Information Technology — Programming languages — …...multiples of a byte address 3.1.3...

BaselineEdition TR24772–3

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ISO/IECJTC1/SC22/WG23N0770

DraftdocumentforworkinggroupreviewDate:2018-01-04

ISO/IECTR24772–3

Edition1

ISO/IECJTC1/SC22/WG23

Secretariat:ANSI

InformationTechnology—Programminglanguages—Guidancetoavoidingvulnerabilitiesinprogramminglanguages–Part3–VulnerabilitydescriptionsfortheprogramminglanguageC

Élémentintroductif—Élémentprincipal—Partien:Titredelapartie

Warning

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Contents Page

Foreword...............................................................................................................................................v

Introduction..........................................................................................................................................vi

1.Scope..................................................................................................................................................1

2.Normativereferences.........................................................................................................................1

3.Termsanddefinitions,symbolsandconventions................................................................................13.1Termsanddefinitions.......................................................................................................................1

4.Languageconcepts.............................................................................................................................6

5.AvoidingprogramminglanguagevulnerabilitiesinC...........................................................................7

6.SpecificGuidanceforCVulnerabilities................................................................................................86.1General.............................................................................................................................................86.2Typesystem[IHN]............................................................................................................................86.3Bitrepresentations[STR]..................................................................................................................96.4Floating-pointarithmetic[PLF].......................................................................................................106.5Enumeratorissues[CCB].................................................................................................................106.6Conversionerrors[FLC]...................................................................................................................126.7Stringtermination[CJM].................................................................................................................146.8Bufferboundaryviolation[HCB].....................................................................................................156.9Uncheckedarrayindexing[XYZ]......................................................................................................166.10Uncheckedarraycopying[XYW]...................................................................................................176.11Pointertypeconversions[HFC].....................................................................................................186.12Pointerarithmetic[RVG]...............................................................................................................186.13NULLpointerdereference[XYH]...................................................................................................196.14Danglingreferencetoheap[XYK].................................................................................................206.15Arithmeticwrap-arounderror[FIF]...............................................................................................216.16Usingshiftoperationsformultiplicationanddivision[PIK]...........................................................226.17Choiceofclearnames[NAI]..........................................................................................................226.18Deadstore[WXQ].........................................................................................................................236.19Unusedvariable[YZS]...................................................................................................................246.20Identifiernamereuse[YOW]........................................................................................................246.21Namespaceissues[BJL].................................................................................................................256.22Initializationofvariables[LAV].....................................................................................................256.23Operatorprecedenceandassociativity[JCW]...............................................................................256.24Side-effectsandorderofevaluationofoperands[SAM]...............................................................266.25Likelyincorrectexpression[KOA]..................................................................................................276.26Deadanddeactivatedcode[XYQ].................................................................................................286.27Switchstatementsandstaticanalysis[CLL]...................................................................................286.28Demarcationofcontrolflow[EOJ]................................................................................................296.29Loopcontrolvariables[TEX].........................................................................................................306.30Off-by-oneerror[XZH]..................................................................................................................31

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6.31Structuredprogramming[EWD]....................................................................................................326.32Passingparametersandreturnvalues[CSJ]..................................................................................336.33Danglingreferencestostackframes[DCM]...................................................................................346.34Subprogramsignaturemismatch[OTR].........................................................................................346.35Recursion[GDL]............................................................................................................................356.36Ignorederrorstatusandunhandledexceptions[OYB]..................................................................356.37Type-breakingreinterpretationofdata[AMV]..............................................................................366.38Deepvs.shallowcopying[YAN]....................................................................................................366.38.1Applicabilitytolanguage............................................................................................................366.39Memoryleak[XYL].......................................................................................................................376.40Templatesandgenerics[SYM]......................................................................................................376.41Inheritance[RIP]...........................................................................................................................376.42ViolationsoftheLiskovsubstitutionprincipleorthecontractmodel[BLP]..................................386.43Redispatching[PPH].....................................................................................................................386.44Polymorphicvariables[BKK].........................................................................................................386.45Extraintrinsics[LRM]....................................................................................................................386.46Argumentpassingtolibraryfunctions[TRJ]..................................................................................386.47Inter-languagecalling[DJS]...........................................................................................................396.48Dynamically-linkedcodeandself-modifyingcode[NYY]...............................................................396.49Librarysignature[NSQ].................................................................................................................406.50Unanticipatedexceptionsfromlibraryroutines[HJW]..................................................................406.51Pre-processordirectives[NMP].....................................................................................................406.52Suppressionoflanguage-definedrun-timechecking[MXB]..........................................................416.53Provisionofinherentlyunsafeoperations[SKL]............................................................................416.54Obscurelanguagefeatures[BRS]..................................................................................................426.55Unspecifiedbehaviour[BQF]........................................................................................................426.56Undefinedbehaviour[EWF]..........................................................................................................436.57Implementation–definedbehaviour[FAB]....................................................................................436.58Deprecatedlanguagefeatures[MEM]...........................................................................................446.59Concurrency–Activation[CGA]....................................................................................................446.60Concurrency–Directedtermination[CGT]....................................................................................456.61Concurrentdataaccess[CGX].......................................................................................................456.62Concurrency–Prematuretermination[CGS]................................................................................456.63Lockprotocolerrors[CGM]...........................................................................................................466.64UncontrolledFormatStrings[SHL]...............................................................................................46

7.LanguagespecificvulnerabilitiesforC..............................................................................................46

8.Implicationsforstandardization.......................................................................................................46

Bibliography.........................................................................................................................................49

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Foreword

ISO(theInternationalOrganizationforStandardization)andIEC(theInternationalElectrotechnicalCommission)formthespecializedsystemforworldwidestandardization.NationalbodiesthataremembersofISOorIECparticipateinthedevelopmentofInternationalStandardsthroughtechnicalcommitteesestablishedbytherespectiveorganizationtodealwithparticularfieldsoftechnicalactivity.ISOandIECtechnicalcommitteescollaborateinfieldsofmutualinterest.Otherinternationalorganizations,governmentalandnon-governmental,inliaisonwithISOandIEC,alsotakepartinthework.Inthefieldofinformationtechnology,ISOandIEChaveestablishedajointtechnicalcommittee,ISO/IECJTC1.

InternationalStandardsaredraftedinaccordancewiththerulesgivenintheISO/IECDirectives,Part2.

ThemaintaskofthejointtechnicalcommitteeistoprepareInternationalStandards.DraftInternationalStandardsadoptedbythejointtechnicalcommitteearecirculatedtonationalbodiesforvoting.PublicationasanInternationalStandardrequiresapprovalbyatleast75%ofthenationalbodiescastingavote.

Inexceptionalcircumstances,whenthejointtechnicalcommitteehascollecteddataofadifferentkindfromthatwhichisnormallypublishedasanInternationalStandard(“stateoftheart”,forexample),itmaydecidetopublishaTechnicalReport.ATechnicalReportisentirelyinformativeinnatureandshallbesubjecttorevieweveryfiveyearsinthesamemannerasanInternationalStandard.

Attentionisdrawntothepossibilitythatsomeoftheelementsofthisdocumentmaybethesubjectofpatentrights.ISOandIECshallnotbeheldresponsibleforidentifyinganyorallsuchpatentrights.

ISO/IECTR24772,waspreparedbyJointTechnicalCommitteeISO/IECJTC1,Informationtechnology,SubcommitteeSC22,Programminglanguages,theirenvironmentsandsystemsoftwareinterfaces.

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Introduction

ThisdocumentprovidesguidancefortheprogramminglanguageC,sothatapplicationdevelopersconsideringorusingCwillbebetterabletoavoidtheprogrammingconstructsthatleadtovulnerabilitiesandtheirattendantconsequences.Thisguidancecanalsobeusedbydeveloperstoselectsourcecodeevaluationtoolsthatcandiscoverandeliminatesuchconstructsintheirsoftware,orthedevelopersofsuchtools.

ThisdocumentisintendedtobeusedwithTR24772–1,whichdiscussesprogramminglanguagevulnerabilitiesinalanguageindependentfashion.

Itshouldbenotedthatthisdocumentisinherentlyincomplete.Itisnotpossibletoprovideacompletelistofprogramminglanguagevulnerabilitiesbecausenewweaknessesarediscoveredcontinually.Anysuchreportcanonlydescribethosethathavebeenfound,characterized,anddeterminedtohavesufficientprobabilityandconsequence.

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TechnicalReport ISO/IECTR24772:2015(E)

©ISO/IEC2017–Allrightsreserved 1

InformationTechnology—ProgrammingLanguages—Guidancetoavoiding

vulnerabilitiesinprogramminglanguages—Vulnerabilitydescriptionsfor

theprogramminglanguageC

1.Scope

Thisdocumentspecifiessoftwareprogramminglanguagevulnerabilitiestobeavoidedinthedevelopmentofsystemswhereassuredbehaviourisrequiredforsecurity,safety,mission-criticalandbusiness-criticalsoftware.Ingeneral,thisguidanceisapplicabletothesoftwaredeveloped,reviewed,ormaintainedforanyapplication.

Thisdocumentdescribesthewaythatthevulnerabilitieslistedinthelanguage-independentTR24772–1aremanifestedoravoidedintheClanguage.

2.Normativereferences

Thefollowingreferenceddocumentsareindispensablefortheapplicationofthisdocument.Fordatedreferences,onlytheeditioncitedapplies.Forundatedreferences,thelatesteditionofthereferenceddocument(includinganyamendments)applies.

ISO/IEC9899:2011—ProgrammingLanguages—CISO/IECTR24731-1:2007—ExtensionstotheClibrary—Part1:Bounds-checkinginterfacesISO/IECTR24731-2:2010—ExtensionstotheClibrary—Part2:DynamicAllocationFunctionsISO/IEC9899:2011/Cor.1:2012—Programminglanguages—C

ISO/IEC9945:2009--InformationTechnology--PortableOperatingSystemInterface(POSIX)withTC1:2013

3.Termsanddefinitions,symbolsandconventions

3.1Termsanddefinitions

Forthepurposesofthisdocument,thetermsanddefinitionsgiveninISO/IEC2382,inTR24772–1,in9899:2011andthefollowingapply.Othertermsaredefinedwheretheyappearinitalictype.

Thefollowingtermsareinalphabeticalorder,withgeneraltopicsreferencingtherelevantspecificterms.

access:readormodifythevalueofanobject

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Note:Modifyincludesthecasewherethenewvaluebeingstoredisthesameasthepreviousvalue.Expressionsthatarenotevaluateddonotaccessobjects

alignmentrequirementthatobjectsofaparticulartypebelocatedonstorageboundarieswithaddressesthatareparticularmultiplesofabyteaddress3.1.3

argument

expressioninthecomma-separatedlistboundedbytheparenthesesinafunctioncallexpression,orasequenceofpreprocessingtokensinthecomma-separatedlistboundedbytheparenthesesinafunction-likemacroinvocation

Note1:AlsocalledactualargumentNote2:Anargumentreplacesaformalparameterasthecallisrealized.

behaviourexternalappearanceoraction

Note:See:implementation-definedbehavior,locale-specificbehaviour,undefinedbehaviour,unspecifiedbehaviour

bitunitofdatastorageintheexecutionenvironmentlargeenoughtoholdanobjectthatmayhaveoneoftwovalues

Note:Itneednotbepossibletoexpresstheaddressofeachindividualbitofanobject

byteaddressableunitofdatastoragelargeenoughtoholdanymemberofthebasiccharactersetoftheexecutionenvironmentNote:Itispossibletoexpresstheaddressofeachindividualbyteofanobjectuniquely.Abyteiscomposedofacontiguoussequenceofbits,thenumberofwhichisimplementation-defined.Theleastsignificantbitiscalledthelow-orderbit;themostsignificantbitiscalledthehigh-orderbit.

characterabstractmemberofasetofelementsusedfortheorganization,control,orrepresentationofdata

Note:See:single-bytecharacter,multibytecharacter,widecharacter

correctlyroundedresult:representationintheresultformatthatisnearestinvalue,subjecttothecurrentroundingmode,towhattheresultwouldbegivenunlimitedrangeandprecision3.1.9

diagnosticmessage:messagebelongingtoanimplementation-definedsubsetoftheimplementation’smessageoutput

Note:TheCStandardrequiresdiagnosticmessagesforallconstraintviolations.

3.1.10

formalparameter:objectdeclaredaspartofafunctiondeclarationordefinitionthatacquiresavalueonentrytothefunction,oranidentifierfromthecomma-separatedlistboundedbytheparenthesesimmediatelyfollowingthemacronameinafunction-likemacrodefinition3.1.11

implementation:particularsetofsoftware,runninginaparticulartranslationenvironmentunderparticularcontroloptions,thatperformstranslationofprogramsfor,andsupportsexecutionoffunctionsin,aparticularexecutionenvironment3.1.12

implementation-definedbehaviour: behaviourwheremultipleoptionsarepermittedbythestandardandwhereeachimplementationdocumentshowthechoiceismade

Note:Anexampleofimplementation-definedbehaviouristhepropagationofthehigh-orderbitwhenasignedintegerisshiftedright.

3.1.13

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implementation-definedvalue:valuenotspecifiedinthestandardwhereeachimplementationdocumentshowthechoiceforthevalueisselected3.1.14

implementationlimit:restrictionimposedupontheprogrambytheimplementation3.1.15

indeterminatevalue:unspecifiedvalueoratraprepresentation3.1.16

locale-specificbehaviour:behaviourthatdependsonlocalconventionsofnationality,culture,andlanguagethateachimplementationdocuments

Note:Anexample,locale-specificbehaviouriswhethertheislower()functionreturnstrueforcharactersotherthanthe26lowercaseLatinletters

3.1.17

memorylocation: objectofscalar1type,oramaximalsequenceofadjacentbit-fieldsallhavingnonzerowidth3.1.18

multibytecharacter:sequenceofoneormorebytesrepresentingamemberoftheextendedcharactersetofeitherthesourceortheexecutionenvironment,wheretheextendedcharactersetisasupersetofthebasiccharacterset3.1.19

object:regionofdatastorageintheexecutionenvironment,thecontentsofwhichcanrepresentvalues3.1.20

1Integertypes,FloatingtypesandPointertypesarecollectivelycalledscalartypesintheCStandard

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parameter:actualargument,argument,orformalparameter3.1.21

recommendedpractice:specificationthatisstronglyrecommendedasbeinginkeepingwiththeintentoftheCStandard,butthatmaybeimpracticalforsomeimplementations3.1.22

runtime-constraint:requirementonaprogramwhencallingalibraryfunction3.1.23

single-bytecharacter:bitrepresentationthatfitsinabyte3.1.24

traprepresentation:objectrepresentationthatneednotrepresentavalueoftheobjecttype3.1.25

undefinedbehaviour:useofanon-portableorerroneousprogramconstructoroferroneousdata,forwhichtheCstandardimposesnorequirements

Note:Undefinedbehaviourrangesfromignoringthesituationcompletelywithunpredictableresults,tobehavingduringtranslationorprogramexecutioninadocumentedmannercharacteristicoftheenvironment(withorwithouttheissuanceofadiagnosticmessage),toterminatingatranslationorexecution(withtheissuanceofadiagnosticmessage).Anexampleof,undefinedbehaviouristhebehaviouronintegeroverflow.

3.1.26

unspecifiedbehaviour:useofanunspecifiedvalue,orotherbehaviourwheretheCStandardprovidestwoormorepossibilitiesandimposesnofurtherrequirementsonwhichischoseninanyinstance

Note:Forexample,unspecifiedbehaviouristheorderinwhichtheargumentsofafunctionareevaluated.

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3.1.27

unspecifiedvalue:validvalueoftherelevanttypewheretheCStandardimposesnorequirementsonwhichvalueischoseninanyinstance

Note:Anunspecifiedvaluecannotbeatraprepresentation.3.1.28

value:meaningofthecontentsofanobjectwheninterpretedashavingaspecifictype

Note:Seeimplementation-definedvalue,indeterminatevalue,unspecifiedvalue,traprepresentation

3.1.29

widecharacter:bitrepresentationcapableofrepresentinganycharacterinthecurrentlocaleNote:TheCStandardusesthenamewchar_tforobjectsofthistype

4.Languageconcepts

TheCprogramminglanguagewasdevelopedintheearly1970’satBellLabs,insupportofthedevelopmentoftheUnixoperatingsystem.Itsfirstpublishedspecificationwasin1978inthebook“TheCprogramminglanguage”[15].ThefirstISOstandardforCwaspublishedin1990andupdatedin1999and2011.

