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1、TheELFObjectFileFmatbyDissectionMay011995ByEricYoungdaleTheExecutableLinkingFmathasbeenapopulartopiclatelyaspeopleaskwhythekernelconfigurationaskswhethernottoconfigureloadingELFexecutables.SinceELFwilleventuallybethecomm

2、onobjectfilefmatfLinuxbinariesitisappropriatetodocumentitabit.LastmonthEricintroducedustoELFthismonthhegivesusaguidedtourofrealELFfiles.LastmonthwereachedapointwherewerebeginningtodissectsomerealELFfiles.FthisIwillusethe

3、readelfutilitywhichIwrotewhenIwasfirsttryingtoundersttheELFfmatitself.LateronitbecameavaluabletoolfdebuggingthelinkerasIaddedsupptfELF.Thesourcestoreadelfshouldbeontsx11.mit.eduinpublinuxpackagesGCCsrcinpublinuxBETAibcs2

4、.Letusstartwithaverysimpleprogram—thehellowldprogramweusedlastmonth.largo%cathello.cmain()printf(“HelloWldn“)largo%gccelfchello.cOnmylaptopthegccelfcomminvokestheELFversionofgcc—onceELFbecomesthedefaultfmatyouwillbeablet

5、ousetheregulargcccommwhichproducestheELFfilehello.o.EachELFfilestartswithaheader(structelfhdrinusrincludelinuxelf.h)thereadelfutilitycandisplaythecontentsofallofthefields:largo%readelfhhello.oELFmagic:7f454c4601010100000

6、0000000000000Typemachineversion=131Entryphoffshoffflags=004400ehsizephentsizephnum=5200shentsizeshnumshstrndx=40118the.shstrtabsection.Letusdumpthesectiontablefthisfileseefigure1.Youwillnoticesectionsfnearlyeverythingwhi

7、chwehavealreadydiscussed.Eachsectionhasanidentifierwhichspecifieswhatthesectioncontains(ingeneralyoushouldneverhavetoactuallyknowthenameofasectioncompareittoanything).Afterthetypethereisaseriesofnumbers.Thefirstoftheseis

8、theaddressinvirtualmemywherethissectionshouldbeloaded.Sincethisisa.ofileitisnotintendedtobeloadedintovirtualmemythisfieldisnotfilledin.Nextistheoffsetwithinthefileofthesectionthenisthesizeofthesection.Afterthiscomeaserie

9、sofnumbers—Iwontparsetheseindetailfyoubuttheycontainthingsliketherequiredalignmentofthesectionasetofflagswhichindicatewhetherthesectionisreadonlywritableexecutable.Thereadelfprogramiscapableofperfmingdisassembly:largo%re

10、adelfi1hello.o0x00000000pushl%ebp0x00000001movl%esp%ebp0x00000003pushl$0x00x00000008call0x080075590x0000000daddl$4%esp0x00000010movl%ebp%esp0x00000012popl%ebp0x00000013retThe.rel.textsectioncontainstherelocationsfthe.tex

11、tsectionofthefilewecandisplaythemasfollows:largo%readelfrhello.elocationsectiondata:.rel.text(0x2entries)Tag:00004Value00301R_386_32(0)Tag:00009Value00b02R_386_PC32(0printf)Thisindicatesthatthe.textsectionhastworelocatio

12、ns.Asexpectedthereisarelocationfprintfindicatingthatwemustpatchtheaddressofprintfintooffset9fromthebeginningofthe.textsectionwhichhappenstobetheoperofthecallinstruction.Thereisalsoarelocationsothatwepassthecrectaddressto

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