Calling a function in an injected DLL?

Question

Using C++, I have an application which creates a remote process and injects a DLL into it. Is there a way to get the remote application to execute a function exported from the DLL, from the application which created it? And is it possible to send parameters to that function? Please note that I am trying to stay away from doing anything within DllMain.

Answer 1

Note:
For a much better answer, please see my update posted below!

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Okay so here's how I was able to accomplish this:

BOOL RemoteLibraryFunction( HANDLE hProcess, LPCSTR lpModuleName, LPCSTR lpProcName, LPVOID lpParameters, SIZE_T dwParamSize, PVOID *ppReturn )
{
    LPVOID lpRemoteParams = NULL;

    LPVOID lpFunctionAddress = GetProcAddress(GetModuleHandleA(lpModuleName), lpProcName);
    if( !lpFunctionAddress ) lpFunctionAddress = GetProcAddress(LoadLibraryA(lpModuleName), lpProcName);
    if( !lpFunctionAddress ) goto ErrorHandler;

    if( lpParameters )
    {
        lpRemoteParams = VirtualAllocEx( hProcess, NULL, dwParamSize, MEM_RESERVE|MEM_COMMIT, PAGE_EXECUTE_READWRITE );
        if( !lpRemoteParams ) goto ErrorHandler;

        SIZE_T dwBytesWritten = 0;
        BOOL result = WriteProcessMemory( hProcess, lpRemoteParams, lpParameters, dwParamSize, &dwBytesWritten);
        if( !result || dwBytesWritten < 1 ) goto ErrorHandler;
    }

    HANDLE hThread = CreateRemoteThread( hProcess, NULL, 0, (LPTHREAD_START_ROUTINE)lpFunctionAddress, lpRemoteParams, NULL, NULL );
    if( !hThread ) goto ErrorHandler;

    DWORD dwOut = 0;
    while(GetExitCodeThread(hThread, &dwOut)) {
        if(dwOut != STILL_ACTIVE) {
            *ppReturn = (PVOID)dwOut;
            break;
        }
    }

    return TRUE;

ErrorHandler:
    if( lpRemoteParams ) VirtualFreeEx( hProcess, lpRemoteParams, dwParamSize, MEM_RELEASE );
    return FALSE;
}

//...
CStringA targetDll = "injected.dll"

    // Inject the target library into the remote process
PVOID lpReturn = NULL;
RemoteLibraryFunction( hProcess, "kernel32.dll", "LoadLibraryA", targetDll.GetBuffer(MAX_PATH), targetDll.GetLength(), &lpReturn );
HMODULE hInjected = reinterpret_cast<HMODULE>( lpReturn );

    // Call our exported function
lpReturn = NULL;
RemoteLibraryFunction( hProcess, targetDll, "Initialize", NULL, 0, &lpReturn );
BOOL RemoteInitialize = reinterpret_cast<BOOL>( lpReturn );

This can also be used to send parameters to a remote function via a pointer to a struct or union, and gets around having to write anything in DllMain.