Cisanimperativelanguagethatsupportsstructuredprogrammingandhasastatictypesystem.Ithasoftenbeendescribedasa‘high-levelassembler’,inthatthesemanticgapbetweenaprogramandtheexecutablecodeissmall(asinatraditionalassembler),buthavingtheadvantagesofahigh-levellanguage:machineindependenceandstructuredprogrammingcontrolconstructs.

Thesmallsemanticgapbetweenprogramandexecutablecodemeansthattheresultingexecutablesarecompactandfast,makingCapopularlanguagefordevelopingoperatingsystemsandembeddedapplications.Thereisadesiretomaintainthisadvantageofthelanguage.Consequentlyasthelanguagehasdevelopedthereisastrategyofavoidingtheadditionofoverheadsthatdonotdirectlycontributetothebehavioroftheapplicationandtomaintainbackwardscompatibility,asembeddedsystemsinparticularcanbeindevelopmentandmaintenanceforaverylongtime.Thisdocumentproposesrestrictionsthatshouldbeimposedondevelopmentinanenvironmentwhererun-timefailureisunacceptable.Somekeyfeaturesofthelanguageare:

• DuetoCbeinga‘high-levelassembler’andhavingbeenaroundforlongerthanmostotherhigh-levellanguages,ithasbecomeacommonexchangeformatbetweenotherlanguages.Inparticular,manylanguagesimplementtheCfunctioncallingmodel(atleastasaselectableoption),sothatthirdpartylibrariescanbeusedinmanylanguageenvironments

• ChasaparticularlycloserelationshipwithC++.InitiallyC++wasastrictsupersetofC,withonlyoneexceptionofafeatureinCnotbeinginC++.Whilstovertheyearstherehasbeensomedivergence,therelationshipisstillclose

• AnunusualfeatureofCisthepreprocessor.Thisallowstextualmanipulationofthecodebeforethecompilerconsiderstheprogram.Itisusedto:allowchangestothecodetomatchspecificimplementationenvironments,implementin-linefunctionsandimplementcode‘short-cuts’byallowingcomponentstatementstobeconstructedthatwouldnotbesyntacticallylegalusingafunctiondefinition

• SinceC11,thelanguagehashadanativethreadingmodel.Previously,parallelismcouldonlybeachievedusingthird-partylibrariesnotincludedinthestandard

• Unlikesomeotherlanguages,Cusestheterms‘pointer’and‘reference’synonymously.Similarly,theterms‘passbyreference’,‘passbypointer’and‘passbyaddress’alsohavethesamemeaning

5.AvoidingprogramminglanguagevulnerabilitiesinCInadditiontothegenericprogrammingrulesfromTR24772-1clause5.4,additionalrulesfromthissectionapplyspecificallytotheCprogramminglanguage.Therecommendationsofthissectionarerestatementsofrecommendationsfromclause6ofthisdocument,butrepresentonesstatedfrequently,orthatareconsideredasparticularlynoteworthybytheauthors.Clause6ofthisdocumentcontainsthefullsetofrecommendations,aswellasexplanationsoftheproblemsthatledtotherecommendationsbeingmade.

Index Reference1 Useamacrotoensurethatthesizeofmemoryallocatedwithmallocmatchesthe

intendedtypeoftheobject

2 UseboundscheckinginterfacesfromAnnexKofC11[4]infavourofnon-boundscheckinginterfaces,suchasstrcpy_sinsteadofstrcpy.

3 UsecommonlyavailablefunctionssuchasthePOSIXfunctionshtonl(),htons(),ntohl()andntohs()toconvertfromhostbyteordertonetworkbyteorderandviceversa

4 Performrangecheckingbeforecopyingmemory(usingmechanismssuchasmemcpyandmemmove),unlessitcanbeshownthatarangeerrorcannotoccur.Boundscheckingisnotperformedautomatically,butintheinterestofspeedandefficiency,rangecheckingonlyneedstobedonewhenitcannotbestaticallyshownthatanaccessoutsideofthearraycannotoccur.

5 Checkthatapointerisnotnullbeforedereferencing,unlessitcanbeshownstaticallythatthepointercannotbenull.

6 Afteracalltofree,setthepointertonulltopreventmultipledeallocationoruseofadanglingreferenceviathispointer,asillustratedinthefollowingcode:

free(ptr);ptr=NULL;

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7 Donotreaduninitializedmemory,includingmemoryallocatedbyfunctionssuchasmalloc.

8 Checkthattheresultofanoperationonanunsignedintegervaluewillnotcausewrapping,unlessitcanbeshownthatwrappingcannotoccur.Anyofthefollowingoperatorshavethepotentialtowrap:

a+ba–ba*ba++++aa----aa+=ba-=ba*=ba<<ba<<=b-a

9 Checkthattheresultofanoperationonasignedintegervaluewillnotcauseanoverflow,unlessitcanbeshownthatoverflowcannotoccur.Anyofthefollowingoperatorshavethepotentialtooverflow,whichisundefinedbehaviorinC:

a+ba–ba*ba/ba%ba++++aa----aa+=ba-=ba*=ba/=ba%=ba<<ba<<=b-a

10 Ensurethatatypeconversionresultsinavaluethatcanberepresentedintheresultingtype.

6.SpecificGuidanceforCVulnerabilities

6.1General

ThisclausecontainsspecificadviceforCaboutthepossiblepresenceofvulnerabilitiesasdescribedinTR24772-1,andprovidesspecificguidanceonhowtoavoidtheminCcode.ThissectionmirrorsTR24772-1clause6inthatthevulnerability“TypeSystem[IHN]”isfoundin6.2ofTR24772–1,andCspecificguidanceisfoundinclause6.2andsubclausesinthisTR.

6.2Typesystem[IHN]

6.2.1ApplicabilitytolanguageCisastaticallytypedlanguage.InsomewaysCisbothstronglyandweaklytypedasitrequiresallvariablestobetyped,butsometimesallowsimplicitorautomaticconversionbetweentypes.Forexample,Ccanimplicitlyconvertalonginttoanintandpotentiallydiscardmanysignificantdigits.Notethatintegersizesareimplementationdefinedsothatinsomeimplementations,theconversionfromalonginttoanintwillnotdiscardanydigitssincetheyarethesamesize.Insomeimplementations,allintegertypescouldbeimplementedasthesamesize.Callowsimplicitconversionsasinthefollowingexample: short a = 1023; int b; b = a;

Ifanimplicitconversioncouldresultinalossofdatasuchasinaconversionfroma32-bitinttoa16-bitshortint: int a = 100000; short b; b = a;

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manycompilerswillissueawarningmessage.Chasasetofrulestodeterminehowconversionbetweendatatypeswilloccur.Forinstance,everyintegertypehasanintegerconversionrankthatdetermineshowconversionsareperformed.Therankingisbasedontheconceptthateachintegertypecontainsatleastasmanybitsasthetypesrankedbelowit.6.2.2Guidancetolanguageusers

• FollowtheadviceprovidedinTR24772-1subclause6.2.5.• Beawareoftherulesfortypingandconversionstoavoidvulnerabilities.• Donotcasttoaninappropriatetype.

6.3Bitrepresentations[STR]

6.3.1ApplicabilitytolanguageCsupportsavarietyofsizesforintegerssuchasshortint,int,longintandlonglongint.Eachmayeitherbesignedorunsigned.Calsosupportsavarietyofbitwiseoperatorsthatfacilitatebitmanipulations,suchasleftandrightshiftsandbitwise&and|.Somebitmanipulationscancauseunexpectedresultsthroughmiscalculatedshiftsorplatformdependentvariations.Forinstance,rightshiftingasignedintegerisimplementationdefinedinC,whileshiftingbyanamountgreaterthanorequaltothesizeofthedatatypeisundefinedbehaviour.Forinstance,onahostwhereanintisofsize32bits, unsigned int foo(const int k) { unsigned int i = 1; return i << k; }isundefinedforvaluesofkgreaterthanorequalto32.Thestoragerepresentationforinterfacingwithexternalconstructscanalsocauseunexpectedresults.Byteordersmaybeinlittle-endianorbig-endianformatandunknowinglyswitchingbetweenthetwocanunexpectedlyaltervalues.

6.3.2Guidancetolanguageusers

InadditiontothegeneraladviceofTR24772-1clause6.3.5:

• Onlyusebitwiseoperatorsonunsignedintegervaluesastheresultsofsomebitwiseoperationsonsignedintegersareimplementationdefinedorundefined

• Whereavailable,usefunctionssuchasthePOSIXstandardfunctionshtonl(),htons(),ntohl()andntohs()toconvertfromhostbyteordertonetworkbyteorderandviceversa.Thiswouldbeneededtointerfacebetweenani80x86architecture,wheretheLeastSignificantByteisfirst,andsomethingwithnetworkbyteorder,asusedontheInternet,wheretheMostSignificantByteisfirst.Usebitwiseoperationsonlyasalastresort.

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• Incaseswherethereisapossibilitythatashiftisgreaterthanthesizeofthevariable,performacheckasthefollowingexampleshows,oramoduloreductionbeforetheshift:

unsigned int i; unsigned int k; unsigned int shifted_i; … if (k < sizeof(unsigned int)*CHAR_BIT) shifted_i = i << k; else // handle error condition

6.4Floating-pointarithmetic[PLF]

6.4.1ApplicabilitytolanguageCpermitsthefloating-pointdatatypesfloat,doubleandlongdouble.Duetotheapproximatenatureoffloating-pointrepresentations,theuseoffloating-pointdatatypesinsituationswhereequalityistobetestedorwhereroundingcouldaccumulateovermultipleiterationsmayleadtounexpectedresultsandpotentialvulnerabilities.

Aswithmostdatatypes,Cisflexibleinhowfloat,doubleandlongdoublecanbeused.Forinstance,Callowstheuseoffloating-pointtypestobeusedasloopcountersandinequalitystatements,eventhoughinmostcasesthesewillnothavetheexpectedbehaviour.Forexample

float x; for (x=0.0; x!=1.0; x+=0.00000001)

mayormaynotterminateafter10,000,000iterations.Therepresentationsusedforxandtheaccumulatedeffectofmanyiterationsmaycause xtonotbeidenticalto1.0causingthelooptocontinuetoiterateforever.

Similarly,theBooleantest

float x=1.336f; float y=2.672f; if (x == (y/2))

mayormaynotevaluatetotrue.Giventhatxandyareconstantvalues,itisexpectedthatconsistentresultswillbeachievedonthesameplatform.However,itisquestionablewhetherthelogicperformsasexpectedwhenafloatthatistwicethatofanotheristestedforequalitywhendividedby2asabove.

6.4.2GuidancetolanguageusersFollowthegeneraladviceofTR24772-1clause6.4.5:

6.5Enumeratorissues[CCB]

6.5.1Applicabilitytolanguage

TheenumtypeinCcomprisesasetofnamedintegerconstantvaluesasintheexample: enum abc {A,B,C,D,E,F,G,H} var_abc;

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Deleted: .Eventhoughaloopmaybeexpectedtoonlyiterateafixednumberoftimes,dependingonthevaluescontainedinthefloating-pointtypeandontheloopcounterandterminationcondition,theloopcouldexecuteforever.Forinstanceiteratingatimesequenceusing10nanosecondsastheincrement:

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ThevaluesofthemembersofabcwouldbeA=0,B=1,C=2,andsoon.Callowsexplicitvaluestobeassignedtotheenumerationtypemembers,sothatthatmemberisassignedtheindicatedvalueandthenextmemberwilltakethenextvalue(unlessalsoexplicitlyassignedavalue).Sothedeclaration: enum abc {A,B,C=6,D,E,F=7,G,H} var_abc;isequivalentto: enum abc {A=0, B=1, C=6, D=7, E=8, F=7, G=8, H=9} var_abc;

Notethatthishasgapsinthesequenceofvaluesandrepeatedvalues.Thereareanumberofissuesthatcanarisewithenumerationtypes:

• Ctreatsenumerationmembersidenticallytointegers.Soanenumerationmembercanbeusedinanintegerexpression(usingitsassociatedvalue)andanintegercanbeassignedtoanenumerationtypeobject,evenifthereisnomemberassociatedwiththatvalue.Thisbecomesanissueifanenumerationtypeobjectisusedtocontrolaswitchstatement.Usingtheexampleabove2,iftheswitchhaseightcasestatements,forcase A:tocase H:thentherearetwoscenarioswheretheswitchmaynotbehaveasexpected:

o theusermayexpectallpossiblevaluestobecovered.However,ifthecontrolexpressionisavariableassignedH+1,thenthecodewill‘fallthough’,withoutexecutinganyofthecasestatements

o theaboveissuecanbeaddressedbyprovidingadefaultclause.However,inthesafetydomain,itiscommonpracticetoprovideadefaultclauseevenifthecode(apparently)canonlyeverhaveenumerationmembervaluesforthecontrolexpression.Theargumentisthatthisprotectsagainstunexpectedcorruptionofthecontrolvariable,saybyabuffetoverrun.However,ifthecompileralsothinksthecontrolvaluecanonlyeverbeoneoftheenumerationmembers,itispermittedtooptimizeawaythedefaultclause,meaningthattheexpectedprotectionmaynotexist.

• Thecodemayinitiallyhavebeenwrittenusingthedefaultassignmentofvalues(0..Numberofmembers–1).Ifanarrayisdeclaredwithbounds[Last_member + 1].Thishasoneelementforeachenumerationtypemember.Ifmaintenanceofthecodethenoccursthatmodifiestheassignmentofvalues,twoissuescanarise:

o amembermaybecreatedthathasavaluegreaterthanLast_member‘s,sotherewillbeundefinedbehaviorifthismemberisusedtoindexthearray

o thevaluescoveredbythemodifiedenumerationtypemembers,maynotformacontinuoussequencefrom0toNumberofmembers–1,witheithergapsinthesequenceorrepeatedvalues.Ifthemembersareusedtoinitializeandaccessthearray,thensomemembersofthearraywillremainuninitializediftherearegaps.Ifsomefinalprocessingisperformedonthearray,usinganintegercountfrom0toNumberofmembers–1,againthereislikelytobeundefinedbehavior.Iftherearerepeatedvalues,theresultisunlikelytobethatexpected.

2 defaultinitialization.TheexamplewhereDandFbothequal7wouldcauseacompilererrorduetorepeatedcaseselector.

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6.5.2GuidancetolanguageusersInadditiontothegeneraladviceofTR24772-1clause6.5.5:

• Enumerationtypedeclarationsshouldbeinoneofthefollowingthreeformats:o noexplicitvalues:

e.g. enum abc {A,B,C,D,E,F,G,H} var_abc;.o asingleexplicitvalueforthefirstmember:

e.g. enum abc {A=5,B,C,D,E,F,G,H} var_abc; o allvaluesexplicit:

e.g. enum abc { A=0, B=1, C=6, D=7, E=8, F=7, G=8, H=9} var_abc;

• Avoidusingloopsthatiterateoveranenumthathasrepresentationspecifiedfortheenums,unlessitcanbeguaranteedthattherearenogapsorrepetitionofrepresentationvalueswithintheenumdefinition.

• Useanenumeratedtypetoselectfromalimitedsetofchoicestomakepossibletheuseoftoolstodetectomissionsofpossiblevaluessuchasinswitchstatements.

• Ifa‘precautionary’defaultstatementisaddedtoswitchstatementcontrolledbyanenumerationtype,makethecontrollingobjectvolatile,sothecompilercannotoptimizeitaway(arguably,acompliantcompilershouldn’toptimizeitawayanyway,butanumberofexamplehavebeenfoundthatdo).

