=Aprogramdoessomethingusefulwithoutinteractingdirectlywiththeuser(sits - PDF document

DAEMONS  Daemon =Aprogramdoessomethingusefulwithoutinteractingdirectlywiththeuser(sitsinthe background )Technicallynotpartofthekernel,butstillpartoftheOSGoodexample: network(TCP)servers A(Unix)daemonisdifferentfromanormalprogramInparticular,aserverdoesnotinteractwithauserItcommunicatesinsteadwithotherprogramsmaybeoveranetworkItalsospawnsthreads/processes(whicharenotunderimmediateusercontrol)Oneisfacedthuswithabunchofnewissues,includingpreventinguserstoaffectserver'sexecutioninotherwaysthantheonesspeci-edprovidingamechanismfortheservertoreportstatusanderrorsresourcemanagementaccesscontrolandothersecurityissues CS409,FALL2013DAEMONS/1 DAEMONS=BACKGROUND Anormalprogramrunsin foreground Itisattachedtoa terminal (moregeneral,a“tty”)Itreceivesuser input fromthatterminalItprints output (using cout , printf ,...)and errormessages (using cerr , perror ,...)tothesameterminalAdaemonrunsin background IsnotattachedtoanyterminalInstead,itislauncheduponboot,maybeevenbeforeterminalsarebornThus,itdoesnotacceptuserinputItmustsendtheoutputtosomethingelsethanaterminal CS409,FALL2013DAEMONS/2 PROGRAMMINGADAEMON Theeasyway:putthedaemoninthebackgroundexplicitly bbserv-cbb.conf-fbbfile& Thehardway:thedaemonputsitselfintothebackgroundStartwithaprocessthatdoestheserverinitializationItprintswhatevermessagesitwants(totheterminal)Itthengoesinthebackgroundfortherestofthejobintmain(...){ Initializestuff(networkserver:socketbinding,preparationofthelesystem) intbgpid=fork();if(bgpid0){perror("startupfork");return1;}if(bgpid) //parentdies! return0; Childcontinuesandbecomesthedaemon } CS409,FALL2013DAEMONS/3 THEHARDWAY(CONT'D) OK,butwhy?Adaemonisstartedupbythe initprocess initstartsthedaemonsinaspecicorder–e.g.,remotelesystemaccessshouldbestartedbeforeanybodyneedsitinitcannotputeverythingintothebackgroundfromtheverystart–ithastomakesurethatthedaemonactuallystartedbeforemovingforwardOntheotherhand,ifthedaemonnevergetstothebackground,initnevergetsachancetogoaheadandstarttheotherservicesErgo,adaemonthatexpectstobelaunchedbyinit(theyallshould!)–sitsintheforegrounduntilitmakessurethatthestartupsucceeded–goesthenintobackgroundfortheactualwork CS409,FALL2013DAEMONS/4 TALKINGTODAEMONS WehaversttondtheprocessidofthedaemonprocessWedo psaux ,wegetalotoflineslikethisUSERPID%CPU%MEMVSZRSSTTYSTATSTARTTIMECOMMANDbruda133190.00.12572816pts/1S12:150:00bbservbbbandthenwehuntforourdaemonbetweenthemWedo psaux|grep name ,wegetonlythelinesthatcontain nameWealreadyhavethepid(useful!)–Buthow?Wecouldthensendsignalstothedaemon kill pid sendsSIGQUITto pid(which may terminate) kill-KILL pid sendsSIGKILLto pid(which will terminate) kill-HUP pid sendsSIGHUPto pid(whichnormallyrestarts) CS409,FALL2013DAEMONS/5 LONELYDAEMONS Daemonsarelonely.ItdoesnotmakesensetorunmultiplecopiesofadaemononthesamemachineHowdowepreventmultiplecopiestorun? Eachdaemonhasawell-knownassociatedlockleDifferentdaemonsusedifferentlockles,butadaemonwillalwaysusethesamelockleImmediatelyuponstartupthedaemontriestoacquirealockonthisleIfitsucceeds,itgoesaheadwiththerestIfitfails,itterminates(thereisanothercopyrunning)–Anerrormessagewouldbenicetoo...Whenthedaemonexits,itreleasesthelockontheleanddeletestheleLooselyspeaking,eachdaemonrunsinacriticalregionThelockleisalsoagoodplacetoholdtheprocessidofthedaemon! CS409,FALL2013DAEMONS/6 LONELYDAEMONS Daemonsarelonely.ItdoesnotmakesensetorunmultiplecopiesofadaemononthesamemachineHowdowepreventmultiplecopiestorun?Eachdaemonhasawell-knownassociated lockle Differentdaemonsusedifferentlockles,butadaemonwillalwaysusethesamelockleImmediatelyuponstartupthedaemontriestoacquirealockonthisleIfitsucceeds,itgoesaheadwiththerestIfitfails,itterminates(thereisanothercopyrunning)–Anerrormessagewouldbenicetoo...Whenthedaemonexits,itreleasesthelockontheleanddeletestheleLooselyspeaking,eachdaemonrunsinacriticalregion