Combining gcc generated .obj with FASM generated .obj

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kamac

Joined: 06 Dec 2013
Posts: 12

kamac 07 Dec 2013, 18:07

Hi.

I'm trying to merge C++ code with FASM code.

For this purpose, I have following C++ code:

Code:

#include <sstream>
#include <string>

const char* strToInt(int i) {
        std::stringstream out;
        out << i;
        return out.str().c_str();
}

And the following call to it in ASM:

Code:

format MS COFF
extrn '__imp__printf' as printf:DWORD
extrn '__imp__ExitProcess@4' as ExitProcess:DWORD
extrn strToInt

section '.text' code
dbgTxt: db "Number: %d",0
_start:
    call main
main:
    push ebp
    mov ebp,esp
    push esi
    push edi

    push 78
    call strToInt
    add esp,4
    push eax
    push dbgTxt
    call [printf]
    add esp,8

    call [ExitProcess]
    pop edi
    pop esi
    mov esp,ebp
    pop ebp
    ret

I compile the C++ code with:

Code:

g++ -c cppFile.cpp

Which produces me a cppFile.o

Now, I compile ASM:

Code:

FASM testApp.ASM testApp.OBJ

Then, I link it with ld.exe:

Code:

ld.exe testApp.OBJ cppFile.OBJ libkernel32.a libmsvcrt.a libuser32.a libstdc++.a

And this is where the problem is. Linking throws me:

Code:

testApp.OBJ:(.text+0x3b): undefined reference to `intToStr'
cppFile.OBJ:cppFile.cpp:(.text+0x9e): undefined reference to `_Unwind_Re
sume'
libstdc++.a(eh_personality.o):(.text$_ZL28read_encoded_value_with_basehjPKhPj+0x
c1): undefined reference to `abort'
libstdc++.a(eh_personality.o):(.text$_ZL15get_ttype_entryP16lsda_header_infom+0x
61): undefined reference to `abort'
ld.exe: libstdc++.a(eh_personality.o): bad reloc address 0x61 in section `.text$
_ZL15get_ttype_entryP16lsda_header_infom'
ld.exe: final link failed: Invalid operation

I'm using mingw. Any thoughts?

07 Dec 2013, 18:07

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20759
Location: In your JS exploiting you and your system

revolution 07 Dec 2013, 18:13

At a minimum you will have to define strToInt, printf and ExitProcess using extrn.

07 Dec 2013, 18:13

kamac

Joined: 06 Dec 2013
Posts: 12

kamac 07 Dec 2013, 18:19

But it's there already.

Code:

extrn '__imp__printf' as printf:DWORD 
extrn '__imp__ExitProcess@4' as ExitProcess:DWORD 
extrn strToInt

From what I read, it could be due to mixing of SJLJ and DW2.. But that's all I know.

07 Dec 2013, 18:19

eax

Joined: 02 Dec 2013
Posts: 1

eax 07 Dec 2013, 19:04

great question, i was wondering about foreign functions with FASM myself.

07 Dec 2013, 19:04

dancho

Joined: 06 Mar 2011
Posts: 74

dancho 08 Dec 2013, 19:55

how to mix C++(VS) and fasm(obj) :

1.) in VS create new console project
2.) copy this code

Code:

#include <iostream>
#include <conio.h>
using namespace std;

// proto for the extern func
extern "C" unsigned int AddFunc(unsigned int x, unsigned int y);

int main(void)
{
        
    cout << "10 + 20 = " << AddFunc(10,20) << endl;
        _getch();

    return 0;
}

3.) open fasmw copy and save as AddFunc.asm , compile it

Code:

format MS COFF
include 'win32axp.inc'

public  AddFunc as '_AddFunc'

section '.code' code readable executable
                
                proc AddFunc c p1,p2
                        
                        mov eax,[p1]
                        add eax,[p2]
                        
                        ret
                endp            
                
                
section '.data' data readable writeable
                nop

4.) copy AddFunc.obj into main VS directory
5.) open project properties and under linker there is an input item,
subitem additional dependencies , write AddFunc.obj here
6.) compile and run

08 Dec 2013, 19:55

kamac

Joined: 06 Dec 2013
Posts: 12

kamac 09 Dec 2013, 16:16

Though, I needed it the other way around Wink

(combine c/c++ .obj files with ASM's .obj files and thus link them together with something like ld)

09 Dec 2013, 16:16

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 09 Dec 2013, 18:06

Given that this is C++ you need the proper compiler "hash".

Comment out the C++ code, declare as extern and make a call to it in C++:

Code:

extern const char* strToInt(int i);
//{
//      return NULL;
//(void)i;
//}
// ...
strToInt(0);

The compiler will complain:

Code:

Error   1       error LNK2001: unresolved external symbol "char const * __cdecl strToInt(int)" (?strToInt@@YAPEBDH@Z)   main.obj        1       1       aluminium

You then take the "?strToInt@@YAPEBDH@Z" and use it for your FASM extrn:

Code:

extrn '?strToInt@@YAPEBDH@Z' as strToInt:DWORD

Uncomment the strToInt in C++ and remove the extern and dummy call. It should now link.

09 Dec 2013, 18:06

dancho

Joined: 06 Mar 2011
Posts: 74

dancho 09 Dec 2013, 18:28

kamac wrote:

Though, I needed it the other way around (combine c/c++ .obj files with ASM's .obj files and thus link them together with something like ld)

ahh yes , sry , just ignore my post then...

09 Dec 2013, 18:28

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