Separate new strucs from C

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donn

Joined: 05 Mar 2010
Posts: 321

donn 21 Dec 2015, 17:10

Hi,

I'm continuing with an attempt to build a Linked List in asm. Also would like it accessible from C.

I'm able to use an asm function to load the List struc with default values. However, when I define a second List in C, and set a struc member, such as isBlankList, it sets the value for both Lists. Is there a way for separate Lists to be defined as needed, from C? What I have so far:

ASM struc definition, C header also has one without default values

Code:

        struc List{
                .firstItem dd ?
                .listSize dd ?
                .index dd ?
                .isBlankList dd 1 ; Used value of 50 when testing as a more unique value
        }

ASM newList function

Code:

newList:

        push ebp
        mov ebp, esp

        mov ecx, [ebp+8]
        mov edx, [ebp+12]
        
        push ebx esi edi

        mov eax, .newListLocal  ; Pass newly defined List address?

        pop edi esi ebx

        mov esp, ebp
        pop ebp

        retn 0

        .newListLocal:          ; This may be the problem. It seems to set default values for
                list List       ; one List, but with two, changes are mirrored to each.

C Lists being defined, and loaded with default values

Code:

        List *asmList = (List*)newList();  
        
        List *asmList2 = (List*)newList(); 


        asmList->isBlankList = 10; // Value of 10 used during testing as a more unique value.
                                   // After setting this, both asmList and asmList2 have 10 
                                   // for the isBlankList member. Beforehand, they had 1
                                   // or whatever was specified in ASM definition.

Thanks a lot

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21 Dec 2015, 17:10

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20518
Location: In your JS exploiting you and your system

revolution 21 Dec 2015, 17:59

The code above allocates memory in the code section statically at compile time. Instead you will have to allocate memory for the list on demand (i.e. dynamically). Perhaps use a "alloc" or "HeapAlloc" or something inside the newList function. Alternatively you can have C allocate the memory and pass the pointer to newList.

Last edited by revolution on 21 Dec 2015, 23:32; edited 1 time in total

21 Dec 2015, 17:59

donn

Joined: 05 Mar 2010
Posts: 321

donn 21 Dec 2015, 22:56

Thanks a lot. Will go the Kernel32 API route for now.

21 Dec 2015, 22:56

donn

Joined: 05 Mar 2010
Posts: 321

donn 21 Feb 2016, 19:09

Got it, solved for now:

Code:

newList:

        push ebp
        mov ebp, esp

        mov ecx, [ebp+8]
        mov edx, [ebp+12]
        
        push ebx esi edi


        call [GetProcessHeap]
        mov [hHeap], eax

        ;Size of List
        push 000001100b
        ;Set allocated to 0s
        push 000001000b
        push [hHeap]
        call [HeapAlloc] 
        mov [hAllocatedMemory], eax
        
        mov dword ebx, [hAllocatedMemory]

        ;Define a local List, members become initialized since we have a definition (shown below)
        list List

        ;Start moving over the local values. Since struc was used, we can reference them with labels. Also, they are initialized.
        mov dword edx, [list]
        mov dword [ebx], edx

        mov dword edx, [list.index]
        mov dword [ebx+4], edx  

        mov dword edx, [list.isBlankList]
        mov dword [ebx+8], edx  

        ;Return the handle
        mov eax, [hAllocatedMemory]
        
        pop edi esi ebx

        mov esp, ebp
        pop ebp

        retn 0

And the struc:

Code:

        struc List{
                .listSize dd 2
                .index dd 1
                .isBlankList dd 50
        }

Plenty new to learn going forward, like maybe using $ for sizing and how to mov the local struc memory values before they get wiped out. Rep and some of the strings instructions may solve that regardless of the struc layout, but want to investigate the options.

Thanks again.

21 Feb 2016, 19:09

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