6.6Conversionerrors[FLC]

Cpermitsimplicitconversions.Thatis,Cwillautomaticallyperformaconversionwithoutanexplicitcast.Forinstance,Callows inti; floatf=1.25f; i=f;Thisimplicitconversionwilldiscardthefractionalpartoffandsetito1.IfthevalueoffisgreaterthanINT_MAX,thentheassignmentofftoiwouldbeundefined.TherulesforimplicitconversionsinCaredefinedintheCstandard.Forinstance,integertypessmallerthanintarepromotedwhenanoperationisperformedonthem.IfallvaluesofBoolean,characterorintegertypecanberepresentedasanint,thevalueofthesmallertypeisconvertedtoanint;otherwise,itisconvertedtoanunsignedint.

Comment [SGM5]: Delete“anyway”,replace“example”with“them”

Integerpromotionsareappliedaspartoftheusualarithmeticconversionstocertainargumentexpressions;operandsoftheunary+,-,and~operators,andoperandsoftheshiftoperators.Thefollowingcodefragmentshowstheapplicationofintegerpromotions: char c1, c2; c1 = c1 + c2;

Integerpromotionsrequirethepromotionofeachvariable(c1andc2)toint.Thetwointvaluesareaddedandthesumistruncatedtofitintothechartype.Integerpromotionsareperformedtoavoidarithmeticerrorsresultingfromtheoverflowofintermediatevalues.Forexample: signed char cresult, c1, c2, c3; c1 = 100; c2 = 3; c3 = 4; cresult = c1 * c2 / c3;

Inthisexample,thevalueofc1ismultipliedbyc2.Theproductofthesevaluesisthendividedbythevalueofc3(accordingtooperatorprecedencerules).Assumingthatsignedcharisrepresentedasan8-bitvalue,theproductofc1andc2(300)cannotberepresentedasasignedchar.However,becauseofintegerpromotions,c1,c2,andc3areeachconvertedtoint,andtheoverallexpressionissuccessfullyevaluated.Theresultingvalueistruncatedandstoredincresult.Becausethefinalresult(75)isintherangeofthesignedchartype,theconversionfromintbacktosignedchardoesnotresultinlostdata.Itispossiblethattheconversioncouldresultinalossofdatashouldthedatabelargerthanthestoragelocation.Alossofdata(truncation)canoccurwhenconvertingfromasignedtypetoanarrowersignedtype.Forexample,thefollowingcodecanresultintruncation: signed long int sl = LONG_MAX; signed char sc = (signed char)sl;

TheCstandarddefinesrulesforintegerpromotions,integerconversionrank,andtheusualarithmeticconversions.Theintentoftherulesistoensurethattheconversionsresultinconsistentnumericalvalues,andthatthesevaluesminimizesurprisesintherestofthecomputation.ArecentinnovationfromISO/IECTR24731-1[9]thathasbeenaddedtotheCstandard9899:2011[4]isthedefinitionofthersize_t type.Extremelylargeobjectsizesarefrequentlyasignthatanobject’ssizewascalculatedincorrectly.Forexample,negativenumbersappearasverylargepositivenumberswhenconvertedtoanunsignedtypelikesize_t.Also,someimplementationsdonotsupportobjectsaslargeasthemaximumvaluethatcanberepresentedbytypesize_t.Forthesereasons,itissometimesbeneficialtorestricttherangeofobjectsizestodetectprogrammingerrors.Forimplementationstargetingmachineswithlargeaddressspaces,itisrecommendedthatRSIZE_MAXbedefinedasthesmallerofthesizeofthelargestobjectsupportedor(SIZE_MAX >> 1),evenifthislimitissmallerthanthesizeofsomelegitimate,butverylarge,objects.ImplementationstargetingmachineswithsmalladdressspacesmaywishtodefineRSIZE_MAXasSIZE_MAX,whichmeansthatthereisnoobjectsizethatisconsideredaruntime-constraintviolation.

6.6.2GuidancetolanguageusersInadditiontothegeneraladviceofTR24772-1subclause6.6.5:

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• Checkthevalueofalargertypebeforeconvertingittoasmallertypetoseeifthevalueinthelargertypeiswithintherangeofthesmallertype.Anyconversionfromatypewithlargerrangetoasmallerrangecouldresultinalossofdata.Insomeinstances,thislossisdesired.Suchcasesshouldbeexplicitlyacknowledgedincomments.Forexample,thefollowingcodecouldbeusedtocheckwhetheraconversionfromanunsignedintegertoanunsignedcharacterwillresultinalossofdata:

unsigned int i; unsigned char c; … if (i <= UCHAR_MAX) { // check against the maximum value // for an object of type unsigned char c = (unsigned char) i; } else { // handle error condition }

• Closeattentionshouldbegiventoallwarningmessagesissuedbythecompilerregardingmultiplecasts.MakingacastinCexplicitwillbothremovethewarningandacknowledgethatthechangeinisintended.

• Ifmixedtypesareusedinanexpression,ensurethateachconversionpreservesthevaluebeforebeingusedasanoperandinanotheroperationinthesameexpression.

• Whenconvertingbetweenwidecharacterandmulti-bytecharactersandstrings,alwaysusetheappropriateconversionfunctions(wctombandwcsrtombsorwcsrtombs_srespectively).Similarlyformulti-bytetowidecharactersandstringsusembrtowcandmbsrtowcsormbsrtowcs_s

6.7Stringtermination[CJM]

AstringinCiscomposedofacontiguoussequenceofcharactersterminatedbyandincludinganullcharacter(abytewithallbitssetto0).ThereforestringsinCcannotcontainthenullcharacterexceptastheterminatingcharacter.Insertinganullcharacterinastringeitherthroughabugorthroughmaliciousactioncantruncateastringunexpectedly.Alternatively,notputtinganullcharacterterminatorinastringcancauseactionssuchasstringcopiestocontinuewellbeyondtheendoftheexpectedstring.Overflowingastringbufferthroughtheintentionallackofanullterminatingcharactercanbeusedtoexposeinformationortoexecutemaliciouscode.

• UsethesaferandmoresecurefunctionsforstringhandlingthataredefinedinnormativeAnnexK3fromISO/IEC9899:2011[4]ortheISOTR24731-2—PartII:Dynamicallocationfunctions.BothofthesedefinealternativestringhandlinglibraryfunctionstothecurrentStandardCLibrary.Thefunctionsverifythatreceivingbuffersarelargeenoughfortheresultingstringsbeingplacedinthemandensurethatresultingstringsarenullterminated.OneimplementationofthesefunctionshasbeenreleasedastheSafeCLibrary.

3 SeecommentscorrectuseofAnnexKfunctionsin6.8.1Bufferboundaryviolation[HCB]

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6.8Bufferboundaryviolation[HCB]

Abufferboundaryviolationconditionoccurswhenanarrayisindexedoutsideitsbounds,orpointerarithmeticresultsinanaccesstostoragethatoccursoutsidetheboundsoftheobjectaccessed.InC,thesubscriptoperator[]isdefinedsuchthatE1[E2]isidenticalto*(E1+E2)andtoE2[E1],sothatinallcasesthevalueinlocation(E1+E2)isreturned.Cdoesnotperformboundscheckingonarrays,sothefollowingcode: int foo(const int i) { int x[] = {0,0,0,0,0,0,0,0,0,0}; return x[i]; }

willreturnwhateverisinlocationx[i]evenif,iwereequalto-10or10(assumingeithersubscriptwasstillwithintheaddressspaceoftheprogram).Thiscouldbesensitiveinformationorevenareturnaddress,whichifalteredcouldchangetheprogramflow.Thefollowingcodeismoreappropriateandwouldnotviolatetheboundariesofthearrayx:

int foo( const int i) { int x[X_SIZE] = {0}; if ( (i < 0) || (i >= X_SIZE) ) { return ERROR_CODE; } else { return x[i]; } }

Abufferboundaryviolationmayalsooccurwhencopying,initializing,writingorreadingabufferifattentiontotheindexoraddressesusedarenottaken.Forexample,inthefollowingmoveoperationthereisabufferboundaryviolation:

char buffer_src[]={“abcdefg”}; char buffer_dest[5]={0}; strcpy(buffer_dest, buffer_src);

Thebuffer_srcislongerthanthebuffer_dest,andthecodedoesnotcheckforthisbeforetheactualcopyoperationisinvoked.Asaferwaytoaccomplishthiscopywouldbetousestrncpy,thatcanbelimitedtocopyamaximumnumberofcharacters:

char buffer_src[]={“abcdefg”]; char buffer_dest[5]={0}; strncpy(buffer_dest, buffer_src, sizeof(buffer_dest) -1); buffer_dest[sizeof(buffer_dest)-1] = 0;

thiswouldnotcauseabufferboundsviolation,however,becausethedestinationbufferissmallerthanthesourcebuffer,thedestinationbufferwillnowhold“abcd”.Notethatthefinalmemberofbuffer_dest isexplicitlyassignedtheterminatorvalue. strncpy doesnotautomaticallyterminatestringsiflongerthantheindicatednumberofcharacters,sothismanualassignmenttothelastcharacterofthedestinationbuffershouldalwaysbemade.

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AfurtheralternativeistousetheequivalentfunctionfromnormativeannexKofC11[4]‘Bounds-checkinginterfaces’:

char buffer_src[]={“abcdefg”]; char buffer_dest[5]={0}; if ( strcpy_s(buffer_dest, sizeof(buffer_dest), buffer_src) ) { /* Error Handler */ }

Ifthesourcestringincludingtheterminatorissmallerthantheindicateddestinationbuffersize,thenthesourcestringiscopiedtothedestinationbuffer.If,asintheexample,thesourcestringistoobig,thefirstelementofthedestinationstringisassigned0(i.e.thedestinationbecomesanemptystring).Notethatstrcpy_sandrelatedfunctionsreturn0onsuccessandanon-zerovalueonerrors.Whencallingthesefunction,theerrorvalueshouldalwaysbechecked.

• Validateallinputvalues.• Checkanyarrayindexbeforeuseifthereisapossibilitythevaluecouldbeoutsidetheboundsofthe

array.• Uselengthrestrictivefunctionssuchasstrncpy()insteadofstrcpy(),unlessitcanbeshownthe

destinationbufferisbigenough,andnotingtherequirementtoensuretodestinationstringisterminated.Alsonotethatthismayleadtotruncationofthesourcestring.

• Usestackguardingadd-onstodetectoverflowsofstackbuffers.• Donotusethedeprecatedfunctions,suchasgets().• UsethesaferandmoresecurefunctionsforstringhandlingfromthenormativeannexKofC11[4],

Bounds-checkinginterfaces,butalwayscheckeachcallforareturnederrorcondition.

6.9Uncheckedarrayindexing[XYZ]

Cdoesnotperformboundscheckingonarrays,soalthougharraysmaybeaccessedoutsideoftheirbounds,thevaluereturnedisundefinedandinsomecasesmayresultinaprogramtermination.Forexample,inCthefollowingcodeisvalid,though,forexample,ifihasthevalue10,theresultisundefined: int foo(const int i) { int t; int x[] = {0,0,0,0,0}; t = x[i]; return t; }

Thevariabletwilllikelybeassignedwhateverisinthelocationpointedtobyx[10] (assumingthatx[10]isstillwithintheaddressspaceoftheprogram).

Deleted: orotherlanguagefeatures

Deleted: <#>Beawarethattheuseofallofthesemeasuresmaystillnotbeabletostopallbufferoverflowsfromhappening.However,theuseofthemcanmakeitmuchrarerforabufferoverflowtooccurandmuchhardertoexploitit.

Deleted: <#>Thefunctionsverifythatoutputbuffersarelargeenoughfortheintendedresultandreturnafailureindicatoriftheyarenot.Optionally,failingfunctionscallaruntime-constraint

handlertoreporttheerror.Dataisneverwrittenpasttheendofanarray.Allstringresultsarenullterminated.Inaddition,thesefunctionsarere-entrant:theyneverreturnpointerstostaticobjectsownedbythefunction.AnnexKalsocontainsfunctionsthataddressinsecuritieswiththeCinput-outputfacilities.

• Performrangecheckingbeforeaccessinganarray.Intheinterestofspeedandefficiency,rangecheckingonlyneedstobedonewhenitcannotbestaticallyshownthatanaccessoutsideofthearraycannotoccur.

• UsethesaferandmoresecurefunctionsforstringhandlingfromthenormativeannexKofC11[4],Bounds-checkinginterfaces4.Thesearealternativestringhandlinglibraryfunctions.Thefunctionsverifythatreceivingbuffersarelargeenoughfortheresultingstringsbeingplacedinthemandensurethatresultingstringsarenullterminated.

6.10Uncheckedarraycopying[XYW]

Abufferoverflowoccurswhensomenumberofbytesiscopiedfromonebuffertoanotherandtheamountbeingcopiedisgreaterthanisallocatedforthedestinationbuffer.Intheinterestofeaseandefficiency,Clibraryfunctionssuchas memcpy(void * restrict s1, const void * restrict s2, size_t n)

and memmove(void *s1, const void *s2, size_t n)areusedtocopythecontentsfromoneareatoanother.memcpy()andmemmove()simplycopymemoryandnochecksaremadeastowhetherthedestinationareaislargeenoughtoaccommodatethenbytesofdatabeingcopied.Itisassumedthatthecallingroutinehasensuredthatadequatespacehasbeenprovidedinthedestination.Problemscanarisewhenthedestinationbufferistoosmalltoreceivetheamountofdatabeingcopiedoriftheindicesbeingusedforeitherthesourceordestinationarenottheintendedindices.Aseparateissueisthatmemcpyassumesthatthememoryblockspointedtobys1ands2arenon-overlapping.Ifthisassumptionisfalse,theprogram’sbehaviourisundefined.Thisrestrictiondoesnotapplytomemmove.

• Performrangecheckingbeforecallingamemorycopyingfunctionsuchasmemcpy()andmemmove().Thesefunctionsdonotperformboundscheckingautomatically.Intheinterestofspeedandefficiency,rangecheckingonlyneedstobedonewhenitcannotbestaticallyshownthatanaccessoutsideofthearraycannotoccur.

• UsethesaferandmoresecurefunctionsforstringhandlingfromthenormativeannexKofC11[4],Bounds-checkinginterfaces5.

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6.11Pointertypeconversions[HFC]

Callowscastingthevalueofapointertoandfromanotherdatatype.Theseconversionscancauseunexpectedchangestopointervalues.

Ifapointeriscasttoadifferenttypeandthenpointerarithmeticapplied(includingarrayindexing)thenthememoryaccessedmaynotbethatintended.Inparticularcastingapointertoastructtoabasictype(likeint)andthenattemptingtoexaminethemembersofthestructbyincrementingthepointermaynotgivetheexpectedresultsbecauseofthepossiblepresenceofpaddingbytes.Theonesafepointerconversionisfromapointertosomeobjecttypetovoid*andthenbacktotheoriginalpointertype.Thestandardguaranteesthistorestoretheoriginalpointer.Onespecificrecommendationisthatamacroisusedtoensurethatwhenmallocisusedtoallocatespaceforanobjectorarrayofaparticulartype,theresultofmallociscasttotheappropriatepointertype.ThatisforanobjectoftypeT: #define makeObjectOfTypeT(T) (T*)malloc(sizeof(T))

orforanarrayofNelements: #define makeArrayOfTypeT(T, N) (T*)malloc(sizeof(T) * N)

• FollowtheadviceprovidedbyTR24772-1clause6.11.5.• Maintainthesametypetoavoiderrorsintroducedthroughconversions.• Useamacrotocastthevaluereturnedbymalloctothecorrecttype• Heedcompilerwarningsthatareissuedforpointerconversioninstances.

6.12Pointerarithmetic[RVG]

WhenperformingpointerarithmeticinC,thesizeofthevaluetoaddtoapointerisautomaticallyscaledtothesizeofthetypeofthepointed-toobject.Forinstance,whenaddingavaluetothebyteaddressofa4-byteinteger,thevalueisscaledbyafactor4andthenaddedtothepointer.TheeffectofthisscalingisthatifapointerPpointstothei-thelementofanarrayobject,then(P)+Nwillpointtothei+n-thelementofthearray.Failingtounderstandhowpointerarithmeticworkscanleadtomiscalculationsthatresultinseriouserrors,suchasbufferoverflows.