Thelockleisalsoagoodplacetoholdtheprocessidofthedaemon! CS409,FALL2013DAEMONS/6 LONELYDAEMONS Daemonsarelonely.ItdoesnotmakesensetorunmultiplecopiesofadaemononthesamemachineHowdowepreventmultiplecopiestorun?Eachdaemonhasawell-knownassociated lockle Differentdaemonsusedifferentlockles,butadaemonwillalwaysusethesamelockleImmediatelyuponstartupthedaemontriestoacquirealockonthisleIfitsucceeds,itgoesaheadwiththerestIfitfails,itterminates(thereisanothercopyrunning)–Anerrormessagewouldbenicetoo...Whenthedaemonexits,itreleasesthelockontheleanddeletestheleLooselyspeaking,eachdaemonrunsinacriticalregionThelockleisalsoagoodplacetoholdthe processidofthedaemon ! CS409,FALL2013DAEMONS/6 GRUMPYDAEMONS Exceptforthesignalstheylike,daemonsdonotwanttotalktoyouIfyouleavetheminthesatetypicalforanormalprogram,theymightevengetangryandrefusetodotheworkThishappenswhentheytrytoaccessstandardinput(descriptor0)Sowehaveto close descriptor0Whattheheck,weclose all thedescriptorsexceptstandardoutputandstandarderror! for(inti=0;igetdtablesize();i++)if(i!=1&&i!=2)close(i); Closingdescriptorsisveryimportant, wethuspreventthedaemonfromcon-sumingresourcesunnecessarily Butnotethatweclosethem beforeopeningbackthosedescriptorsweactuallyneed (suchaswhoknowswhatleonwhichthedaemondoesitsstuff)Closingdescriptor0doesnotmakeourdaemonhappythough!( why? ) CS409,FALL2013DAEMONS/7 GRUMPYDAEMONS(CONT'D) Thedaemonmaystilltrytoaccessdescriptor0ManylibraryfunctionsassumethattherstthreedescriptorsareopenWejustexchangeoneerrorforanother!Soweopendescriptor0againThistime,descriptor0willpointtoaspecialdevicewhichdoesnothing(“bitbucket”)Thisdeviceiscalled,suggestively, /dev/null –readingfrom/dev/nullalwaysreturnanendofle–anythingyouwriteto/dev/nullissimplydiscardedfor(inti=0;igetdtablesize();i++)if(i!=1&&i!=2)close(i); //Wecloseddescriptor0already,sothis//willbethefirstoneavailable!intfd=open("/dev/null",O_RDWR); CS409,FALL2013DAEMONS/8 DETACHEDDAEMONS EachUnixprocessinheritsaconnectiontoits controllingtty Auserthatstartedaprocesscancontrolitbyissuingappropriatecontrolcom-mandstothatprocess'controllingttyUnlikenormalprograms,daemonsshouldnotreceivesignalsgeneratedbythepro-cessthatstarteditSignalingfromthettytothepieceofcodethatstartsthedaemonisacceptable(sometimesdesired),signalingtothedaemonitselfisnotAdaemonmust detachitself fromthecontrollingtty #includesys/ioctl.h&#x-1.3;瘁intfd=open("/dev/tty",O_RDWR);ioctl(fd,TIOCNOTTY,0);close(fd); CS409,FALL2013DAEMONS/9 DETACHEDDAEMONSANDTHEIROUTPUT OK,sowehavenownoterminal,wheredoweputtheoutput? InitializationcodeoutputstowhateverisinheritedfromtheparentprocessWethenredirectstandardoutput(descriptor1)andstandarderror(descriptor2)toles//Wecloseeverything!!for(inti=getdtablesize()-1;i�=0;i--)close(i);intfd=open("/dev/null",O_RDWR);//Descriptor0//Wenowre-opendescriptors1and2,inthisorder:–Samele:fd=open("global-output-file",O_RDWR);dup(fd);–Differentles:fd=open("output-file",O_RDWR);fd=open("error-file",O_RDWR); CS409,FALL2013DAEMONS/10 DETACHEDDAEMONSANDTHEIROUTPUT OK,sowehavenownoterminal,wheredoweputtheoutput?InitializationcodeoutputstowhateverisinheritedfromtheparentprocessWethen redirect standardoutput(descriptor1)andstandarderror(descriptor2)to les //Wecloseeverything!!for(inti=getdtablesize()-1;i�=0;i--)close(i);intfd=open("/dev/null",O_RDWR);//Descriptor0//Wenowre-opendescriptors1and2,inthisorder:–Samele:fd=open("global-output-file",O_RDWR);dup(fd);–Differentles:fd=open("output-file",O_RDWR);fd=open("error-file",O_RDWR); CS409,FALL2013DAEMONS/10 DAEMONSDON'TLIKESIGNALS Thereisnosignalfromthecontrollingtty,butnonethelessadaemonmayreceivesignals(e.g.,fromyouwhenyouusethecommandkill)Somesignals(e.g.,SIGHUP,maybe)havesomemeaningtothedaemonOnesignal always