how to get the address of the entry point in an API
 

I read the microsoft docs about PE and tried to understand how to get the address of the entry point in an API but i still don't got it.I know how to get each imported dll and imported functions for each dll but don't know how to get the [xxxx] address from jmp dword ptr [xxxx] that is used to call an import function.
Can someone enlight me ... I know only the RVA's to the names of the functions


// Get a pointer to the found module's import address table (IAT)
// =====IMAGE_THUNK_DATA *pThunk;
pThunk = MakePtr(PIMAGE_THUNK_DATA, pDosHeader, pImportDesc->FirstThunk);
//This is what i was talkin about earlier...
//In pThunk, if it was image loaded in memory, you'll get the address to
//entry point of functions
//but in a disk file, It's a function name

I think you might be wanting: LoadLibrary yourdll and then GetProcAddress the api you want. It will return the entry point of an api. Check out an api help file or search for these commands.

Hello:

Just spend a little time reading these tutorials, sure you are finding there your answer:

http://spiff.tripnet.se/~iczelion/tutorials.html

In that web, go to "PE tutorials", and there, "Import table" and "Export table". You can find some tools to test all that these tutorials are teaching you.

Good luck! :cool:

Nacho_dj

Look at this code. It part of programm
which takes ntdll.dll(on disk file) and generates something like this
/*w2k3callx.h*/
MagicFoo (NtAcceptConnectPort, 24) //0
MagicFoo (NtAccessCheck, 32) //1
MagicFoo (NtAccessCheckAndAuditAlarm, 44) //2
MagicFoo (NtAccessCheckByType, 44) //3
MagicFoo (NtAccessCheckByTypeAndAuditAlarm, 64) //4
MagicFoo (NtAccessCheckByTypeResultList, 44) //5
MagicFoo (NtAccessCheckByTypeResultListAndAuditAlarm, 64) //6
MagicFoo (NtAccessCheckByTypeResultListAndAuditAlarmByHandle, 68) //7
MagicFoo (NtAddAtom, 12) //8
....

#####################################
...
#define MAKESECTVA(rva,sectva) (DWORD)rva-(DWORD)sectva
#define MAKERAW(rva,sraw,setcva) (DWORD)sraw+MAKESECTVA(rva,setcva)
...
void Export :: processdll(std::string dllname)
{
FILE *f_dll= fopen(dllname.c_str(),"rb");
struct pe_header_t hdr;

DWORD sectVA=0;

IMAGE_DOS_HEADER ddh;
IMAGE_NT_HEADERS32 hdr2;


char *sectdata;
char *exportData;

if(f_dll)
{
fread(&ddh,sizeof(ddh),1,f_dll);

fseek(f_dll,ddh.e_lfanew,FILE_BEGIN);

fread(&hdr2,sizeof(hdr2),1,f_dll);


#ifdef INFORMATE
printf("\n\tINFO:export va=%x(hex) ",hdr2.OptionalHeader.DataDirectory[0].VirtualAddress);
printf("\tsize=%d(decimal)",hdr2.OptionalHeader.DataDirectory[0].Size);
#endif
sectdata=(char *)malloc(sizeof(IMAGE_SECTION_HEADER)*hdr2.FileHeader.NumberOfSections);
fread(sectdata,sizeof(IMAGE_SECTION_HEADER)*hdr2.FileHeader.NumberOfSections,1,f_dll);

//PIMAGE_SECTION_HEADER sects = IMAGE_FIRST_SECTION32(&hdr2);
PIMAGE_SECTION_HEADER sects=(PIMAGE_SECTION_HEADER)sectdata;
BOOL wasfound=FALSE;
for(int i=0;i<hdr2.FileHeader.NumberOfSections;i++)
{
if(sects->VirtualAddress<=hdr2.OptionalHeader.DataDirectory[0].VirtualAddress &&
sects->VirtualAddress+sects->Misc.VirtualSize>hdr2.OptionalHeader.DataDirectory[0].VirtualAddress)
{
wasfound=TRUE;
break;
}
sects++;
}

if(wasfound)
{

exportData=(char *)malloc(hdr2.OptionalHeader.DataDirectory[0].Size);
if(exportData)
{
fseek(f_dll,
sects->PointerToRawData+
hdr2.OptionalHeader.DataDirectory[0].VirtualAddress-
sects->VirtualAddress
,FILE_BEGIN);
fread(exportData,
hdr2.OptionalHeader.DataDirectory[0].Size,
1,
f_dll);
PIMAGE_EXPORT_DIRECTORY pexp=(PIMAGE_EXPORT_DIRECTORY)exportData;
#ifdef INFORMATE
printf("\n\tINFO:exports number=%d(decimal)",pexp->NumberOfFunctions);
#endif

