CODEWITHSUNDEEP
c++program-entry-pointportable-executablesystems-programmingentry-point
Dmitry Katkevich
Dmitry Katkevich

Reputation: 923

Call "main" function programmatically in Windows

I have a third-party console application. I need run it from my application but I cannot run it as a separate process (because I need to work with its dependencies: fill Import tables manually, setup hooks etc.). So probably I should call main function of this executable manually. Here is how I'm trying to do this:

  1. Load this EXE using auto hMod = LoadLibrary("console_app.exe")
  2. Fill Import table of this exe manually
  3. Get entry point of this EXE and call it

And I'm stuck with the last step.

Here is how I'm trying to call entry point:

void runMain(HINSTANCE hInst)
{
    typedef BOOL(WINAPI *PfnMain)(int, char*[]);

    auto imageNtHeaders = ImageNtHeader(hInst);
    auto pfnMain = (PfnMain)(DWORD_PTR)(imageNtHeaders->OptionalHeader.AddressOfEntryPoint + (DWORD_PTR)hInst);

    char* args[] = { R"(<console_app_path>)", R"(arg1)", R"(arg2)" };
    pfnMain(3, args);
}

It works. But it works as if there is no arguments.

Where am I wrong? How can I run an executable inside my process with arguments? Thanks.

UPDATE:

I've investigated how my particular third-party exe gets cmd arguments and found that:

  1. It doesn't import GetCommandLine at all and do not call it
  2. After call _initterm call argc and argv arguments are available through cs:argc and cs:argv (see pictures below) enter image description here enter image description here
  3. CMD arguments that I pass to my main console app are transferred to child EXE too.

Can you explain, please, what _initterm actually do and where CMD arguments are actually stored?

Upvotes: 12

Views: 2332

Answers (2)

MSalters
MSalters

Reputation: 180245

You're calling the entry point of the application, not int main(int, char**). Now you may have read that the entry point of a C++ program is int main(int, char**) but that's just a C++ perspective.

The Win32 perspective is different; the entry point is a int (*)(void);. The Visual Studio linker looks for int mainCRTStartup(void); and uses that, unless you specify another entry point with /ENTRY. The default implementation of mainCRTStartup calls GetCommandLine() to fill in argv[] before calling main(argc,argv). There are also other things in mainCRTStartup which you might want to happen: run global ctors, initialize the CRT state, ...

Of course, that's assuming the other program was compiled with Visual C++, but whatever language it's been written in, it must be calling GetCommandLine.

Now, for your problem, here's an interesting observation: GetCommandLine() returns a writeable pointer. You can overwrite the existing command line. Of course, if you control the import tables, you decide what GetCommandLine means. (Remember, as usual there are A and W variants).

One warning: the MSVCRT isn't designed to be initialized twice, neither the static version nor the DLL one. So practically speaking you can't use it, and that will hurt.

[edit] Your update shows a call to _initterm. That's a MSVCRT function, as I already hinted. Specifically,

/***
*crtexe.c - Initialization for console EXE using CRT DLL
*
*       Copyright (c) Microsoft Corporation. All rights reserved.
*
...
/*
 * routine in DLL to do initialization (in this case, C++ constructors)
 */
extern int __cdecl _initterm_e(_PIFV *, _PIFV *);
extern void __cdecl _initterm(_PVFV *, _PVFV *);

The MSVCRT DLL calls GetCommandLine() on behalf of the EXE.

Upvotes: 13

RbMm
RbMm

Reputation: 33794

the entrypoint of executable (EP) have no arguments - so you and can not direct call it with arguments.

usual application got arguments by parsing command line. [w]mainCRTStartup do this - if you have console application linked to c/c++ runtime - this is real EP .

so if you Fill Import table of this exe manually - set exception for GetCommandLineA and GetCommandLineW functions - redirect it to self implementation and return your custom command line.

but if app used not static linked CRT it can import __getmainargs or __wgetmainargs or even _acmdln, or _wcmdln from msvcrt.dll - so already task become complex.

and you assume that relocs exits in EXE, you not handle TLS if it exist, you not handle application manifest, possible dl redirections, etc.

but I cannot run it as a separate process

this is not true. you can and must run it as separate process - this is the best solution.

exec your app by CreateProcess with CREATE_SUSPENDED flag. here you free easy set any CommandLine which you need. you not need manually and not fully correct load EXE but system do this task for you.

after process is created you need inject self DLL to it by using QueueUserAPC (but not CreateRemoteThread !!) and finally call ResumeThread

as result your DLL will be loaded and executed in first EXE thread, just before application EP - and here you can do all needed tasks

Upvotes: 2

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