#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <unistd.h>

#include <sys/mman.h>

// if defined, uses mmap(), otherwise uses malloc()
// if using malloc() will mprotect cause trouble with dlmalloc? 
// i.e. if you PROT_NONE and then free(), would the next time malloc
// allocates that memory still be protected? That's bad..
#define USE_MMAP

int main()
{
	char *p, *buf, *prot, *saved_p;
	int i;
	int buffer_size = 1024;
	int page_size = getpagesize();
	int total = buffer_size + page_size;

	// round total up to the nearest page
	total = (total + page_size - 1) & ~(page_size-1);

#ifdef USE_MMAP
	p = mmap(0, total, PROT_READ|PROT_WRITE|PROT_EXEC, 
		MAP_PRIVATE|MAP_ANON, -1, 0);

	if (p == MAP_FAILED) {
		perror("mmap");
		exit(1);
	}
#else
	saved_p = p = malloc(total+page_size);
	// realign p to page
	p = (void *) (((unsigned int) p + page_size - 1) & ~(page_size-1));
#endif

	printf("p: %p\n", p);

	buf = p;
	prot = p + total - page_size;

	if (mprotect(prot, page_size, PROT_NONE) < 0) {
		perror("mprotect");
		exit(1);
	}

	printf("Set buf to 0\n"); fflush(stdout);
	for (i = 0; i < buffer_size; i++) {
		buf[i] = 0;
	}

	printf("Overflowing...\n"); fflush(stdout);
	for (i = total - page_size; i < total; i++) {
		printf("i: %d\n", i); fflush(stdout);
		buf[i] = 0;
	}

	printf("We shouldn't reach here.\n"); fflush(stdout);

#ifdef USE_MMAP
	munmap(p, total);
#else
	free(saved_p);
#endif

	return 0;
}