InC,arrayshaveastrongrelationshiptopointers.ThefollowingexamplewillillustratearithmeticinCinvolvingapointerandhowtheoperationisdonerelativetothesizeofthepointer'starget.Considerthefollowingcodesnippet: int buf[5]; int *buf_ptr = buf;

Deleted: PointersinCrefertoaspecifictype,suchasinteger.Ifsizeof(int)is4bytes,andptr isapointertointegersthatcontainsthevalue0x5000,thenptr++wouldmakeptrequalto0x5004.However,ifptrwereapointertochar,thenptr++wouldmakeptrequalto0x5001.Itisthedifferenceduetodatasizescoupledwithconversionsbetweenpointerdatatypesthatcauseunexpectedresultsandpotentialvulnerabilities.Duetoarithmeticoperations,pointersmaynotmaintaincorrectmemoryalignmentormayoperateuponthewrongmemoryaddresses.

Deleted: Inparticular,makecastsexplicitinthereturnvalueofmalloc ... [6]Comment [SGM7]: Inotherplaceswesay“InadditiontotheadviceprovidedbyTR24772-1clause6.11.5”followedbythebullets.

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wheretheaddressofbufis0x1234,aftertheassignmentbuf_ptrpointstobuf[0].Adding1tobuf_ptrwillresultinbuf_ptr == 0x1238onahostwhereanintis4bytes;buf_ptrwillthenpointtobuf[1].Notrealizingthataddressoperationswillbeintermsofthesizeoftheobjectbeingpointedtocanleadtoaddressmiscalculationsandundefinedbehaviour.Indexinganarrayisimplementedbypointerarithmetic,sothataccessinganarrayelementarray[n]createsapointerequivalenttoarray+nandaccessingthememoryatthataddress6.12.2Guidancetolanguageusers

• FollowtheadviceprovidedbyTR24772-1clause6.12.5.• Consideranoutrightbanonpointerarithmetic(otherthanbyuseoftheindexoperator)duetoitserror-

pronenature.• Verifythatallpointersareassignedavalidmemoryaddressforuse.

6.13NULLpointerdereference[XYH]

6.13.1ApplicabilitytolanguageCallowsmemorytobedynamicallyallocatedprimarilythroughtheuseofmalloc(),calloc(),andrealloc().Eachwillreturntheaddresstotheallocatedmemory.Duetoavarietyofsituations,thememoryallocationmaynotoccurasexpectedandanullpointerwillbereturned.Otheroperationsorfaultsinlogiccanresultinamemorypointerbeingsettonull.Usingthenullpointerasthoughitpointedtoavalidmemorylocationcausesundefinedbehaviour.Spacefor10000integerscanbedynamicallyallocatedinCinthefollowingway: int *ptr = malloc(10000*sizeof(int)); // allocate space for 10000 ints

malloc()willreturntheaddressofthememoryallocatedoranullpointerifinsufficientmemoryisavailablefortheallocation.ItisgoodpracticeaftertheattemptedallocationtocheckwhetherthememoryhasbeenallocatedviaaniftestagainstNULL: if (ptr != NULL) // check to see that the memory could be allocated

Memoryallocationsusuallysucceed,soneglectingthistestandusingthememorywillusuallywork.Thatiswhyneglectingthenulltestwillfrequentlygounnoticed.Anattackercanintentionallycreateasituationwherethememoryallocationwillfailleadingtoundefinedbehaviour.

• FollowtheadviceprovidedbyTR24772-1clause6.13.5.• Createaspecificcheckthatapointerisnotnullbeforedereferencingit.Asthiscanbeexpensiveinsome

cases(suchasinaforloopthatperformsoperationsoneachelementofalargesegmentofmemory),judiciouscheckingofthevalueofthepointeratkeystrategicpointsinthecodeisrecommended.

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6.14Danglingreferencetoheap[XYK]

Callowsmemorytobedynamicallyallocatedprimarilythroughtheuseofofmalloc(),calloc(),andrealloc(). Callowsaconsiderableamountoffreedominaccessingthedynamicmemory.Pointerstothedynamicmemorycanbecreatedtoperformoperationsonthememory.Oncethememoryisnolongerneeded,itcanbereleasedthroughtheuseoffree().However,freeingthememorydoesnotpreventtheattempteduseofthepointerstothememoryandissuescanariseifoperationsareperformedaftermemoryhasbeenfreed.Considerthefollowingsegmentofcode: int foo() { int *ptr = malloc (100*sizeof(int));/* allocate space for 100 integers */ if (ptr != NULL) { /* check to see that the memory could be allocated */ /* perform some operations on the dynamic memory */ free (ptr); /* memory is no longer needed, so free it */ /* program continues performing other operations */ ptr[0] = 10; /* ERROR – memory being used after released */ … } … }

TheuseofmemoryinCafterithasbeenfreedisundefinedbehaviour.Dependingontheexecutionpathtakenintheprogram,freedmemorymayhavebeenreallocatedviaanothercallofmalloc()orotherdynamicmemoryallocation.Ifthememoryhasnotbeenreallocated,useofthememorymaybeunnoticed.However,ifthememoryhasbeenreallocated,alteringofthedatacontainedinthememorywillalmostcertainlyresultindatacorruption.Determiningthatadanglingmemoryreferenceisthecauseofaproblemandlocatingitcanbedifficult.Settingandusinganotherpointertothesamesectionofdynamicallyallocatedmemorycanalsoleadtoundefinedbehaviour.Considerthefollowingsectionofcode: int foo() { int *ptr = malloc (100*sizeof(int));/* allocate space for 100 integers */ if (ptr != NULL) { /* check to see that the memory could be allocated */ int ptr2 = &ptr[10]; /* set ptr2 to point to the 10th element of the allocated memory */ … /* perform some operations on the memory */ free (ptr); /* memory is no longer needed */ ptr = NULL; /* set ptr to NULL to prevent ptr from being used again */ … /* program continues performing other operations */ ptr2[0] = 10; /* ERROR – memory is being used after it has been released via ptr2 */ …

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} return (0); }

Dynamicmemorywasallocatedviaamalloc()andthenlaterinthecode,ptr2wasusedtopointtoanaddressinthedynamicallyallocatedmemory.Afterthememorywasfreedusingfree(ptr)andthegoodpracticeofsettingptrtoNULLwasfollowedtoavoidadanglingreferencebyptrlaterinthecode,adanglingreferencestillexistedusingptr2.6.14.2Guidancetolanguageusers

• FollowtheadviceprovidedbyTR24772-1clause6.14.2.• SetafreedpointertoNULLimmediatelyafterafree()call,asillustratedinthefollowingcode:

free (ptr); ptr = NULL;

• Donotcreateanduseadditionalpointerstodynamicallyallocatedmemory.

6.15Arithmeticwrap-arounderror[FIF]

6.15.1ApplicabilitytolanguageGiventhefixedsizeofintegerdatatypes,continuouslyaddingtoanunsignedintegereventuallyresultsinavaluethatcannotberepresented.ForCthisisdefinedto‘wraparound’,soaddingonetothemaximumpositivevalueresultsinzero.Thishappenswithoutanydetectionornotificationmechanism.Continuouslyaddingtoasignedintegeruntilitreachesavaluethatcannotberepresentedresultsinundefinedbehaviour.Similarly,repeatedlysubtractingfromanunsignedintegerleadstowraparound,orundefinedbehaviourforsignedintegers.Forexample,considerthefollowingcodeforashort intcontaining16bits: int foo( short int i ) { i++; return i; }

Callingfoowiththevalueof32767wouldcauseundefinedbehaviour,suchaswrappingto-32768,trapping,oranyotherbehaviour.Manipulatingavalueinthiswaycanresultinunexpectedresultssuchasoverflowingabuffer.Forunsignedintegers,thewraparoundbehaviouriswelldefined,andmaybewhattheprogrammerintended.However,theprogrammermayhaveexpectednormalarithmeticbehaviour,andbeenunawarethatthevaluewasgettingtoobigtorepresent.Asitsimpossibleforthecompilerorananalysistooltodeterminewhattheprogrammerintended,itisbettertowarnifwraparoundmayoccur.

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Comment [SGM11]: Itis

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InC,bitshiftingbyavaluegreaterthanthesizeofthedatatypeorbyanegativenumberisundefinedbehaviourforbothsignedandunsignedintegers.Thefollowingcode,whereaintis16bits,wouldbeundefinedwhenj >= 16orjisnegative: int foo( int i, const int j ) { return i>>j; }

• FollowtheadviceprovidedbyTR24772-1clause6.15.2.• Checkthattheresultofanoperationonanunsignedintegervaluewillnotcausewrapping,unlessitcan

beshownthatwrappingcannotoccur.Anyofthefollowingoperatorshavethepotentialtowrap:a+ba–ba*ba++++aa----aa+=ba-=ba*=ba<<ba<<=b-a

• Checkthattheresultofanoperationonasignedintegervaluewillnotcauseanoverflow,unlessitcanbeshownthatoverflowcannotoccur.Anyofthefollowingoperatorshavethepotentialtooverflow,whichisundefinedbehaviorinC:a+ba–ba*ba/ba%ba++++aa----aa+=ba-=ba*=ba/=ba%=ba<<ba<<=b-a

• Usedefensiveprogrammingtechniquestocheckwhetheranoperationwilloverfloworunderflowthereceivingdatatype.Thesetechniquescanbeomittedifitcanbeshownatcompiletimethatoverfloworunderflowisnotpossible.

• Thenumberofbitstobeshiftedbyashiftoperatorshouldliebetween1and(n-1),wherenisthesizeofthedatatype.

6.16Usingshiftoperationsformultiplicationanddivision[PIK]

TheissuesforCarewelldefinedinTR24772-1clause6.16UsingShiftOperationsforMultiplicationandDivision

[PIK].Alsoseeclause6.15ArithmeticWrap-aroundError[FIF].

TheguidanceforCusersiswelldefinedinTR24772-1clause6.16UsingShiftOperationsforMultiplicationand

Division[PIK].Alsosee,6.15ArithmeticWrap-aroundError[FIF].

6.17Choiceofclearnames[NAI]

ThepossibleconfusionofnameswithtypographicallysimilarcharactersisnotspecifictoC,butCisaspronetoitasanyotherlanguage.Dependinguponthelocalcharacterset,avoidhavingnamesthatonlydifferbycharactersthatmaybeconfused,suchas‘O’and‘0’

Deleted: Cisoftenusedforbitmanipulation.PartofthisisduetothecapabilitiesinCtomaskbitsandshiftthem.AnotherpartisduetotherelativeclosenessChastoassemblyinstructions.Manipulatingbitsonasignedvaluecaninadvertentlychangethesignbitresultinginanumberpotentiallygoingfromapositivevaluetoanegativevalue. ... [8]Deleted: thatis

Deleted: Onlyconductbitmanipulationsonunsigneddatatypes.

ForC,themaximumsignificantnamelengthisimplementationdefined.Ifaprogramincludesnamesthatarelongerthanthedefinedmaximum,thecompilerwilltruncatethemtothemaximum.So,iftwonamesinaprogramonlydifferincharactersafterthemaximum,theywillbetreatedasthesame.Forfunctionsthisisusuallydetectedbythecompilerasanattemptedredeclaration,butforvariablesdeclaredindifferentbutoverlappingscopesthismayleadtothewrongvariablebeingused,asin:

int long_name_ending in_A = … { int long_name_ending in_B = … /* Use of long_name_ending in_A here will actually use long_name_ending in_B */ }

• FollowtheadviceprovidedbyTR24772-1clause6.17.2.• Usenamesthatareclearandnon-confusing.• Useconsistencyinchoosingnames.• Keepnamesshortandconciseinordertomakethecodeeasiertounderstand.• Donotdeclarenameslongerthanthemaximumdefinedbytheimplementation.• Choosenamesthatarerichinmeaning.• Donotusesnamesthatonlydifferbyamixtureofcaseorthepresenceorabsenceofanunderscore

character.• Avoiddifferentiatingthroughcharactersthatarecommonlyconfusedvisuallysuchas‘O’and‘0’,‘I’(lower

case‘L’),‘l’(capital‘I’)and‘1’,‘S’and‘5’,‘Z’and‘2’,and‘n’and‘h’.

6.18Deadstore[WXQ]

BecauseCisanimperativelanguage,programsinCcancontaindeadstores(locationsthatarewrittenbutneversubsequentlyread,orover-writtenwithoutaninterveningread).Thiscanresultfromanerrorintheinitialdesignorimplementationofaprogram,orfromanincompleteorerroneousmodificationofanexistingprogram.However,itmayalsobeintendedbehaviour,forexamplewheninitializingasparsearray.Itmaybemoreefficienttocleartheentirearraytozero,thenassignthenon-zerovalues,sothepresenceofdeadstoresshouldberegardedasawarningofapossibleerror,ratherthananactualerror.

Astoreintoavolatile-qualifiedvariablegenerallyshouldnotbeconsideredadeadstorebecauseaccessingsuchavariablemaycauseadditionalsideeffects,suchasinput/output(memory-mappedI/O)orobservabilitybyadebuggeroranotherthreadofexecution.

6.18.2Guidancetolanguageusers• FollowtheadviceprovidedbyTR24772-1clause6.18.2.• Usecompilersandanalysistoolstoidentifydeadstoresintheprogram.• Declarevariablesasvolatilewhentheyareintentionaltargetsofastorewhosevaluedoesnotappearto

beused.

Deleted: Cissomewhatsusceptibletoerrorsresultingfromtheuseofsimilarlyappearingnames.Cdoesrequirethedeclarationofvariablesbeforetheyareused.However,Callowsscopingsothatavariablethatisnotdeclaredlocallymayberesolvedtosomeouterblockandahumanreviewermaynotnoticethatresolution.Variable

Deleted: specificDeleted: andsooneimplementationmayresolvenamestoonelengthwhereasanotherimplementationmayresolvenamestoanotherlengthresultinginunintendedbehaviour

Deleted: Aswiththegeneralcase,callstothewrongsubprogramorreferencestothewrongdataelement(whenmissedbyhumanreview)canresultinunintendedbehaviour.

Comment [SGM12]: Badadvice.Changetoadifferentcompiler(oradifferentversionofthesamecompiler)changesthemaximumlength.Themainpointistoensurethatnamesdiffernearthebeginningofthestrings.

Deleted: <#>Keepinmindthatcodewillbereusedandcombinedinwaysthattheoriginaldevelopersneverimagined.... [9]Deleted: <#>entiatenamesthroughonly

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6.19Unusedvariable[YZS]

Variablesmaybedeclared,butneverusedwhenwritingcodeortheneedforavariablemaybeeliminatedinthecode,butthedeclarationmayremain.Mostcompilerswillreportthisasawarningandthewarningcanbeeasilyresolvedbyremovingtheunusedvariable.

• FollowtheadviceprovidedbyTR24772-1clause6.19.2.• Resolveallcompilerwarningsforunusedvariables.Havinganunusedvariableincodeindicatesthat

eitherwarningswereturnedoffduringcompilationorwereignoredbythedeveloper.

6.20Identifiernamereuse[YOW]

Callowsscopingsothatavariablethatisnotdeclaredlocallymayberesolvedtosomeouterblockandthatresolutionmaycausethevariabletooperateonanentityotherthantheoneintended.Inthefollowingexample,becausethevariablenamevar1wasreused,theprintedvalueofvar1maybeunexpected.

int var1; /* declaration in outer scope */ var1 = 10; { int var2; int var1; /* declaration in nested (inner) scope */ var2 = 5; var1 = 1; /* var1 in inner scope is 1 */ }

print (“var1=%d\n”, var1); /* will print “var1=10” as var1 refers */ /* to var1 in the outer scope */

Removingthedeclarationofvar2willresultinadiagnosticmessagebeinggeneratedmakingtheprogrammerawareofanundeclaredvariable.However,removingthedeclarationofvar1intheinnerblockwillnotresultinadiagnosticasvar1willberesolvedtothedeclarationintheouterblockandaprogrammermaintainingthecodecouldveryeasilymissthissubtlety.Theremovingofinnerblockvar1willresultintheprintingofvar1=1insteadofvar1=10. 6.20.2Guidancetolanguageusers

• FollowtheadviceprovidedbyTR24772-1clause6.20.2.

