Reverse Engineering Asked by Mah35h on December 18, 2020
Attached is the part of a disassembled main from a x86 binary file, generated by ghidra.
**************************************************************
* FUNCTION *
**************************************************************
undefined main(undefined1 param_1)
undefined AL:1 <RETURN> XREF[1]: 0804835e(W)
undefined1 Stack[0x4]:1 param_1 XREF[1]: 08048309(*)
undefined4 EAX:4 str_in XREF[1]: 0804835e(W)
undefined4 Stack[0x0]:4 local_res0 XREF[1]: 08048310(R)
undefined4 Stack[-0x10]:4 local_10 XREF[6]: 08048358(R),
08048363(W),
0804836d(R),
08048388(R),
08048393(W),
0804839d(R)
undefined4 Stack[-0x14]:4 local_14 XREF[2]: 0804831a(W),
08048366(R)
undefined4 Stack[-0x18]:4 local_18 XREF[2]: 08048321(W),
08048396(R)
undefined4 Stack[-0x2c]:4 local_2c XREF[3]: 08048369(W),
08048399(W),
080483ac(W)
undefined4 Stack[-0x30]:4 local_30 XREF[12]: 08048328(*),
08048334(*),
08048340(*),
0804834c(*),
0804835b(*),
08048370(*),
0804837c(*),
0804838b(*),
080483a0(*),
080483b4(*),
080483c2(*),
080483d0(*)
main XREF[2]: Entry Point(*),
_start:08048167(*)
08048309 8d 4c 24 04 LEA ECX=>param_1,[ESP + 0x4]
0804830d 83 e4 f0 AND ESP,0xfffffff0
08048310 ff 71 fc PUSH dword ptr [ECX + local_res0]
08048313 55 PUSH EBP
08048314 89 e5 MOV EBP,ESP
08048316 51 PUSH ECX
08048317 83 ec 24 SUB ESP,0x24
0804831a c7 45 f4 MOV dword ptr [EBP + local_14],DAT_080a6b19 = 6Ah j
19 6b 0a 08
08048321 c7 45 f0 MOV dword ptr [EBP + local_18],s_the_ripper_080a6b1e = "the ripper"
1e 6b 0a 08
Same code from gdb
0x08048309 <+0>: lea ecx,[esp+0x4]
0x0804830d <+4>: and esp,0xfffffff0
0x08048310 <+7>: push DWORD PTR [ecx-0x4]
0x08048313 <+10>: push ebp
0x08048314 <+11>: mov ebp,esp
0x08048316 <+13>: push ecx
0x08048317 <+14>: sub esp,0x24
=> 0x0804831a <+17>: mov DWORD PTR [ebp-0xc],0x80a6b19
Why is ghidra changeing [ebp-0xc] to [EBP + local_14].
Similar question I found is Ghidra interpreting stack pointers wrongly but reading the answer, I’m not getting the meaning of [EBP + local_14] Here, is ghidra just renaming -0xc to a easily readable name like local_14? I’m not getting how to make sense of this exactly.
In the function header, it is shown that Stack[-0x10]:4 local_10. I assume it means that local_10 is 4 byte variable at Stack[-0x10], where Stack is the stack pointer upon entry to function. But why is it added to ebp. What’s the meaning of that representation used by ghidra?
Since local variables are usually placed on the stack in x86 and esp register can change during function execution, it is more convenient to save the value of esp register on function entry and access data relatively to that value. ebp register is used for this purpose. So you will often see
push ebp
mov ebp, esp
lines at the begining of functions. In the example you have provided it is the case - all local variables are accessed this way, through ebp.
Now, there are two different naming conventions for local variables:
[ebp - xxx] is accessed, it will be displayed as [ebp + local_xxx]. Here, local_xxx = -xxx, so for instance, local_18 = -0x18.esp value at the beginning of a function. In your example, two dwords are pushed on the stack before mov ebp, esp line. It means, that if some local variable was called local_xxx in the first convention, in the second one it will be named local_xxx+0x8, for instance local_18 in the first one will be local_20 in the second one, used by Ghidra.Why do we add 0x8 in the second one? Because two dwords (8 bytes) were pushed onto the stack before esp value was saved into ebp and in x86 architecture stack "grows downwards", which means if you push something onto it, this value will be saved there and esp will be decreased accordingly (in this case, twice, by 4 bytes). So, in your particular example, you have the instruction
mov DWORD PTR [ebp-0xc],0x80a6b19
which would be displayed as
mov DWORD PTR [ebp+local_c],0x80a6b19
in the first convention and
mov DWORD PTR [ebp+local_14],0x80a6b19
in the second one, implemented in Ghidra, since 0xc + 0x8 = 0x14.
Correct answer by bart1e on December 18, 2020
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