PDWORD address_t,name_t;
unsigned short *ordinal_t;
PDWORD raddress_t,rname_t;
unsigned short *rordinal_t;
raddress_t=address_t=(PDWORD)malloc(pexp->NumberOfFunctions*sizeof(DWORD));
fseek(f_dll,
MAKERAW(pexp->AddressOfFunctions,
sects->PointerToRawData,
sects->VirtualAddress
)
,FILE_BEGIN);
fread(address_t,pexp->NumberOfFunctions*sizeof(DWORD),1,f_dll);
rname_t=name_t=(PDWORD)malloc(pexp->NumberOfNames*sizeof(DWORD));
int offset=MAKERAW(pexp->AddressOfNames,
sects->PointerToRawData,
sects->VirtualAddress);
fseek(f_dll,
offset
,FILE_BEGIN);
fread(name_t,pexp->NumberOfNames*sizeof(DWORD),1,f_dll);
rordinal_t=ordinal_t=(unsigned short *)malloc(pexp->NumberOfNames*sizeof(DWORD));

offset=MAKERAW(pexp->AddressOfNameOrdinals,
sects->PointerToRawData,
sects->VirtualAddress);

fseek(f_dll,
offset
,FILE_BEGIN);

fread(ordinal_t,pexp->NumberOfNames*sizeof(unsigned short),1,f_dll);

for(int i=0;i<pexp->NumberOfFunctions;i++,address_t++,name_t++,ordinal_t++)
{
int ianumber=0;
unsigned char funcdata[15];
char funcname[1024];
std::string funcname2;

//(*address_t)
fseek(f_dll,
MAKERAW((*name_t),
sects->PointerToRawData,
sects->VirtualAddress)
,FILE_BEGIN);
fread(funcname,1024,1,f_dll);

if(used->find(dllname,funcname))
{
printf("\n\tFound %s",funcname);

//磬 滂耜?铕滂磬臌 脲驵?raw, ?? 徨?OrdinalBase
DWORD ordinal=*ordinal_t;



PIMAGE_SECTION_HEADER sects2=(PIMAGE_SECTION_HEADER)sectdata;
BOOL wasfound=FALSE;
for(int i=0;i<hdr2.FileHeader.NumberOfSections;i++)
{
if(sects2->VirtualAddress<=(raddress_t[ordinal]) &&
sects2->VirtualAddress+sects2->Misc.VirtualSize>(raddress_t[ordinal]))
{
wasfound=TRUE;
break;
}
sects2++;
}

if(wasfound)
{
fseek(f_dll,
MAKERAW(raddress_t[ordinal],
sects2->PointerToRawData,
sects2->VirtualAddress)
,FILE_BEGIN);

fread(funcdata,15,1,f_dll);
printf("\n\t");

/*for(int j=0;j<8;j++)
printf("%x ",funcdata[j]);*/

DWORD api_num=*((PDWORD)(&funcdata[1]));
DWORD ret_size=0x666;
if(funcdata[0xc]==(unsigned char)0xc2)
{
ret_size=*((unsigned short *)(&funcdata[13]));
}
else if(funcdata[0xc]==(unsigned char)0xc3)
ret_size=0;

if(ret_size!=0x666)
used->output(dllname,funcname,api_num,ret_size);
else
used->outputAlarm(dllname,funcname,api_num,ret_size);
}
#ifdef INFORMATE
else
{
printf("\nERROR:Section with function was not found in - %s",dllname.c_str());
}
#endif




//funcdata 耦溴疰栩 RVA 磬 趔黻鲨?





}

memset(funcname,0,strlen(funcname));
//(_dlldata [dllname])[funcname]=ianumber;
}
free(rordinal_t);
free(raddress_t);
free(rname_t);
free(exportData);
}
}
#ifdef INFORMATE
else
{
printf("\nERROR:Section with export data was not found in - %s",dllname.c_str());
}
#endif

free(sectdata);
fclose(f_dll);
}
#ifdef INFORMATE
else
{
printf("\nERROR:File access error - %s",dllname.c_str());
}
#endif
}

When the program is executed it looks for the IAT wich contains the address of the names of the apis to be imported, then the dlls are loaded with loadlibrary and the address of the apis are replaced with the result of getprocaddress.

nice source of code ..can you share it where you found it??

bye nEO

Found!? :)
I wrote it some time ago.
I wanted to port strace to w2k3