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[Assembly] Access violation - code c0000005 00000000?When run simple inline ASM in windows - Printable Version

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+---- Thread: [Assembly] Access violation - code c0000005 00000000?When run simple inline ASM in windows (/showthread.php?tid=4492322)

Access violation - code c0000005 00000000?When run simple inline ASM in windows - mr.cracker - Oct 15, 2014

i was running a simple program inside windows 8.1
Code
//cl /EHsc ConnectBack.cpp
    #include <windows.h>
    #include <stdio.h>    

    DWORD __cdecl connect_back()
    {  

    __asm{
        pushad

        connectback:
            jmp startup_bnc
    find:
        cld
        mov ebx, 0x00000002
        mov eax, ebx
        ret            



    find_function:      
        pushad                          


    find_function_loop:

    compute_hash:
        xor edi, edi
        xor eax, eax
        cld
    compute_hash_again:

    mov ebp, 0x08
    compute_hash_finished:
    find_function_compare:
        mov eax, 0x00000001
        mov [esp + 0x1c], eax

    find_function_finished:
            popad
            ret



    startup_bnc:
            call find
            mov edx, eax
            mov eax, 0xec0e4e8e
            push eax
            push edx
            call find_function
            add esp, 0x08

    finished:
        mov [esp + 0x1c], eax
        popad
        ret

}
}

    int main()
    {
    DWORD addr = connect_back();
    printf("EAX Now: 0x%08x\n ", addr);

    return 0;
    }
error from windbg
Code
(e18.11e8): Access violation - code c0000005 (!!! second chance !!!)
00000000 ??              ???
0:000:x86> k
ChildEBP RetAddr  
WARNING: Frame IP not in any known module. Following frames may be wrong.
0101feac 00000000 0x0
how can i return without any error ? i used 'pushad' in the beginning because i change 'ebp, ebx' which should be preserved in C function. Am i right ? But i don't know how to return 'eax' perfectly? How does my EIP changes ?

* - mr.cracker - Oct 16, 2014

i got answer..when i call "connect_back" function my system does the following,

Code
push ebp
mov ebp, esp
push ebx
push esi
push edi
so when i simply call "ret" , the return address is actually after 16 bytes long from the current value. i.e before i call "ret" my "esp" points to in the following order ->

ebp ebx esi edi "return address"
so we don't wanna use "pushad" which is in top of the code(no need with that). and use "return ;", so that the loader itself pop out the 4 registers implicitly.

my changed code is, below

Code
#include <windows.h>
#include <stdio.h>

DWORD __cdecl connect_back()
{

__asm{

    connectback:
      jmp startup_bnc
find:
    cld
    mov ebx, 0x0000002
    mov eax, ebx
    ret

find_function:      
    pushad            

find_function_loop:

compute_hash:
    xor edi, edi
    ;xor eax, eax
    cld
compute_hash_again:

    mov ebp, 0x08
compute_hash_finished:
find_function_compare:
    ;mov eax, 0x0000001
    mov [esp + 0x1c], eax

find_function_finished:
      popad
      ret

startup_bnc:
      call find
      mov edx, eax
      mov eax, 0xec0e4e8e
      push eax
      push edx
      call find_function
      add esp, 0x08

finished:

}
return ;
}

int main()
{
DWORD addr = connect_back();
printf("hellooo\n");
printf("EAX Now: 0x%08x\n ", addr);

return 0;
}