Deleted: ThisistrivialinCasonesimplyneedstoremovethedeclarationofthevariable.

Deleted: BDeleted: inthefollowingexample

• Ensurethatadefinitionofanentitydoesnotoccurinascopewhereadifferententitywiththesamenameisaccessibleandcanbeusedinthesamecontext.Alanguage-specificprojectcodingconventioncanbeusedtoensurethatsucherrorsaredetectablewithstaticanalysis.

6.21Namespaceissues[BJL]

DoesnotapplytoCbecauseCrequiresuniquenamesandhasasingleglobalnamespace.Adiagnosticmessageisrequiredforduplicatenamesinasinglecompilationunit.

6.22Initializationofvariables[LAV]

Local,automaticvariablescanassumeunexpectedvaluesiftheyareusedbeforetheyareinitialized.TheCStandardspecifies,"Ifanobjectthathasautomaticstoragedurationisnotinitializedexplicitly,itsvalueisindeterminate".Inthecommoncase,onarchitecturesthatmakeuseofaprogramstack,thisvaluedefaultstowhichevervaluesarecurrentlystoredinstackmemory.Whileuninitializedmemorymaycontainzeros,thisisnotguaranteed.Consequently,uninitializedmemorycancauseaprogramtobehaveinanunpredictableorunplannedmannerandmayprovideanavenueforattack.

Manyimplementationswillissueadiagnosticmessageindicatingthatavariablehasbeenusedthatwasnotinitialized.

• FollowtheadviceprovidedbyTR24772-1clause6.22.2.• Heedcompilerwarningmessagesaboutuninitializedvariables.Thesewarningsshouldberesolvedas

recommendedtoachieveacleancompileathighwarninglevels.• Donotusememoryallocatedbyfunctionssuchasmalloc()beforethememoryisinitializedasthe

memorycontentsareindeterminate.

6.23Operatorprecedenceandassociativity[JCW]

OperatorprecedenceandassociativityinCareclearlydefined.

Mixedlogicaloperatorsareallowedwithoutparentheses.

• FollowtheguidanceprovidedinTR24772-1clause6.23.5• Useparenthesesanytimearithmeticoperators,logicaloperators,andshiftoperatorsaremixedinan

expression,orwheretheexpressioniscomplexandmaybedifficulttoparseforreviewormaintenance.

Deleted: <#>Ensurethatadefinitionofanentitydoesnotoccurinascopewhereadifferententitywiththesamenameisaccessibleandhasatypethatpermitsittooccurinatleastonecontextwherethefirstentitycanoccur. ... [10]

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Comment [SGM13]: Iamunhappyaboutthisone.C’slargenumberofprecedencescauseconfusionfortheaverageprogrammer.Yes,theyareclearlydefined,buttheyarecomplex.

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6.24Side-effectsandorderofevaluationofoperands[SAM]

6.24.1ApplicabilitytolanguageCallowsexpressionstohavesideeffects.Iftwoormoresideeffectsmodifythesameexpressionasin: int v[10]; int i; /* … */ i = v[i++];

thebehaviourisundefinedandthiscanleadtounexpectedresults.Eitherthe“i++”isperformedfirstortheassignmenti=v[i] isperformedfirst,orsomeotherunspecifiedbehaviouroccurs.Becausetheorderofevaluationcanhavedrasticeffectsonthefunctionalityofthecode,thiscangreatlyimpactportabilityandleadtounexpectedbehaviour.ThereareseveralsituationsinCwheretheorderofevaluationofsubexpressionsortheorderinwhichsideeffectstakeplaceisunspecifiedincluding:

• Theorderinwhichtheargumentstoafunctionareevaluated(C,Section6.5.2.2,"Functioncalls").• Theorderofevaluationoftheoperandsinanassignmentstatement(C,Section6.5.16,"Assignment

operators").• Theorderinwhichanysideeffectsoccuramongtheinitializationlistexpressionsisunspecified.In

particular,theevaluationorderneednotbethesameastheorderofsubobjectinitialization(C,Section6.7.9,“Initialization").

Becausetheseareunspecifiedbehaviours,testingmaygivethefalseimpressionthatthecodeisworkingandportable,whenitcouldjustbethatthevaluesprovidedcauseevaluationstobeperformedinaparticularorderthatcausessideeffectstooccurasexpected.Thereisalsoacommonmisconceptionthatbracketinginfluencestheorderofevaluation.Thisisnottrue.IfA,BandCarefunctionsthatreturnintegers,thenin: ( A() + B() ) * C()

thebracketsdon’taffecttheorderofevaluationofA,BandC,butdoaffecttheorderinwhichtheresultsofthesefunctionsarecombined.A,BandCmaybeevaluatedinanyorder,andiftheymodifycommonvariablestheresultisunspecified.6.24.2Guidancetolanguageusers

• FollowtheguidanceprovidedinTR24772-1clause6.24.5• ExpressionsshouldbewrittensothatthesameeffectswilloccurunderanyorderofevaluationthattheC

standardpermitssincesideeffectscanbedependentonanimplementationspecificorderofevaluation.• BecomefamiliarwithAnnexCoftheCstandardISO/IEC9899:2011[4],whichisalistofthesequence

pointsthatenforceanorderingofcomputations.

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6.25Likelyincorrectexpression[KOA]6.25.1ApplicabilitytolanguageChasseveralinstancesofoperatorswhicharesimilarinstructure,butvastlydifferentinmeaning,forexampleconfusingthecomparisonoperator“==”withassignment“=”.Usinganexpressionthatissyntacticallycorrect,butwhichmayjustbeanullstatementcanleadtounexpectedresults.Consider:

int x, y; /* … */ if (x = y){ /* … */ }

Afairamountofanalysismayneedtobedonetodeterminewhethertheprogrammerintendedtodoanassignmentaspartoftheifstatement(perfectlyvalidinC)orwhethertheprogrammermadethecommonmistakeofusingan“=”insteadofa“==”.Inordertopreventthisconfusion,itissuggestedthatanyassignmentsincontextsthatareeasilymisunderstoodbemovedoutsideoftheBooleanexpression.Thiswouldchangetheexamplecodetothesemanticallyequivalent:

int x,y; /* … */ x = y; if (x == 0) { /* … */ }

Thiswouldclearlystatewhattheprogrammermeantandthattheassignmentofytoxwasintended.Itisalsonotunknownforprogrammerstoinsertthe“;”statementterminatorprematurely.However,inadvertentlydoingthiscandrasticallyalterthemeaningofcode,eventhoughthecodeisvalid,asinthefollowingexample: int a,b; /* … */ if (a == b); // the semi-colon will make this a null statement { /* … */ }

Becauseofthemisplacedsemi-colon,thecodeblockfollowingtheifwillalwaysbeexecuted.Inthiscase,itisextremelylikelythattheprogrammerdidnotintendtoputthesemi-colonthere.6.25.2Guidancetolanguageusers

• FollowtheguidanceprovidedinTR24772-1clause6.25.5• Simplifystatementswithinterspersedcommentstoclarifyprogrammingfunctionalityandhelpfuture

maintainersunderstandtheintentandnuancesofthecode.• Avoidassignmentsembeddedwithinotherstatements,asthesecanbeproblematic.Eachofthe

followingwouldbeclearerandhavelesspotentialforproblemsiftheembeddedassignmentswereconductedoutsideoftheexpressions:

int a,b,c,d;

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/* … */ if ((a == b) || (c = (d-1))) // the assignment to c may not // occur if a is equal to b

or: int a,b,c; /* … */ foo (a=b, c);

EachisavalidCstatement,buteachmayhaveunintendedresults.• Givenullstatementsasourcelineoftheirown.This,combinedwithenforcementbystaticanalysis,

wouldmakeclearertheintentionthatthestatementwasmeanttobeanullstatement.• Considertheadoptionofacodingstandardthatlimitstheuseoftheassignmentstatementwithinan

expression.6.26Deadanddeactivatedcode[XYQ]6.26.1ApplicabilitytolanguageCallowstheusualsourcesofdeadcode(describedin6.26ofTR24772-1)thatarecommontomostconventionalprogramminglanguages.Cusessomeoperatorsthatcanbeconfusedwithotheroperators.Forinstance,thecommonmistakeofusinganassignmentoperatorinaBooleantestasin:

int a; /* … */ if (a = 1) { … } else { … }

cancauseportionsofcodetobecomedeadcode,becausetheelseportionoftheifstatementcannotbereached.6.26.2Guidancetolanguageusers

• ApplytheguidanceprovidedinTR24772-1clause6.26.5.• Eliminatedeadcode.• Usecompilersandanalysistoolstoassistinidentifyingunreachablecode.• Use“//”commentsyntaxinsteadof“/*…*/”commentsyntaxtoavoidtheinadvertentcommentingout

sectionsofcode.

6.27Switchstatementsandstaticanalysis[CLL]6.27.1ApplicabilitytolanguageBecauseofthewayinwhichtheswitch-casestatementinCisstructured,itcanberelativelyeasytounintentionallyomitthebreakstatementbetweencasescausingunintendedexecutionofstatementsforsomecases.Ccontainsaswitchstatementoftheform:

Deleted: totheextentpossiblefromCprograms

Deleted: <#>Deletedeactivatedcodefromprogramsduetothepossibilityofaccidentallyactivatingit.

char abc; /* … */ switch (abc) { case 1: sval = “a”; break; case 2: sval = “b”; break; case 3: sval = “c”; break; default: printf (“Invalid selection\n”); }

Ifthereisn’tadefaultcaseandtheswitchedexpressiondoesn’tmatchanyofthecases,thencontrolsimplyshiftstothenextstatementaftertheswitchstatementblock.Unintentionallyomittingabreakstatementbetweentwocaseswillcausesubsequentcasestobeexecuteduntilabreakortheendoftheswitchblockisreached.Thiscouldcauseunexpectedresults.6.27.2Guidancetolanguageusers

• ApplytheguidanceprovidedinTR24772-1subclause6.27.5• Onlyadirectfallthroughshouldbeallowedfromonecasetoanother.Thatis,everynonemptycase

statementshouldbeterminatedwithabreakstatementasillustratedinthefollowingexample:int i; /* … */ switch (i) { case 1: case 2: i++; /* fall through from case 1 to 2 is permitted */ break; case 3: j++; case 4: /* fall through from case 3 to 4 is not permitted */ /* as it is not a direct fall through due to the */ /* j++ statement */ }

• Ifdirectfallthroughfromonenonemptycasetoanotherisrequired,thenthisshouldbeclearlydocumentedbyacomment,preferablyonerecognizedbytheanalysistoolused.

• Adoptastylethatpermitsyourlanguageprocessorandanalysistoolstoverifythatallcasesarecovered.Wherethisisnotpossible,useadefaultclausethatdiagnosestheerror.

• Acodingstandardthatrequiresthedefaultclausetobeeitherthefirstorlastclauseintheswitchstatementcanassistthemaintenanceofcomplexswitchstatements

6.28Demarcationofcontrolflow[EOJ]

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6.28.1ApplicabilitytolanguageClacksakeywordtobeusedasanexplicitterminator.Therefore,itmaynotbereadilyapparentwhichstatementsarepartofaloopconstructoranifstatement.Considerthefollowingsectionofcode:

int foo(int a, const int *b) { int i=0, count = 0; /* … */ a = 0; for (i=0; i<10; i++) a += b[i]; count++; printf(“%d %d\n”, a, count); }

Theprogrammermayhaveintendedbotha+=b[i];andcount++;tobethebodyoftheloop,butasthereisnoenclosingbrackets,thesecondstatementisonlyperformedonce.IfstatementsinCarealsosusceptibletocontrolflowproblemssincethereisn’tarequirementfortheretobeanelsestatementforeveryifstatement.AnelsestatementinCalwaysbelongtothemostrecentifstatementwithoutanelse.However,thesituationcouldoccurwhereitisnotreadilyapparenttowhichifstatementanelsebelongsduetothewaythecodeisindentedoraligned.6.28.2Guidancetolanguageusers

• FollowtherulesprovidedinTR24772-1clause6.28.5.• Enclosethebodiesofif,else,while,for,andsimilarinbraces.Thiswillreduceconfusionand

potentialproblemswhenmodifyingthesoftware.Forexample:int a,b,i; /* … */ if (i == 10){ a = 5; /* this is correct */ b = 10; }

else a = 10; b = 5;

Iftheassignmentstobwereaddedlaterandwereexpectedtobepartofeachifandelseclause(theyareindentedassuch),theabovecodeisincorrect:theassignmenttobthatwasintendedtobeintheelseclauseisunconditionallyexecuted.

6.29Loopcontrolvariables[TEX]6.29.1Applicabilitytolanguage

Deleted: ;

Deleted: {Atfirstitmayappearthatawillbeasumofthenumbersb[0]tob[9].However,eventhoughthecodeisarrangedsothatthea = a + b[i]codeappearstobewithintheforloop,the“;”attheendoftheforstatementcausesthelooptobeonanullstatement(the“;”)andthea = a + b[i];statementtoonlybeexecutedonce.Inthiscase,thismistakemaybereadilyapparentduringdevelopmentortesting.Moresubtlecasesmaynotbeasreadilyapparentleadingtounexpectedresults.

Deleted: Atfirstitmayappearthatawillbeasumofthenumbersb[0]tob[9].However,eventhoughthecodeisarrangedsothatthea = a + b[i]codeappearstobewithintheforloop,the“;”attheendoftheforstatementcausesthelooptobeonanullstatement(the“;”)andthea = a + b[i];statementtoonlybeexecutedonce.Inthiscase,thismistakemaybereadilyapparentduringdevelopmentortesting.Moresubtlecasesmaynotbeasreadilyapparentleadingtounexpectedresults.

Deleted: inC

Callowsthemodificationofloopcontrolvariableswithintheloop,butcancauseunexpectedbehaviour.Sincethemodificationofaloopcontrolvariablewithinaloopisinfrequentlyencountered,reviewersofCcodemaynotexpectitandhencemissnoticingthemodificationornotrecognizeitssignificance.Modifyingtheloopcontrolvariablecancauseunexpectedresults,asin: int a,i; for (i=1; i<10; i++){ … if (a > 7) i = 10; … }

whichwouldcausetheforlooptoexitonceaisgreaterthan7regardlessofthenumberofiterationsthathaveoccurred.Cdoesn’trequiretheloopcontrolvariabletobeanintegertype.If,forexample,itisafloatingpointtype,thetestforcompletionshouldnotuseequalityorinequality,asfloatingpointroundingmayleadtomathematicallyinexactresults,andhenceanunterminatedloop.Thefollowingmaylooptentimesorindefinitely: float j; for (j = 0.0f; j != 10.0f; j += 1.0f){ … }

Thefollowingislittlebetter: float j; for (j = 0.0f; j < 10.0f; j += 1.0f){ … } Rounding may cause this loop to be performed ten or eleven times. To ensure this loop is performed ten times, j needs to be initialized to 0.5f.

• ApplytheguidanceofTR24772-1clause6.29.5.• Donotmodifyaloopcontrolvariablewithinaloop.• Donotusefloatingpointtypesasaloopcontrolvariable

6.30Off-by-oneerror[XZH]6.30.1ApplicabilitytolanguageArraysareacommonplaceforoffbyoneerrorstomanifest.InC,arraysareindexedstartingat0,causingthecommonmistakeofloopingfrom0tothesizeofthearrayasin: int foo() { int a[10]; int i; for (i=0, i<=10, i++) …

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Deleted: .TDeleted: houghthisisusuallynotconsideredgoodprogrammingpracticeasit

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Deleted: .EventhoughthecapabilityexistsinC,itisstillconsideredtobeapoorprogrammingpractice.

