不同libc版本的fastbin_dup.c源码有点小区别：主要是有tcache的，需要先填充
以下为有tcache的源码示例：

#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);

	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);
}

编译：
gcc -g fastbin_dup.c -o fastbin_dup

tcache[count]特点：
count= 7
优先分配-用malloc(8)的大小即可free后占用

7个tcache
fastbin_dup注意事项：
头结点不能同时free两次。

释放b把头结点变成b，即可再次free(a):
达成利用：
执行结果：

fastbin_dup总结：
有tcache先malloc(8)*7，在free掉7个占满tcache;
double free的chunk不能是头结点，需要利用一个中间替换头结点；