> [!NOTE]
> 注意以这种方式链接列表可以避免需要一个数组来注册每一个块。
### 块指针
当使用malloc时，返回一个可以写入的内容的指针（就在头部之后），然而，在管理块时，需要一个指向头部（元数据）开始的指针。\
为这些转换使用这些函数：
```c
// https://github.com/bminor/glibc/blob/master/malloc/malloc.c
/* Convert a chunk address to a user mem pointer without correcting the tag. */
#define chunk2mem(p) ((void*)((char*)(p) + CHUNK_HDR_SZ))
/* Convert a user mem pointer to a chunk address and extract the right tag. */
#define mem2chunk(mem) ((mchunkptr)tag_at (((char*)(mem) - CHUNK_HDR_SZ)))
/* The smallest possible chunk */
#define MIN_CHUNK_SIZE (offsetof(struct malloc_chunk, fd_nextsize))
/* The smallest size we can malloc is an aligned minimal chunk */
#define MINSIZE \
(unsigned long)(((MIN_CHUNK_SIZE+MALLOC_ALIGN_MASK) & ~MALLOC_ALIGN_MASK))
```
### 对齐和最小大小
指向块的指针和 `0x0f` 必须为 0。
```c
// From https://github.com/bminor/glibc/blob/a07e000e82cb71238259e674529c37c12dc7d423/sysdeps/generic/malloc-size.h#L61
#define MALLOC_ALIGN_MASK (MALLOC_ALIGNMENT - 1)
// https://github.com/bminor/glibc/blob/a07e000e82cb71238259e674529c37c12dc7d423/sysdeps/i386/malloc-alignment.h
#define MALLOC_ALIGNMENT 16
// https://github.com/bminor/glibc/blob/master/malloc/malloc.c
/* Check if m has acceptable alignment */
#define aligned_OK(m) (((unsigned long)(m) & MALLOC_ALIGN_MASK) == 0)
#define misaligned_chunk(p) \
((uintptr_t)(MALLOC_ALIGNMENT == CHUNK_HDR_SZ ? (p) : chunk2mem (p)) \
& MALLOC_ALIGN_MASK)
/* pad request bytes into a usable size -- internal version */
/* Note: This must be a macro that evaluates to a compile time constant
if passed a literal constant. */
#define request2size(req) \
(((req) + SIZE_SZ + MALLOC_ALIGN_MASK < MINSIZE) ? \
MINSIZE : \
((req) + SIZE_SZ + MALLOC_ALIGN_MASK) & ~MALLOC_ALIGN_MASK)
/* Check if REQ overflows when padded and aligned and if the resulting
value is less than PTRDIFF_T. Returns the requested size or
MINSIZE in case the value is less than MINSIZE, or 0 if any of the
previous checks fail. */
static inline size_t
checked_request2size (size_t req) __nonnull (1)
{
if (__glibc_unlikely (req > PTRDIFF_MAX))
return 0;
/* When using tagged memory, we cannot share the end of the user
block with the header for the next chunk, so ensure that we
allocate blocks that are rounded up to the granule size. Take
care not to overflow from close to MAX_SIZE_T to a small
number. Ideally, this would be part of request2size(), but that
must be a macro that produces a compile time constant if passed
a constant literal. */
if (__glibc_unlikely (mtag_enabled))
{
/* Ensure this is not evaluated if !mtag_enabled, see gcc PR 99551. */
asm ("");
req = (req + (__MTAG_GRANULE_SIZE - 1)) &
~(size_t)(__MTAG_GRANULE_SIZE - 1);
}
return request2size (req);
}
```
请注意，在计算所需的总空间时，仅添加了 `SIZE_SZ` 1 次，因为 `prev_size` 字段可以用来存储数据，因此只需要初始头部。
### 获取块数据并更改元数据
这些函数通过接收指向块的指针来工作，并且对于检查/设置元数据非常有用：
- 检查块标志
```c
// From https://github.com/bminor/glibc/blob/master/malloc/malloc.c
/* size field is or'ed with PREV_INUSE when previous adjacent chunk in use */
#define PREV_INUSE 0x1
/* extract inuse bit of previous chunk */
#define prev_inuse(p) ((p)->mchunk_size & PREV_INUSE)
/* size field is or'ed with IS_MMAPPED if the chunk was obtained with mmap() */
#define IS_MMAPPED 0x2
/* check for mmap()'ed chunk */
#define chunk_is_mmapped(p) ((p)->mchunk_size & IS_MMAPPED)
/* size field is or'ed with NON_MAIN_ARENA if the chunk was obtained
from a non-main arena. This is only set immediately before handing
the chunk to the user, if necessary. */
#define NON_MAIN_ARENA 0x4
/* Check for chunk from main arena. */
#define chunk_main_arena(p) (((p)->mchunk_size & NON_MAIN_ARENA) == 0)
/* Mark a chunk as not being on the main arena. */
#define set_non_main_arena(p) ((p)->mchunk_size |= NON_MAIN_ARENA)
```
- 大小和指向其他块的指针
```c
/*
Bits to mask off when extracting size
Note: IS_MMAPPED is intentionally not masked off from size field in
macros for which mmapped chunks should never be seen. This should
cause helpful core dumps to occur if it is tried by accident by
people extending or adapting this malloc.
