Easyhook: unmanaged hooking, how to call original function / change return status?

Question

So I have a hook function at winspool.drv!WritePrinter, which is successfully hooked with unmanaged C++ remotely injected to spoolsv.exe.

Currently, the hook seems to either replace original function, or corrupt the stack in an undetectable way: after hooking, WritePrinter calls result in no printer activity outside the hook.

I've figured out there's at least one way to call original function, so-called LhGetOldProc. However, using it leads to crashes, don't sure if this is easyhook-related error or it's just bad casting.

So, how do I properly call original function in Easyhook unmanaged version?

Hook callback with LhGetOldProc:

UCHAR *uc = NULL;
LhGetOldProc(hhW, &uc);
typedef BOOL (*wp)(_In_   HANDLE, _In_   LPVOID, _In_   DWORD cbBuf, _Out_  LPDWORD);
wp my_wp = reinterpret_cast<wp>(reinterpret_cast<long>(uc)); // http://stackoverflow.com/questions/1096341/function-pointers-casting-in-c

BOOL res ;
if (my_wp == 0x0) {
 return -1;
} else {
 res  = my_wp(hPrinter, pBuf, cbBuf, pcWritten); // crash
}

Hook code:

HMODULE                 hSpoolsv = LoadLibraryA("winspool.drv");
TRACED_HOOK_HANDLE      hHook = new HOOK_TRACE_INFO();
NTSTATUS                NtStatus;
UNICODE_STRING*         NameBuffer = NULL;
HANDLE                  hRemoteThread;
FORCE(LhInstallHook(GetProcAddress(hSpoolsv, "WritePrinter"), WritePrinterHookA, 0x0, hHook));
ULONG ACLEntries[1] = { (ULONG) - 1 };
FORCE(LhSetExclusiveACL(ACLEntries, 1, hHook));

hhW = hHook;

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TIL: in 2013, CodePlex (where EasyHook discussion list is) doesn't accept third level domains for e-mail when registering with Microsoft account. Not going to use Firebug to bypass the form.

Answer 1

The stack gets corrupted because your function pointer has the wrong calling convention.

The default calling convention is __cdecl which expects the caller to clean the stack.

typedef BOOL (* wp)(_In_   HANDLE ....);

equals:

typedef BOOL (__cdecl* wp)(_In_   HANDLE ...);

but the winapi functions use __stdcall calling convention which expects the callee to clean the stack. you will have to typedef a __stdcall function:

typedef BOOL (__stdcall* wp)(_In_   HANDLE ....);