hassomemeaningtoanyUnixprogram,namelySIGKILLSignalswithmeaningsshouldhaveassociated signalhandlers (exceptSIGKILL) signal( signal, handler-function); SomeothersignalsdonothaveanymeaningSignalsthatarenotneeded shouldbeignored Thereisapredenedfunctionthatdoesexactlythis:SIG_IGN signal( signal,SIG_IGN); CS409,FALL2013DAEMONS/11 SIGPIPE NotablesignalSenttoanetworkserverwhenaclientquitsunexpectedly(withoutshuttingdownthesocket)WhenunhandledaSIGPIPEbringsdownthewholeprocessAservermustnotdiewhenaclientmisbehavesErgo,thissignalshouldalwaysbeexplicitlyhandledignoringitisneformostapplications,sincethesocket also receivesanendofle CS409,FALL2013DAEMONS/12 DAEMONSARENOTGREGARIOUS Unixplaceseachprocessina processgroup ItcanthentreatasetofrelatedprocessesasoneentityAdaemoninheritsmembershipinaprocessgroup But :usually,adaemonoperatesindependentlyfromanyprocessgroupE.g.,itshouldnotreceivesignalssenttoitsparent'sgroupThedaemonmustthus leave itsparent'sgroup: setpgid( what-process, to-what-group); Theprocessidofthecurrentprocess(whichispassedtosetpgid)canbeobtainedbyusingthefunctiongetpidTocreateanew,privategroupwepass0assecondargumentofsetpgrp.Sowedo: setpgrp(getpid(),0); CS409,FALL2013DAEMONS/13 SECUREDAEMONS Daemonsmayrunwith rootprivileges Inotherwords,theycandowhatevertheypleasewithyoursystemSo youtheprogrammer havetomakesuretheydonotdothingsthatinterferewithnormalsystemoperationCarefulprogrammingisonewayofkeepingthematbayInparticular,itiscrucialthatyoucheckforarraybounds,andthatyoudonotaccessmemoryareasyoudonotownNotcheckingfortheseisthemostusualcauseforissuingsecurityupdates(andforpeoplecrackingintoyoursystem)ThisisofcourseacomplexproblemInaddition,youshouldbecarefulaboutwhatdaemonswritetodiskandwhere CS409,FALL2013DAEMONS/14 DAEMONSANDTHEIRDIRECTORIES Whenaprogramislaunched,itinheritsanenvironmentvariablecalledthe currentworkingdirectory Whenaprogramcreatesoropensale,itlooksinthiscurrentworkingdirectoryDaemonsarelaunchedbytheinitscript,whichworksinadirectorywhosecontentshouldnotbemodiedDaemonshavethishabittowriteondiskYoucanspecifythedirectorytheywriteintobyprovidingabsolutepathstoyourles But adaemonthatencountersanerrorconditionmight dumpcore (writetodiskamemoryimagefordebuggingpurposes...inthecurrentworkingdirectory!) But adaemonstartedbythesystemadministratorwillhavethecurrentdirectoryasthehomedirectoryoftheadministrator(verybad!) But adaemonworkinginsomedirectorywillpreventthatdirectorytobeunmountedevenifthedaemondoesnotreallyusethedirectoryforanything Conclusion: Youshouldmoveadaemontoa known,safedirectory .Youthendo: chdir("/"); CS409,FALL2013DAEMONS/15 CONFIDENTIALDAEMONS Somedatathatiswrittentolesislogdata,whichshouldbeinspectablebymanypeopleSomeotherdatashouldnotbeaccessibletoanybodyelse(e.g.,passwords)EachleinaUnixlesystemhasasetofpermissionsYoucanspecifyatcreationtimethepermissionsoftheleyoucreateYoucanalsospecifyasetofpermissionsthatwillneverbeset(the umask ) read permission setexecute permission set rxw others:group:owner: rwxr-xrw-111101110756 740 permissions for the file (declared): umask (denied permissions): actual permissions for the file: Bitwise AND with 037 CS409,FALL2013DAEMONS/16 SETTINGAUMASK Youdonotwanttorunintothepossibilityofcreatingaleownedbytheadministratorandwithallthepermissionsset(777)Notevenbychance!So,besidessettingsuitablepermissionsforeachleyoucreate,itisaverygoodideatoprovideasuitableumaskforthedaemonasawholeToseta(new)umask,youusethesystemcall umask Itisverycomfortabletoworkwithnumbersin octal whenyoudealwithleper-missions–Thiswayadigitcorrespondswithasetofpermissionsforagivengroupofusers–InC/C++aliteralintegerwhoserstdigitis0isconsideredtobeinbase8–Sowhenyoucallumask,itislikelythatyoudonotwanttowriteumask(137);butrather umask(0137); Alwayskeepinmindthattheumaskspeciespermissionsthatare denied CS409,FALL2013DAEMONS/17