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return (0); }

StringsinCarealsoanothercommonsourceoferrorsinCduetotheneedtoallocatespaceforandaccountforthestringterminator.Acommonmistakeistoexpecttostoreannlengthstringinannlengtharrayinsteadoflengthn+1toaccountfortheterminating‘\0’.Interfacingwithotherlanguagesthatdonotuseterminatorsinstringscanalsoleadtoanoffbyoneerror.Cdoesnotflagaccessesoutsideofarraybounds,soanoffbyoneerrormaynotbedetectable.Severaltoolscanbeusedtohelpdetectaccessesbeyondtheboundsofarrays.However,suchtoolswillnothelpinthecasewhereonlyaportionofthearrayisusedandtheaccessisstillwithintheboundsofthearray.Loopingonemoreoronelessisusuallydetectablebygoodtesting.DuetothestructureoftheClanguage,thismaybethemainwaytoavoidthisvulnerability.Unfortunatelysomecasesmaystillslipthroughthedevelopmentandtestphaseandmanifestthemselvesduringoperationaluse.6.30.2Guidancetolanguageusers

• FollowtheguidanceofTR24772-1clause6.30.5.• Usecarefulprogramming,testingofboundaryconditionsandstaticanalysistoolstodetectoffbyone

errorsinC.

6.31Structuredprogramming[EWD]6.31.1ApplicabilitytolanguageItisaseasytowritestructuredprogramsinCasitisnotto.Ccontainsthegotoandlongjmpstatements,whichcancreateunstructuredcode.Calsohascontinue,break,andreturnthatcancreatecomplicatedcontrolflowwhenusedinanundisciplinedmanner.Unstructured{spaghetti}codecanbemoredifficultforCstaticanalyzerstoanalyzeandissometimesusedonpurposetoobfuscatethefunctionalityofsoftware.Codethathasbeenmodifiedmultipletimesbyanassortmentofprogrammerstoaddorremovefunctionalityortofixproblemscanbepronetobecomeunstructured.

BecauseunstructuredcodeinCcancauseproblemsforanalyzers(bothautomatedandhuman),problemswiththecodemaynotbedetectedasreadilyoratallaswouldbethecaseifthesoftwarewaswritteninastructuredmanner.

• FollowtheguidanceofTR24772-1clause6.31.5.• Writeclearandconcisestructuredcodetomakecodeasunderstandableaspossible.

Restricttheuseofgoto,continue,breakandlongjmptoencouragemorestructuredprogramming.• IEC61508[12]highlyrecommendstheuseofnomorethanonereturnstatementinafunction.At

times,thisguidancecanhavetheoppositeeffect,suchasinthecaseofanifcheckofparametersatthestartofafunctionthatrequirestheremainderofthefunctiontobeencasedintheifstatementinordertoreachthesingleexitpoint.If,forexample,theuseofmultipleexitpointscanarguablymakeapieceof

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codeclearer,thentheyshouldbeused.However,thecodeshouldbeabletowithstandacritiquethatarestructuringofthecodewouldhavemadetheneedformultipleexitpointsunnecessary.

6.32Passingparametersandreturnvalues[CSJ]6.32.1ApplicabilitytolanguageCusescallbyvalueparameterpassing.Theparameterisevaluatedanditsvalueisassignedtotheformalparameterofthefunctionthatisbeingcalled.Aformalparameterbehaveslikealocalvariableandcanbemodifiedinthefunctionwithoutaffectingtheactualargument.Anobjectcanbemodifiedinafunctionbypassingtheaddresstotheobjecttothefunction,forexample void swap(int *x, int *y) { int t = *x; *x = *y; *y = t; }Wherexandy areintegerpointerformalparameters,and*xand*y intheswap()functionbodydereferencethepointerstoaccesstheintegers.Ifitisnotintendedthatthefunctionshouldbeabletomodifytheobjectwhoseaddressispassedtothefunction,thepointershouldbemadeconstant,asconstint *p.Cmacrosuseacallbynameparameterpassing;acalltothemacroreplacesthemacrobythebodyofthemacro.Thisiscalledmacroexpansion.Macroexpansionisappliedtotheprogramsourcetextandamountstothesubstitutionoftheformalparameterswiththeactualparameterexpressions.Formalparametersareoftenparenthesizedtoavoidsyntaxissuesaftertheexpansion.Callbynameparameterpassingreevaluatestheactualparameterexpressioneachtimetheformalparameterisread.C11introducedtherestrictkeyword.Thismaybeappliedtofunctionpointerparameters.Whereafunctionhastwoormorepointerparametersmarkedwithrestrict,theprogrammeristellingthecompilerthatthefunctionwillneverbecalledwitharraysthathaveoverlappingaccess.Thisallowsthecompilertomakeuseofoptimizationsthatmayleadtoincorrectresultsifthearraysdooverlap,e.g.acopyfunctionlikestrncpythatcopiesafixednumberofcharactersfromasourcestringtoatarget.Ifthetargetoverlapsthesource,theresultdependsuponwhetherthecopyingwasperformedfromthestartofthestringtotheendorviceversa.Conversely,wherealibraryfunctionisdeclaredwithrestrictparameters,theprogrammerisbeingtoldnevertocallitsothataccesseswithinthefunctionoverlap.Thereisnocompileorrun-timecheckthattheparameterarraysareactuallynon-overlapping,socautionshouldbetakenwhenusingfunctionswithrestrictparameters.6.32.2Guidancetolanguageusers

• FollowtheguidanceofTR24772-1clause6.32.5.• Donotuseexpressionswithsideeffectsinparameterstofunction-likemacros,unlessitcanbeshown

thattheparameterisusedonlyonceinsidethemacro.• Donotuseexpressionswithsideeffectsformultipleparameterstofunctions,theorderinwhichthe

parametersareevaluatedandhencethesideeffectsoccurisunspecified.

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• Usecautionwhenpassingtheaddressofanobject.Theobjectpassedcouldbeanalias6.AliasescanbeavoidedbyfollowingtherespectiveguidelinesofTR24772-1subclause6.32.5.

• Donotuseafunctionthatincludestherestrictkeywordunlessitcanbeestablishedthatthearrayparameterstothefunctioncanneveroverlap.

6.33Danglingreferencestostackframes[DCM]6.33.1ApplicabilitytolanguageCallowstheaddressofavariabletobestoredinapointervariable.Shouldthispointervariablepointto,forexample,theaddressofalocalvariablethatwaspartofastackframe,thenusingthisaddressafterthefunctioncontainingthelocalvariablehasterminatedleadstoundefinedbehaviour,asthememorywillhavebeenmadeavailableforfurtherallocationandmayindeedhavebeenallocatedforsomeotheruse.Thesameistrueforapointertomemoryallocatewithmallocetc.andwhichhassubsequentlybeenfreed.6.33.2Guidancetolanguageusers

• Donotassigntheaddressofanobjecttoanyentitywhichpersistsaftertheobjecthasceasedtoexist.Thisisdoneinordertoavoidthepossibilityofadanglingreference.Inparticular,neverreturntheaddressofalocalvariableastheresultofafunctioncall.

• Longlivedpointersthatcontainblock-localaddressesshouldbeassignedthenullpointervaluebeforeexecutingareturnfromtheblock.

6.34Subprogramsignaturemismatch[OTR]

6.34.1ApplicabilitytolanguageFunctionsinCmaybecalledwithmoreorlessthanthenumberofparametersthereceivingfunctionexpects.However,mostCcompilerswillgenerateawarningoranerroraboutthissituation.Ifthenumberofargumentsdoesnotequalthenumberofparameters,thebehaviourisundefined.Thiscanleadtounexpectedresultswhenthecountortypesoftheparametersdiffersfromthecallingtothereceivingfunction.Iftoofewargumentsaresenttoafunction,thenthefunctioncouldstillpoptheexpectednumberofargumentsfromthestackleadingtounexpectedresults.Callowsafunctiontotakeavariablenumberofarguments,asintheprintf()function.Thisisspecifiedinthefunctiondefinitionbyterminatingthelistofparameterswithanellipsis(...).Noinformationaboutthenumberortypesoftheparametersexpectedissupplied,andthecompilerwillacceptanynumberandtypeofparametersinthecall.

6Analiasisavariableorformalparameterthatreferstothesamelocationasanothervariableorformalparameter.

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Afunctiondefinitionmaybeforwardreferencedbyafunctiondeclaration,thatmayomittheparameterlist.Ifafunctionthatacceptsavariablenumberofargumentsisdefinedwithoutaparametertypelistthatendswiththeellipsisnotation,thebehaviourisundefined.Ccompilerswillattempttoperformanimplicitconversionfromthetypeofanactualparametertothetypeoftheformalparameter.Soforsqrt()thatisdefinedtoexpectadouble: double sqrt(double)

thecall: root2 = sqrt(2);

convertstheinteger2intothedoublevalue2.0.6.34.2Guidancetolanguageusers

• FollowtheguidelinesofTR24772-1clause6.34.5.• Useafunctionprototypetodeclareafunctionwithitsexpectedparameterstoallowthecompilerto

checkforamatchingcountandtypesoftheparameters.• Donotusethevariableargumentfeatureexceptinrareinstances.Thevariableargumentfeature

suchasisusedinprintf()isdifficulttouseinatypesafemanner.6.35Recursion[GDL]6.35.1ApplicabilitytolanguageCpermitsrecursion,henceissubjecttotheproblemsdescribedinTR24772-1subclause6.35.6.35.2Guidancetolanguageusers

• ApplytheguidancedescribedinTR24772-1clause6.35.5.

6.36Ignorederrorstatusandunhandledexceptions[OYB]

TheCstandarddoesnotincludeexceptionhandling,thereforeonlyerrorstatuswillbecovered.

Cprovidestheheaderfile<errno.h>thatdefinesthemacrosEDOM, EILSEQ and ERANGE,whichexpandtointegerconstantexpressionswithtypeint,distinctpositivevaluesandwhicharesuitableforusein#ifpreprocessingdirectives.Calsoprovidestheintegererrnothatmaybesettoanonzerovaluebyanylibraryfunctiontoindicatethatanerrorhasoccurred(iftheuseoferrnoisnotdocumentedinthedescriptionofthelibraryfunctionintheCStandard,errnocouldbeusedwhetherornotthereisanerror).Thoughthesevaluesaredefined,inconsistenciesinrespondingtoerrorconditionscanleadtovulnerabilities.errnoandthedefinedmacrosmayalsobeusedbyuserdefinedfunctions,butforclarity,suchuseshouldbeconsistentwiththeusebylibraryfunctions.

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Clibraryfunctionsmayalsoreturnerrorindicatorvalues.

• Checkthereturnederrorstatusuponreturnfromafunction.• Seterrnotozerobeforealibraryfunctioncallinsituationswhereaprogramintendstocheckerrno

beforeasubsequentlibraryfunctioncall.• Useerrno_ttomakeitreadilyapparentthatafunctionisreturninganerrorcode.Oftenafunctionthat

returnsanerrnoerrorcodeisdeclaredasreturningavalueoftypeint.Althoughsyntacticallycorrect,itisnotapparentthatthereturncodeisanerrnoerrorcode.ThenormativeAnnexKfromISO/IEC9899:2011[4]introducesthenewtypeerrno_tin<errno.h>thatisdefinedtobetypeint.

• Handleanerrorascloseaspossibletotheoriginoftheerrorbutasfaroutasnecessarytobeabletodealwiththeerror.

• Whenafunctionreturnsanerrorvalue,otherthanusingerrno(e.g.mallocthatreturnsNULLiftherequestedmemoryallocationcannotbeperformed),alwayschecktheerrorconditionreturnedafteracall

• Foreachroutine,documentallerrorconditions,matchingerrordetectionandreportingneeds,andprovidesufficientinformationforhandlingtheerrorsituation.

• Usestaticanalysistoolstodetectandreportmissingorineffectiveerrordetectionorhandling.• Whenexecutionwithinaparticularcontextencountersanerror,finalizethecontextbyclosingopenfiles,

releasingresourcesandrestoringanyinvariantsassociatedwiththecontext.

6.37Type-breakingreinterpretationofdata[AMV]

TheprimarywayinCthatareinterpretationofdatacanbeaccomplishedisthroughaunion,whichmaybeusedtointerpretthesamepieceofmemoryinmultipleways.Iftheuseoftheunionmembersisnotmanagedcarefully,thenunexpectedanderroneousresultsmayoccur.

• FollowtheguidelinesofTR24772-1clause6.37.5.• Whenusingunions,implementanexplicitdiscriminantandcheckitsvaluebeforeaccessingthedatain

theunion.

6.38Deepvs.shallowcopying[YAN]6.38.1Applicabilitytolanguage

Thisissuecanarisewhereastructorunioncontainsapointertoanobject.IfAandBaretwostructobjectsofthesametypethathasapointermember,thenthestatementA = B;copiesallthemembersofBtothe

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equivalentmembersofA.Forthepointer,onlythepointeritselfhasbeencopied,soAandBbothnowpointtothesameobject,i.e.shallowcopying.

Iftherequiredbehavioristocopythestructandhaveeachcopypointtoitsownobject,thenafunctionisneededtoimplementdeepcopying,i.e.copyallthemembersofBtoA–otherthanthepointer,andallocatesufficientmemorytomakeacopyoftheobjectpointedtobyBandmakeApointtothisnewobject.

• FollowtheguidelinesofTR24772-1clause6.38.5.• Wherenecessary,createafunctiontocorrectlyperformthedeepcopy

6.39Memoryleak[XYL]

Creliesontheprogrammertoimplementmemorymanagement,allocatingandfreeingdynamicmemoryasrequired,ratherthansupplyingabuiltingarbagecollector.

MemoryisdynamicallyallocatedinCusingthelibrarycallsmalloc(),calloc(),andrealloc().Whentheprogramnolongerneedsthedynamicallyallocatedmemory,itcanbereleasedusingthelibrarycallfree().Shouldtherebeaflawinthelogicoftheprogram,memorymaycontinuetobeallocatedbutnotfreedwhenitisnolongerneeded.Acommonsituationiswherememoryisallocatedwhileinafunction,thememoryisnotfreedbeforetheexitfromthefunctionandthelifetimeofthepointertothememoryhasendeduponexitfromthefunction.

• Usedebuggingtoolssuchasleakdetectorstohelpidentifyunreachablememory.• Allocateandfreememoryinthesamemoduleandatthesamelevelofabstractiontomakeiteasierto

determinewhenandifanallocatedblockofmemoryhasbeenfreed.• Userealloc()onlytoresizedynamicallyallocatedarrays.• UsegarbagecollectorsthatareavailabletoreplacetheusualClibrarycallsfordynamicmemory

allocationwhichallocatememorytoallowmemorytoberecycledwhenitisnolongerreachable.Theuseofgarbagecollectorsmaynotbeacceptableforsomeapplicationsasthedelayintroducedwhentheallocatorreclaimsmemorymaybenoticeableorevenobjectionableleadingtoperformancedegradation.

6.40Templatesandgenerics[SYM]ThisvulnerabilitydoesnotapplytoC,becauseCdoesnotimplementthesemechanisms.6.41Inheritance[RIP]

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ThisvulnerabilitydoesnotapplytoC,becauseCdoesnotimplementstructhierarchies.6.42ViolationsoftheLiskovsubstitutionprincipleorthecontractmodel[BLP]ThisvulnerabilitydoesnotapplytoC,becauseCdoesnotimplementpolymorphism.

6.43Redispatching[PPH]ThisvulnerabilitydoesnotapplytoC,becauseCdoesnotimplementthismechanism.

6.44Polymorphicvariables[BKK]ThisvulnerabilitydoesnotapplytoC,becauseCdoesnotimplementthismechanism.6.45Extraintrinsics[LRM]ThisvulnerabilitydoesnotapplytoC,becauseCdoesnotimplementthesemechanisms.6.46Argumentpassingtolibraryfunctions[TRJ]

ParameterpassinginCiseitherpassbyreferenceorpassbyvalue.Thereisn’taguaranteethatthevaluesbeingpassedwillbeverifiedbyeitherthecallingorreceivingfunctions.Sovaluesoutsideoftheassumedrangemaybereceivedbyafunctionresultinginapotentialvulnerability.

Aparametermaybereceivedbyafunctionthatwasassumedtobewithinaparticularrangeandthenanoperationorseriesofoperationsisperformedusingthevalueoftheparameterresultinginunanticipatedresultsandevenapotentialvulnerability.

