In Demystifying the Execve Shellcode is explained a way to write an execve shellcode:
#include<stdio.h>
#include<string.h>
unsigned char code[] =
"\x31\xc0\x50\x68\x6e\x2f\x73\x68\x68\x2f\x2f\x62\x69\x89\xe3\x50\x89\xe2\x53\x89\xe1\xb0\x0b\xcd\x80";
main()
{
printf("Shellcode Length: %d\n", strlen(code));
int (*ret)() = (int(*)())code;
ret();
}
What does the line int (*ret)() = (int(*)())code;
do?
int (*ret)() = (int(*)())code;
~~~~~~~~~~~~ ~~~~~~~~~~~~~~
1 2
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3
It defines ret
as a pointer to a function which has no parameter ()
and returns int
. So, Those ()
indicates the definition of parameters of a function.
It's for casting code
to a pointer to a function which has no parameter ()
and returns int
.
Casts code
as a function and assigns it to ret
. After that you can call ret();
.
unsigned char code[] = "\x31\xc0\x50\x68\x6e\x2f\...
It is a sequence of machine instructions represented by hex values. It will be injected to the code as a function.