsapropos,man,andinfoareyourfriends.Inparticular,manasciimightcomeinuseful.infogaswillgiveyoumorethanyoueverwantedtoknowabouttheGNUAssembler.Also,thewebmayalsobeatreasuretroveofinformation.Ifyougetstumped,feelfreetoaskyourinstructorforhelp.4