• FollowtheguidelinesofTR24772-1clause6.46.5.• Donotmakeassumptionsaboutthevaluesofparameters.• Donotassumethatthecallingorreceivingfunctionwillberangecheckingaparameter.Therefore,

establishastrategyforeachinterfacetocheckparametersineitherthecallingorreceivingroutines.

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6.47Inter-languagecalling[DJS]

TheCStandarddefinesthecallingconventions,datalayout,errorhandingandreturnconventionsneededtouseCfromanotherlanguage.AdahasdevelopedastandardforinterfacingwithC.FortranhasincludedaClause15thatexplainshowtocallCfunctions.CallsfromCintootherlanguagesbecometheresponsibilityoftheprogrammer.

• FollowtheguidelinesofTR24772-1clause6.47.5.• Minimizetheuseofthoseissuesknowntobeerror-pronewheninterfacingfromC,suchas

1. passingcharacterstrings,2. dimension,boundsandlayoutissuesofarrays,3. interfacingwithotherparameterformatssuchascallbyreferenceorname,4. receivingreturncodes,and5. bitrepresentation.

6.48Dynamically-linkedcodeandself-modifyingcode[NYY]

Mostloadersallowdynamicallylinkedlibrariesalsoknownassharedlibraries.Codeisdesignedandtestedusingasuiteofsharedlibrarieswhichareloadedatexecutiontime.TheprocessoflinkingandloadingisoutsidethescopeoftheCstandard.

Ccanallowself-modifyingcode.InCthereisn’tadistinctionbetweendataspaceandcodespace,executablecommandscanbealteredasdesiredduringtheexecutionoftheprogram.Althoughself-modifyingcodemaybeeasytodoinC,itcanbedifficulttounderstand,testandfixleadingtopotentialvulnerabilitiesinthecode.

Self-modifyingcodecanbedoneintentionallyinCtoobfuscatetheeffectofaprogramorinsomespecialsituationstoincreaseperformance.ModificationofCcodecanoccurifpointersaremisdirectedtoaccessthecodespaceinsteadofdataspaceorcodeisexecutedindataspace.Accidentalmodificationusuallyleadstoaprogramcrash.Intentionalmodificationcanalsoleadtoaprogramcrash,butusedinconjunctionwithothervulnerabilitiescanleadtomoreseriousproblemsthataffecttheentirehost.

• FollowtheguidelinesofTR24772-1clause6.48.5.• Donotuseself-modifyingcodeexceptinrareinstances.Inthoserareinstances,self-modifyingcodeinC

canandshouldbeconstrainedtoaparticularsectionofthecodeandwellcommented.• Verifythatthedynamicallylinkedorsharedcodebeingusedisthesameasthatwhichwastested.• Retestwhenitispossiblethatthedynamicallylinkedorsharedcodehaschangedbeforeusingthe

application.

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6.49Librarysignature[NSQ]

6.49.1ApplicabilitytolanguageIntegratingCandanotherlanguageintoasingleexecutablereliesonknowledgeofhowtointerfacethefunctioncalls,argumentlistsanddatastructuressothatsymbolsmatchintheobjectcodeduringlinking.Bytealignmentscanbeasourceofdatacorruption.

Forinstance,whencallingFortranfromC,severalissuesarise:• NeitherCnorFortrancheckformismatchargumenttypesoreventhenumberofarguments.• CpassesargumentsbyvalueandFortranpassesargumentsbyreference,soaddressesmustbepassedto

Fortranratherthanvaluesintheargumentlist.• MultidimensionalarraysinCarestoredinrowmajororder,whereasFortranstoresthemincolumnmajor

order.• StringsinCareterminatedbyanullcharacter,whereasFortranusesthedeclaredlengthofastring.

ThesearejustsomeoftheissuesthatarisewhencallingFortranprogramsfromC.EachlanguagehasitsdifferenceswithC,sodifferentissuesarisewitheachinterface.Writingalibrarywrapperisthetraditionalwayofinterfacingwithcodefromanotherlanguage.However,thiscanbequitetediousanderror-prone.

• FollowtheguidelinesofTR24772-1clause6.49.5.• Useatool,ifpossible,toautomaticallycreateinterfacewrappers.

6.50Unanticipatedexceptionsfromlibraryroutines[HJW]

SinceCdoesnothaveexceptionsandsocannothandleexceptionspassedfromotherlanguagesystems,thisvulnerabilitydoesnotapply.See6.36foradiscussionofIgnorederrors.SeeTR24772-1clause6.46inthecasewherelibrarieswritteninlanguagesthatuseexceptionsmaybecalled.

6.51Pre-processordirectives[NMP]

6.51.1ApplicabilitytolanguageTheCpre-processorallowstheuseofmacrosthataretext-replacedbeforecompilation.Function-likemacroslooksimilartofunctionsbuthavedifferentsemantics.Becausetheargumentsaretext-replaced,expressionspassedtoafunction-likemacromaybeevaluatedmultipletimes.Thiscanresultinunintendedandunspecifiedbehaviour,iftheargumentshavesideeffectsorarepre-processordirectivesasdescribedbyC§6.10[1].Additionally,theargumentsandbodyoffunction-likemacrosshouldbefullyparenthesizedtoavoidunintendedandunspecifiedbehaviour[2].Thefollowingcodeexampledemonstratesunspecifiedbehaviourwhenafunction-likemacroiscalledwith

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argumentsthathaveside-effects(inthiscase,theincrementoperator)[2]:#define foo(X) ((X) * (X) + (X)) /* ... */ int i = 2; int a = foo(++i);

Theaboveexamplecouldexpandto: int a = ((++i) * (++i) + (++i));

thishasunspecifiedbehaviour,asitsnotknowninwhichorderthecompilerwillevaluatethethree++isubexpressions.Anothermechanismoffailurecanoccurwhentheargumentswithinthebodyofafunction-likemacroarenotfullyparenthesized.Thefollowingexampleshowsamacrowithoutparenthesizedarguments[2]:

#define CUBE(X) (X * X * X) /* ... */ int a = CUBE(2 + 1);

Thisexampleexpandsto: int a = (2 + 1 * 2 + 1 * 2 + 1)

whichevaluatesto7insteadoftheintended27.

6.51.2GuidancetolanguageusersThisvulnerabilitycanbeavoidedormitigatedinCinthefollowingways:

• FollowtheguidelinesofTR24772-1clause6.51.5.• Replacemacro-likefunctionswithinlinefunctionswherepossible.Althoughmakingafunctioninlineonly

suggeststothecompilerthatthecallstothefunctionbeasfastaspossible,theextenttowhichthisisdoneisimplementation-defined.Inlinefunctionsdoofferconsistentsemanticsandallowforbetteranalysisbystaticanalysistools.

• Ensurethatifafunction-likemacromustbeused,thatitsargumentsandbodyareparenthesized.• Donotusepre-processordirectivesorexpressionswithside-effects(suchasassignment,

increment/decrement,volatileaccess,orfunctioncalls)intheparameterofafunction-likemacro.6.52Suppressionoflanguage-definedrun-timechecking[MXB]

DoesnotapplytoCsincetherearenolanguage-definedruntimechecks.6.53Provisionofinherentlyunsafeoperations[SKL]

6.53.1ApplicabilitytolanguageCwasdesignedforimplementingsystemsoftwarewheresome‘unsafe’operationsareinherentandcommon.

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• FollowtheguidelinesofTR24772-1clause6.53.5.

6.54Obscurelanguagefeatures[BRS]

6.54.1Applicabilityoflanguage

Cisarelativelysmalllanguagewithalimitedsyntaxset,lackingmanyofthecomplexfeaturesofsomeotherlanguages.ManyofthecomplexfeaturesinCarenotimplementedaspartofthelanguagesyntax,butratherimplementedaslibraryroutines.Assuch,mostoftheavailablefeaturesinCareusedrelativelyfrequently.

Problemsaremorelikelytoarisefromtheuseofacombinationoffeaturesthatarerarelyusedtogetherorfraughtwithissuesifnotusedcorrectly.Thiscancauseunexpectedresultsandpotentialvulnerabilities.

• ConsidertheguidelinesinTR24772-1clause6.54.5.• Organizationsshouldspecifycodingstandardsthatrestrictorbantheuseoffeaturesorcombinationsof

featuresthathavebeenobservedtoleadtovulnerabilitiesintheoperationalenvironmentforwhichthesoftwareisintended.

6.55Unspecifiedbehaviour[BQF]

6.55.1ApplicabilityoflanguageTheCstandardhasdocumented,inAnnexJ.1,54instancesofunspecifiedbehaviour.Examplesofunspecifiedbehaviourare:

• Theorderinwhichparametersofafunctioncallareevaluated• Theorderinwhichanysideeffectsoccuramongtheinitializationlistexpressionsinaninitializer• Thelayoutofstorageforfunctionparameters

Relianceonaparticularobservedbehaviourthatisunspecifiednotonlyleadstoportabilityproblemswhenthesamecodeiscompiledwithadifferentcompiler,butisnotrequiredtobeconsistentwithinthesameprogram.Manycasesofunspecifiedbehaviourhavetodowiththeorderofevaluationofsubexpressionsandsideeffects.Forexample,inthefunctioncall f1(f2(x), f3(x));

thefunctionsf2andf3maybecalledinanyorder,possiblyyieldingdifferentresults.

• FollowtheguidelinesofTR24772-1clause6.55.5.• Donotrelyonunspecifiedbehaviourbecausethebehaviourcanchangeateachinstance.Anycodethat

makesassumptionsaboutthebehaviourofsomethingthatisunspecifiedshouldbereplaced.

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6.56Undefinedbehaviour[EWF]

6.56.1ApplicabilitytolanguageTheCstandarddoesnotimposeanyrequirementsoncodewithundefinedbehaviour.Typicalundefinedbehavioursincludedoingnothing,producingarbitraryresults,andterminatingtheprogram.TheCstandardhasdocumented,inAnnexJ.2,191instancesofundefinedbehaviourthatexistinC.Oneexampleofundefinedbehaviouroccurswhenthevalueofthesecondoperandofthe/or%operatoriszero.Thisisgenerallynotdetectablethroughstaticanalysisofthecode,butcouldeasilybepreventedbyacheckforazerodivisorbeforetheoperationisperformed.Leavingthisbehaviourasundefinedlessenstheburdenontheimplementationofthedivisionandmodulooperators.Otherexamplesofundefinedbehaviourare:

• Referringtoanobjectoutsideofitslifetime• Theconversiontoorfromanintegertypethatproducesavalueoutsideoftherangethatcanbe

represented• Theuseoftwoidentifiersthatdifferonlyinnon-significantcharacters

Relyingonundefinedbehaviourmakesaprogramunstableandnon-portable.Whilesomecasesofundefinedbehaviourmaybehaveconsistentlyacrossmultipleimplementations,itisstilldangeroustorelyonthem.Relyingonundefinedbehaviourcanresultinerrorsthataredifficulttolocateandonlypresentthemselvesunderspecificcircumstances.Forexample,accessingmemoryafterithasbeendeallocatedbyfree()orrealloc()resultsinundefinedbehaviour,butitmayworkmostofthetime.

6.57Implementation–definedbehaviour[FAB]

6.57.1ApplicabilitytolanguageTheCstandardhasdocumented,inAnnexJ.3,112instancesofimplementation-definedbehaviour.Examplesofimplementation-definedbehaviourare:

• Thenumberofbitsinabyte• Thedirectionofroundingwhenafloating-pointnumberisconvertedtoanarrowerfloating-pointnumber• Therulesforcomposingvalidfilenames

Relyingonimplementation-definedbehaviourcanmakeaprogramlessportableacrossimplementations.However,thisislesstruethanforunspecifiedandundefinedbehaviour.Also,asmanybasicproperties,suchasthesizesofthebasictypes,areimplementationdefined,itisvirtuallyimpossibletoavoidusingimplementationdefinedfeatures.

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Thefollowingcodeshowsanexampleofrelianceuponimplementation-definedbehaviour:unsigned char x = 100; x += (x << 2) + 1; // x = 5x + 1

Sincethewidthofunsignedcharisimplementation-defined,thecomputationonxwillyielddifferentresultsforimplementationswithdifferentwidths.6.57.2Guidancetolanguageusers

• FollowtheguidelinesofTR24772-1clause6.57.5.• Eliminatetotheextentpossibleanyrelianceonimplementation-definedbehaviourfromprogramsin

ordertoincreaseportability.Evenprogramsthatarespecificallyintendedforaparticularimplementationmayinthefuturebeportedtoanotherenvironmentorsectionsreusedforfutureimplementations.

6.58Deprecatedlanguagefeatures[MEM]

6.58.1ApplicabilitytolanguageCdeprecatedonefunction,thefunctiongets()andremoveditfromthestandardin2011.Chasdeprecatedseverallanguagefeaturesprimarilybytighteningtherequirementsforthefeature:

• Implicitintdeclarationsarenolongerallowed.• Functionscannotbeimplicitlydeclared.Theymustbedefinedbeforeuseorhaveaprototype.• Theuseofthefunctionungetc()atthebeginningofabinaryfileisdeprecated.• Areturnwithoutexpressionisnotpermittedinafunctionthatreturnsavalue(andviceversa).

• FollowtheguidelinesofTR24772-1clause6.58.5.• Althoughbackwardcompatibilityissometimesofferedasanoptionforcompilerssoonecanavoid

changestocodetobecompliantwithcurrentlanguagespecifications,updatingthelegacysoftwaretothecurrentstandardisabetteroption.

6.59Concurrency–Activation[CGA]

TheCstandard,inclause7.26.5.1,requiresaconformingimplementationtosetspecificreturncodestoindicatewhetherornotathreadactivationsucceeded;thereforethevulnerabilitydoesnotapplytotheClanguage.

However,iftheprogramfailstocheckthereturncodeandfailstotakeappropriateactiontohandleafailedthreadcreation,thevulnerabilitydescribedinclause6.36applies.

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6.60Concurrency–Directedtermination[CGT]

ThisvulnerabilitydoesnotapplytoCbecauseCdoesnotimplementamechanismtodirectlyterminateathread.Asimilareffectmaybeachievedbyaglobalflagrequestingthatathreadterminateitself,butthethreadisresponsibletoensurethatthatsuchterminationdoesn’toccuruntilallcriticalactivitiesarecompleted.

6.61Concurrentdataaccess[CGX]

Asstatedinclause5.1.2.4oftheCstandard,aprogramthatcontainsadataraceexhibitsundefinedbehaviour.Inadditiontothreads,signalhandlersalsoposeariskofconcurrentdataaccess.Itistheresponsibilityoftheapplicationtouseatomicvariablesormutexestoensurethatonethreadorsignalhandlercannotmodifyanobjectwhileanotherthreadorsignalhandlerisattemptingtoaccessthesameobject.Forsignalhandling,“volatilesig_atomic_t”oratomicvariablescanbeusedtopreventthisvulnerability.

• FollowtheguidelinesofTR24772-1clause6.61.5.• Useatomicvariableswhereappropriatetoavoiddataraces.• Usemutexesappropriatelytoprotectaccessestonon-atomicsharedobjects.Wheremutexesareused,

theprogrammermustshowthattherearenopathsintheprogramwhereareleasecanbemissed,eitherbecauseofconditionalcodeorothermechanisms.

• UsemutexestomodelHoaremonitorsorsimilarhighlevelabstractionsofsynchronization.• Use“volatilesig_atomic_t”toprotectdatasharedwithsignalhandlersinasingle-threadedenvironment.

6.62Concurrency–Prematuretermination[CGS]

ThisvulnerabilityappliestoCbecausethestandarddoesnotprovideamechanismtodeterminewhetherathreadhasterminated.

• FollowtheguidelinesofTR24772-1clause6.62.5.• Uselow-leveloperatingsystemprimitivesorotherAPIswhereavailabletocheckthatarequiredthreadis

stillactive.

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6.63Lockprotocolerrors[CGM]

ApplicationsinCmaycontainlockprotocolerrorssuchasamissingreleaseofamutex.SeeTR24772-1clause6.63fordescriptionsandmitigationsofprotocollockerrors.

