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<title>5.README_MISSING_SYSCALL_OR_IOCTL</title>
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<div class="titlepage"><div><div><h1 class="title">
<a name="dist.readme-missing"></a>5.README_MISSING_SYSCALL_OR_IOCTL</h1></div></div></div>
<div class="literallayout"><p><br>
<br>
DealingwithmissingsystemcallorioctlwrappersinValgrind<br>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br>
You'reprobablyreadingthisbecauseValgrindbombedoutwhilst<br>
runningyourprogram,andadvisedyoutoreadthisfile.Thegood<br>
newsisthat,ingeneral,it'seasytowritethemissingsyscallor<br>
ioctlwrappersyouneed,sothatyoucancontinueyourdebugging.If<br>
yousendtheresultingpatchestome,thenyou'llbedoingafavourto<br>
allfutureValgrinduserstoo.<br>
<br>
Notethatan"ioctl"isjustaspecialkindofsystemcall,really;so<br>
there'snotalotofneedtodistinguishthem(atleastconceptually)<br>
inthediscussionthatfollows.<br>
<br>
Allthismachineryisincoregrind/m_syswrap.<br>
<br>
<br>
Whataresyscall/ioctlwrappers?Whatdotheydo?<br>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br>
Valgrinddoeswhatitdoes,inpart,bykeepingtrackofeverythingyour<br>
programdoes.Whenasystemcallhappens,forexamplearequesttoread<br>
partofafile,controlpassestotheLinuxkernel,whichfulfillsthe<br>
request,andreturnscontroltoyourprogram.Theproblemisthatthe<br>
kernelwilloftenchangethestatusofsomepartofyourprogram'smemory<br>
asaresult,andtools(instrumentationplug-ins)mayneedtoknowabout<br>
this.<br>
<br>
Syscallandioctlwrappershavetwojobs:<br>
<br>
1.Tellatoolwhat'sabouttohappen,beforethesyscalltakesplace.A<br>
toolcouldperformchecksbeforehand,eg.ifmemoryabouttobewritten<br>
isactuallywriteable.Thispartisuseful,butnotstrictly<br>
essential.<br>
<br>
2.Tellatoolwhatjusthappened,afterasyscalltakesplace.Thisis<br>
soitcanupdateitsviewoftheprogram'sstate,eg.thatmemoryhas<br>
justbeenwrittento.Thisstepisessential.<br>
<br>
The"happenings"mostlyinvolvereading/writingofmemory.<br>
<br>
So,let'slookatanexampleofawrapperforasystemcallwhich<br>
shouldbefamiliartomanyUnixprogrammers.<br>
<br>
<br>
Thesyscallwrapperfortime()<br>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br>
Thewrapperforthetimesystemcalllookslikethis:<br>
<br>
PRE(sys_time)<br>
{<br>
/*time_ttime(time_t*t);*/<br>
PRINT("sys_time(%p)",ARG1);<br>
PRE_REG_READ1(long,"time",int*,t);<br>
if(ARG1!=0){<br>
PRE_MEM_WRITE("time(t)",ARG1,sizeof(vki_time_t));<br>
}<br>
}<br>
<br>
POST(sys_time)<br>
{<br>
if(ARG1!=0){<br>
POST_MEM_WRITE(ARG1,sizeof(vki_time_t));<br>
}<br>
}<br>
<br>
Thefirstthingwedohappensbeforethesyscalloccurs,inthePRE()function.<br>
ThePRE()functiontypicallystartswithinvokingtothePRINT()macro.This<br>
PRINT()macroimplementssupportforthe--trace-syscallscommandlineoption.<br>
Next,thetoolistoldthereturntypeofthesyscall,thatthesyscallhas<br>
oneargument,thetypeofthesyscallargumentandthattheargumentisbeing<br>
readfromaregister:<br>
<br>
PRE_REG_READ1(long,"time",int*,t);<br>
<br>
Next,ifanon-NULLbufferispassedinastheargument,tellthetoolthatthe<br>
bufferisabouttobewrittento:<br>
<br>
if(ARG1!=0){<br>
PRE_MEM_WRITE("time",ARG1,sizeof(vki_time_t));<br>
}<br>
<br>
Finally,thereallyimportantbit,afterthesyscalloccurs,inthePOST()<br>
function:if,andonlyif,thesystemcallwassuccessful,tellthetoolthat<br>
thememorywaswritten:<br>
<br>
if(ARG1!=0){<br>
POST_MEM_WRITE(ARG1,sizeof(vki_time_t));<br>
}<br>
