Hi Fellow,
At this bloc article I am going to write about the difference call standard in x86 assembly.
#include <stdio.h>
int test(int first, int second, int third) {
printf("FUnction Called");
return first+second+third;
}
int main()
{
int result = test(1, 2, 3);
printf("Rio Asmara Suryadi %d", result);
}
; Attributes: bp-based frame
; int __cdecl main(int argc, const char **argv, const char **envp)
_main proc near
var_CC= byte ptr -0CCh
var_8= dword ptr -8
argc= dword ptr 8
argv= dword ptr 0Ch
envp= dword ptr 10h
push ebp
mov ebp, esp
sub esp, 0CCh
push ebx
push esi
push edi
lea edi, [ebp+var_CC]
mov ecx, 33h
mov eax, 0CCCCCCCCh
rep stosd
mov ecx, offset unk_41C003
call j_@__CheckForDebuggerJustMyCode@4 ; __CheckForDebuggerJustMyCode(x)
push 3 ; int
push 2 ; int
push 1 ; int
call test
add esp, 0Ch
mov [ebp+var_8], eax
mov eax, [ebp+var_8]
push eax
push offset aRioAsmaraSurya ; "Rio Asmara Suryadi %d"
call j__printf
add esp, 8
xor eax, eax
pop edi
pop esi
pop ebx
add esp, 0CCh
cmp ebp, esp
call j___RTC_CheckEsp
mov esp, ebp
pop ebp
retn
_main endp
1. cdecl
As we can see above that above is using cdecl standard because we can see that the caller handle the stack clear by running this instruction add esp, 0Ch. cdecl function also push the parameter variable from right to left
C codes
int result = test(1, 2, 3);
x86 assembly codes
push 3 ; int
push 2 ; int
push 1 ; int
call test
add esp, 0Ch
2. stdcal
the function call with stdcal would be similar to cdecl but the stack clear will be handled by the stack itself.
3. fastcall
The fastcall calling convention will have a bit different. The first few arguments (typically two) are passed in registers with most commonly used registers being EDX and ECX where the rest of argument will be pushed from right to left. The calling function will do the stack clear.
.LC0:
.string "FUnction Called"
test(int, int, int):
push rbp
mov rbp, rsp
sub rsp, 16
mov DWORD PTR [rbp-4], edi
mov DWORD PTR [rbp-8], esi
mov DWORD PTR [rbp-12], edx
mov edi, OFFSET FLAT:.LC0
mov eax, 0
call printf
mov edx, DWORD PTR [rbp-4]
mov eax, DWORD PTR [rbp-8]
add edx, eax
mov eax, DWORD PTR [rbp-12]
add eax, edx
leave
ret
.LC1:
.string "Rio Asmara Suryadi %d"
main:
push rbp
mov rbp, rsp
sub rsp, 16
mov edx, 3
mov esi, 2
mov edi, 1
call test(int, int, int)
mov DWORD PTR [rbp-4], eax
mov eax, DWORD PTR [rbp-4]
mov esi, eax
mov edi, OFFSET FLAT:.LC1
mov eax, 0
call printf
mov eax, 0
leave
ret
Cyber Security Architect | Red/Blue Teaming | Exploit/Malware Analysis 