• FollowtheguidelinesofTR24772-1clause6.63.5.• Beawareoftheoperationofeachsynchronizationmechanism,suchasthecaseswhereaccessesto

atomicvariablesmayoccurmorethanonceinastatement.

6.64UncontrolledFormatStrings[SHL]

ThestandardClibrariesprovidealargefamilyofinputandoutputfunctionsthatuseacontrolstringtointerpretthedatareadorformattheoutput.ThesestringsincludeallthefeaturedescribedinTR24772-1clause6.64.1.

7.LanguagespecificvulnerabilitiesforC

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8.Implicationsforstandardization

Futurestandardizationeffortsshouldconsider:• Movinginthedirectionovertimetobeingamorestronglytypedlanguage.Muchoftheuseofweak

typingissimplyconveniencetothedeveloperinnothavingtofullyconsiderthetypesandusesofvariables.Strongertypingforcesgoodprogrammingdisciplineandclarityaboutvariableswhileatthesametimeremovingmanyunexpectedruntimeerrorsduetoimplicitconversions.ThisisnottosaythatCshouldbestrictlyastronglytypedlanguage–someadvantagesofCareduetotheflexibilitythatweakertypingprovides.Itissuggestedthatwhenenforcementofstrongtypingdoesnotdetractfrom

Comment [CP19]: Myviewisthatthisshouldbedeleted–ThisisWG14’sjob,notours

thegoodflexibilitythatCoffers(forexample,addinganintegertoacharactertostepthroughasequenceofcharacters)andisonlyaconvenienceforprogrammers(forexample,addinganintegertoafloating-pointnumber),thenthestandardshouldspecifythestrongertypedsolution.

• AcommonwarninginAnnexIshouldbeaddedforfloating-pointexpressionsbeingusedinaBooleantestforequality.

• ModifyingordeprecatingmanyoftheCstandardlibraryfunctionsthatmakeassumptionsabouttheoccurrenceofastringterminationcharacter.

• Defineastringconstructthatdoesnotrelyonthenullterminationcharacter.• Defininganarraytypethatdoesautomaticboundschecking.• Deprecatinglesssafefunctionssuchasstrcpy()andstrcat()whereamoresecurealternativeisavailable.• Definingsaferandmoresecurereplacementfunctionssuchasmemncpy()andmemncmp()to

complementthememcpy()andmemcmp()functions(see6.11.6Implicationsforstandardization)• Defininganarraytypethatdoesautomaticboundschecking.• Definingfunctionsthatcontainanextraparameterinmemcpy()andmemmove()forthemaximum

numberofbytestocopy.Inthepast,somehavesuggestedthatthesizeofthedestinationbufferbeusedasanadditionalparameter.Somecriticsstatethatthissolutioniseasytocircumventbysimplyrepeatingtheparameterthatwasusedforthenumberofbytestocopyastheparameterforthesizeofthedestinationbuffer.Thisanalysisandcriticismiscorrect.Whatisneededisafailsafecheckastothemaximumnumberofbytestocopy.Thereareseveralreasonsforcreatingnewfunctionswithanadditionalparameter.Thiswouldmakeiteasierforstaticanalysistoeliminatethosecaseswherethememorycopycouldnotbeaproblem(suchaswhenthemaximumnumberofbytesisdemonstrablylessthanthecapacityofthereceivingbuffer).Manualanalysisormoreinvolvedstaticanalysiscouldthenbeusedfortheremainingsituationswherethesizeofthedestinationbuffermaynotbesufficientforthemaximumnumberofbytestocopy.Thisextraparametermayalsohelpindeterminingwhichcopiescouldtakeplaceamongobjectsthatoverlap.SuchcopyingisundefinedaccordingtotheCstandard.Itissuggestedthatsaferversionsoffunctionsthatincludearestrictionmax_nonthenumberofbytesntocopy(forexample,void*memncpy(void*restricts1,constvoid*restricts2,size_tn),constsize_tmax_n)beaddedtothestandardinadditiontoretainingthecurrentcorrespondingfunctions(forexample,memcpy(void*restricts1,constvoid*restricts2,size_tn))).Theadditionalparameterwouldbeconsistentwiththecopyingfunctionpairsthathavealreadybeencreatedsuchasstrcpy()/strncpy()andstrcat()/strncat().Thiswouldallowasaferversionofmemorycopyingfunctionsforthoseapplicationsthatwanttousethemintofacilitatebothsaferandmoresecurecodeandmoreefficientandaccuratestaticcodereviews7.

• Restrictionsonpointerarithmeticthatcouldeliminatecommonpitfalls.Pointerarithmeticiserror-proneandtheflexibilitythatitoffersisuseful,butsomeoftheflexibilityissimplyashortcutthatifrestrictedcouldlessenthechanceofapointerarithmeticbasederror.

• Definingastandardwayofdeclaringanattributetoindicatethatavariableisintentionallyunused.• AcommonwarninginAnnexIshouldbeaddedforvariableswiththesamenameinnestedscopes.• Creatingafewstandardizedprecedenceorders.Standardizingonafewprecedenceorderswillhelpto

eliminatetheconfusingintricaciesthatexistbetweenlanguages.Thiswouldnotaffectcurrentlanguages

7ThishasbeenaddressedbyWG14inanoptionallynormativeannexinthecurrentworkingpaper

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asalteringprecedenceordersinexistinglanguagesistooonerous.However,thiswouldsetabasisforthefutureasnewlanguagesarecreatedandadopted.Statingthatalanguageuses“ISOprecedenceorderA”wouldbeusefulratherthanhavingtospellouttheentireprecedenceorderthatdiffersinaconceptuallyminorwayfromsomeotherlanguages,butinamajorwaywhenprogrammersattempttoswitchbetweenlanguages.

• Deprecatingthegotostatement.Theuseofthegotoconstructisoftenspotlightedastheantithesisofgoodstructuredprogramming.ThoughitsdeprecationwillnotinstantlymakeallCcodestructured,deprecatingthegotoandleavinginplacetherestrictedgotovariations(forexample,breakandcontinue)andpossiblyaddingotherrestrictedgoto’scouldassistinencouragingsaferandmoresecureCprogrammingingeneral.

• Defininga“fallthru”constructthatwillexplicitlybindmultipleswitchcasestogetherandeliminatetheneedforthebreakstatement.Thedefaultwouldbeforacasetobreakinsteadoffallingthroughtothenextcase.Grantedthisisamajorshiftinconcept,butifitcouldbeaccomplished,lessunintentionalerrorswouldoccur.

• DefininganidentifiertypeforloopcontrolthatcannotbemodifiedbyanythingotherthantheloopcontrolconstructwouldbearelativelyminoradditiontoCthatcouldmakeCcodesaferandencouragebetterstructuredprogramming.

• DefiningastandardizedinterfacepackageforinterfacingCwithmanyofthetopprogramminglanguagesandareciprocalpackageshouldbedevelopedoftheothertoplanguagestointerfacewithC.

• Joiningwithotherlanguagesindevelopingastandardizedsetofmechanismsfordetectingandtreatingerrorconditionssothatalllanguagestotheextentpossiblecouldusethem.Notethatthisdoesnotmeanthatalllanguagesshouldusethesamemechanismsasthereshouldbeavariety(labelparameters,auxiliarystatusvariables),buteachofthemechanismsshouldbestandardized.

• Sincefaulthandlingandexitingofaprogramiscommontoalllanguages,itissuggestedthatcommonterminologysuchasthemeaningoffailsafe,failhard,failsoft,andsoonalongwithacoreAPIsetsuchasexit,abort,andsoonbestandardizedandcoordinatedwithotherlanguages.

• Deprecatingunions.Theprimaryreasonfortheuseofunionstosavememoryhasbeendiminishedconsiderablyasmemoryhasbecomecheaperandmoreavailable.Unionsarenotstaticallytypesafeandarehistoricallyknowntobeacommonsourceoferrors,leadingtomanyCprogrammingguidelinesspecificallyprohibitingtheuseofunions.

• Creatingarecognizablenamingstandardforroutinessuchthatoneversionofalibrarydoesparametercheckingtotheextentpossibleandanotherversiondoesnoparameterchecking.Thefirstversionwouldbeconsideredsaferandmoresecureandthesecondcouldbeusedincertainsituationswhereperformanceiscriticalandthecheckingisassumedtobedoneinthecallingroutine.Anamingstandardcouldbemadesuchthatthelibrarythatdoesparametercheckingcouldbenamedasusual,say“library_xyz”andanequivalentversionthatdoesnotdocheckingcouldhavea“_p”appended,suchas“library_xyz_p”.Withoutanamingstandardsuchasthis,aconsiderablenumberofwastedcycleswillbeconducteddoingadoublecheckofparametersorevenworse,nocheckingwillbedoneinboththecallingandreceivingroutinesaseachisassumingtheotherisdoingthechecking.

• CreatinganAnnexthatlistsdeprecatedfeatures.

Bibliography

[1] ISO/IECDirectives,Part2,RulesforthestructureanddraftingofInternationalStandards,2004

[2] ISO/IECTR10000-1,Informationtechnology—FrameworkandtaxonomyofInternationalStandardized

Profiles—Part1:Generalprinciplesanddocumentationframework

[3] ISO10241(allparts),Internationalterminologystandards

[4] ISO/IEC9899:2011,Informationtechnology—Programminglanguages—C

[5] ISO/IEC9899:2011/Cor.1:2012,TechnicalCorrigendum1

[6] ISO/IEC/IEEE60559:2011,Informationtechnology–MicroprocessorSystems–Floating-Pointarithmetic

[7] R.Seacord,TheCERTCSecureCodingStandard.Boston,MA:Addison-Westley,2008.

[8] MotorIndustrySoftwareReliabilityAssociation.GuidelinesfortheUseoftheCLanguageinVehicleBasedSoftware,2012(thirdedition)16F

[9] ISO/IECTR24731–1,Informationtechnology—Programminglanguages,theirenvironmentsandsystem

softwareinterfaces—ExtensionstotheClibrary—Part1:Bounds-checkinginterfaces

[10] L.Hatton,SaferC:developingsoftwareforhigh-integrityandsafety-criticalsystems.McGraw-Hill1995

[11] SoftwareConsiderationsinAirborneSystemsandEquipmentCertification.IssuedintheUSAbytheRequirementsandTechnicalConceptsforAviation(documentRTCASC167/DO-178B)andinEuropebytheEuropeanOrganizationforCivilAviationElectronics(EUROCAEdocumentED-12B).December1992.

[12] IEC61508:Parts1-7,Functionalsafety:safety-relatedsystems.1998.(Part3isconcernedwithsoftware).

[13] ISO/IEC15408:1999Informationtechnology.Securitytechniques.EvaluationcriteriaforITsecurity.

[14] Hogaboom,Richard,AGenericAPIBitManipulationinC,EmbeddedSystemsProgramming,Vol12,No7,July1999http://www.embedded.com/1999/9907/9907feat2.htm

[15] Seacord,R.SecureCodinginCandC++.Boston,MA:Addison-Wesley,2005.Seehttp://www.cert.org/books/secure-codingfornewsanderrata.

[16] TheCommonWeaknessEnumeration(CWE)Initiative,MITRECorporation,(http://cwe.mitre.org/)

[17] ISO/IECTS17961,Informationtechnology–Programminglanguages,theirenvironmentsandsystem

softwareinterfaces–Csecurecodingrules

[18] Kernighan,Ritchie,TheCProgrammingLanguage(1stEdition),PrenticeHall1978

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CGM–LockprotocolErrors,45CGS–Concurrency–Prematuretermination,44LanguageVulnerabilities

Concurrency–Prematuretermination[CGS],44LockprotocolErrors[CGM],45

Uncontrolledformatstring[SHL],45rsize_t,13SHL–Uncontrolledformatstring,45size_t,13

[1] Deleted Clive Pygott 12/22/17 3:22:00 PM

Note:Abit-fieldandanadjacentnon-bit-fieldmemberareinseparatememorylocations.Thesameappliestotwobit-fields,ifoneisdeclaredinsideanestedstructuredeclarationandtheotherisnot,orifthetwoareseparatedbyazero-lengthbit-fielddeclaration,oriftheyareseparatedbyanon-bit-fieldmemberdeclaration.Itisnotsafetoconcurrentlyupdatetwobit-fieldsinthesamestructureifallmembersdeclaredbetweenthemarealsobit-fields,nomatterwhatthesizesofthoseinterveningbit-fieldshappentobe.Forexampleastructuredeclaredas struct { char a; int b:5, c:11, :0, d:8; struct { int ee:8; } e; } containsfourseparatememorylocations:Themembera,andbit-fieldsdande.eeareseparatememorylocations,andcanbemodifiedconcurrentlywithoutinterferingwitheachother.Thebit-fieldsbandctogetherconstitutethefourthmemorylocation.Thebit-fieldsbandccan’tbeconcurrentlymodified,butbanda,canbeconcurrentlymodified[CP1]

Everyguidanceprovidedinthissection,andinthecorrespondingPartsection,issupportedmaterialinClause6ofthisdocument,aswellasotherimportantrecommendations.

Example:s=(structfoo*)malloc(sizeof(structfoo));usestheCtypesystemtoenforcethatthepointertotheallocatedspacewillbeofatypethatisappropriateforthesize.Becausemallocreturnsavoid *,withoutthecast,"s"couldbeofanyrandompointertype,withthecast,thatmistakewillbecaught

Theintegerconversionrankisusedintheusualarithmeticconversionstodeterminewhatconversionsneedtotakeplacetosupportanoperationonmixedintegertypes.

Otherconversionrulesexistforotherdatatype-conversions.Soeventhoughtherearerulesinplaceandtherulesareratherstraightforward,thevarietyandcomplexityoftherulescancauseunexpectedresultsandpotentialvulnerabilities.Forexample,thoughthereisaprescribedorderinwhich

conversionswilltakeplace,determininghowtheconversionswillaffectthefinalresultcanbedifficultasinthefollowingexample: long foo (short a, int b, int c, long d, long e, long f) { return (((b + f) * d – a + e) / c); } Theimplicitconversionsperformedinthereturnstatementcanbenontrivialtodiscern,butcangreatlyimpactwhetheranyoftheintermediatevaluesoverflowduringthecomputation.

Beawarethatimplicitcastsmaymaketheresultingtypeofanexpressionfloating-point[CP2]. Donotconvertafloating-pointnumbertoanintegerunlesstheconversionisaspecified

algorithmicrequirementorisrequiredforahardwareinterface[CP3].

Inparticular,makecastsexplicitinthereturnvalueofmallocExample:s = (struct foo*)malloc(sizeof(struct foo)); ThisusestheCtypesystemtoenforcethatthepointertotheallocatedspacewillbeofatypethatisappropriateforthesize.Becausemallocreturnsavoid *,withoutthecast,scouldbeofanyrandompointertype;withthecast,thatmistakewillbecaught

dynamic

Cisoftenusedforbitmanipulation.PartofthisisduetothecapabilitiesinCtomaskbitsandshiftthem.AnotherpartisduetotherelativeclosenessChastoassemblyinstructions.Manipulatingbitsonasignedvaluecaninadvertentlychangethesignbitresultinginanumberpotentiallygoingfromapositivevaluetoanegativevalue.

Keepinmindthatcodewillbereusedandcombinedinwaysthattheoriginaldevelopersneverimagined.

MakenamesdistinguishablewithinthefirstfewcharactersduetoscopinginC.Thiswillalsoassistinavertingproblemswithcompilersresolvingtoashorternamethanwasintended.

[10] Deleted Clive Pygott 12/19/17 9:58:00 AM

Ensurethatadefinitionofanentitydoesnotoccurinascopewhereadifferententitywiththesamenameisaccessibleandhasatypethatpermitsittooccurinatleastonecontextwherethefirstentitycanoccur.

Ensurethatallidentifiersdifferwithinthenumberofcharactersconsideredtobesignificantbytheimplementationsthatarelikelytobeused,anddocumentallassumptions.

Cprovidessignificantoffreedominconstructingstatements.Thisfreedom,ifmisused,canresultinunexpectedresultsandpotentialvulnerabilities.TheflexibilityofCcanobscuretheintentofaprogrammer.

Encouragetheuseofasingleexitpointfromafunction.

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