<br>
ThePOST()functionwon'tbecalledifthesyscallfailed,soyou<br>
don'tneedtoworryaboutcheckingthatinthePOST()function.<br>
(Note:thisissometimesabug;somesyscallsdoreturnresultswhen<br>
they"fail"-forexample,nanosleepreturnstheamountofunslept<br>
timeifinterrupted.TODO:addanotherper-syscallflagforthis<br>
case.)<br>
<br>
Notethatweusethetype'vki_time_t'.Thisisacopyofthekernel<br>
type,with'vki_'prefixed.Ourcopiesofsuchtypesarekeptinthe<br>
appropriatevki*.hfile(s).Wedon'tincludekernelheadersorglibcheaders<br>
directly.<br>
<br>
<br>
Writingyourownsyscallwrappers(seebelowforioctlwrappers)<br>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br>
IfValgrindtellsyouthatsystemcallNNNisunimplemented,dothe<br>
following:<br>
<br>
1.Findoutthenameofthesystemcall:<br>
<br>
grepNNN/usr/include/asm/unistd*.h<br>
<br>
Thisshouldtellyousomethinglike__NR_mysyscallname.<br>
Copythisentrytoinclude/vki/vki-scnums-$(VG_PLATFORM).h.<br>
<br>
<br>
2.Do'man2mysyscallname'togetsomeideaofwhatthesyscall<br>
does.Notethattheactualkernelinterfacecandifferfromthis,<br>
soyoumightalsowanttocheckaversionoftheLinuxkernel<br>
source.<br>
<br>
NOTE:anysyscallwhichhassomethingtodowithsignalsor<br>
threadsisprobably"special",andneedsmorecarefulhandling.<br>
Postsomethingtovalgrind-developersifyouaren'tsure.<br>
<br>
<br>
3.Addacasetothealready-hugecollectionofwrappersin<br>
thecoregrind/m_syswrap/syswrap-*.cfiles.<br>
Foreachin-memoryparameterwhichisreadorwrittenby<br>
thesyscall,dooneof<br>
<br>
PRE_MEM_READ(...)<br>
PRE_MEM_RASCIIZ(...)<br>
PRE_MEM_WRITE(...)<br>
<br>
forthatparameter.Thendothesyscall.Then,ifthesyscall<br>
succeeds,issuesuitablePOST_MEM_WRITE(...)calls.<br>
(There'snoneedforPOST_MEM_READcalls.)<br>
<br>
Also,addittothesyscall_table[]array;useoneofGENX_,GENXY<br>
LINX_,LINXY,PLAX_,PLAXY.<br>
GEN*forgenericsyscalls(insyswrap-generic.c),LIN*forlinux<br>
specificones(insyswrap-linux.c)andPLA*fortheplatform<br>
dependantones(insyswrap-$(PLATFORM)-linux.c).<br>
The*XYvariantifitrequiresaPRE()andPOST()function,and<br>
the*X_variantifitonlyrequiresaPRE()<br>
function.<br>
<br>
Ifyoufindthisdifficult,readthewrappersforothersyscalls<br>
forideas.Agoodtipistolookforthewrapperforasyscall<br>
whichhasasimilarbehaviourtoyours,anduseitasa<br>
startingpoint.<br>
<br>
Ifyouneedstructuredefinitionsand/orconstantsforyoursyscall,<br>
copythemfromthekernelheadersintoinclude/vki.handco.,with<br>
theappropriatevki_*/VKI_*namemangling.Don't#includeany<br>
kernelheaders.Andcertainlydon't#includeanyglibcheaders.<br>
<br>
Testit.<br>
<br>
NotethatacommonerroristocallPOST_MEM_WRITE(...)<br>
with0(NULL)asthefirst(address)argument.Thisusuallymeans<br>
yourlogicisslightlyinadequate.It'sasufficientlycommonbug<br>
thatthere'sabuilt-incheckforit,andyou'llgeta"probably<br>
sanitycheckfailure"forthesyscallwrapperyoujustmade,ifthis<br>
isthecase.<br>
<br>
<br>
4.Oncehappy,sendusthepatch.Prettyplease.<br>
<br>
<br>
<br>
<br>
Writingyourownioctlwrappers<br>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br>
<br>
Isprettymuchthesameaswritingsyscallwrappers,exceptthatall<br>
theactionhappenswithinPRE(ioctl)andPOST(ioctl).<br>
<br>
There'sadefaultcase,sometimesitisn'tcorrectandyouhavetowritea<br>
morespecificcasetogettherightbehaviour.<br>
<br>
Asabove,pleasecreateabugreportandattachthepatchasdescribed<br>
onhttp://www.valgrind.org.<br>
<br>
<br>
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