*/
#define SIZE_BITS (PREV_INUSE | IS_MMAPPED | NON_MAIN_ARENA)
/* Get size, ignoring use bits */
#define chunksize(p) (chunksize_nomask (p) & ~(SIZE_BITS))
/* Like chunksize, but do not mask SIZE_BITS. */
#define chunksize_nomask(p) ((p)->mchunk_size)
/* Ptr to next physical malloc_chunk. */
#define next_chunk(p) ((mchunkptr) (((char *) (p)) + chunksize (p)))
/* Size of the chunk below P. Only valid if !prev_inuse (P). */
#define prev_size(p) ((p)->mchunk_prev_size)
/* Set the size of the chunk below P. Only valid if !prev_inuse (P). */
#define set_prev_size(p, sz) ((p)->mchunk_prev_size = (sz))
/* Ptr to previous physical malloc_chunk. Only valid if !prev_inuse (P). */
#define prev_chunk(p) ((mchunkptr) (((char *) (p)) - prev_size (p)))
/* Treat space at ptr + offset as a chunk */
#define chunk_at_offset(p, s) ((mchunkptr) (((char *) (p)) + (s)))
```
- 确保位
```c
/* extract p's inuse bit */
#define inuse(p) \
((((mchunkptr) (((char *) (p)) + chunksize (p)))->mchunk_size) & PREV_INUSE)
/* set/clear chunk as being inuse without otherwise disturbing */
#define set_inuse(p) \
((mchunkptr) (((char *) (p)) + chunksize (p)))->mchunk_size |= PREV_INUSE
#define clear_inuse(p) \
((mchunkptr) (((char *) (p)) + chunksize (p)))->mchunk_size &= ~(PREV_INUSE)
/* check/set/clear inuse bits in known places */
#define inuse_bit_at_offset(p, s) \
(((mchunkptr) (((char *) (p)) + (s)))->mchunk_size & PREV_INUSE)
#define set_inuse_bit_at_offset(p, s) \
(((mchunkptr) (((char *) (p)) + (s)))->mchunk_size |= PREV_INUSE)
#define clear_inuse_bit_at_offset(p, s) \
(((mchunkptr) (((char *) (p)) + (s)))->mchunk_size &= ~(PREV_INUSE))
```
- 设置页眉和页脚（当使用块编号时）
```c
/* Set size at head, without disturbing its use bit */
#define set_head_size(p, s) ((p)->mchunk_size = (((p)->mchunk_size & SIZE_BITS) | (s)))
/* Set size/use field */
#define set_head(p, s) ((p)->mchunk_size = (s))
/* Set size at footer (only when chunk is not in use) */
#define set_foot(p, s) (((mchunkptr) ((char *) (p) + (s)))->mchunk_prev_size = (s))
```
- 获取块内实际可用数据的大小
```c
#pragma GCC poison mchunk_size
#pragma GCC poison mchunk_prev_size
/* This is the size of the real usable data in the chunk. Not valid for
dumped heap chunks. */
#define memsize(p) \
(__MTAG_GRANULE_SIZE > SIZE_SZ && __glibc_unlikely (mtag_enabled) ? \
chunksize (p) - CHUNK_HDR_SZ : \
chunksize (p) - CHUNK_HDR_SZ + (chunk_is_mmapped (p) ? 0 : SIZE_SZ))
/* If memory tagging is enabled the layout changes to accommodate the granule
size, this is wasteful for small allocations so not done by default.
Both the chunk header and user data has to be granule aligned. */
_Static_assert (__MTAG_GRANULE_SIZE <= CHUNK_HDR_SZ,
"memory tagging is not supported with large granule.");
static __always_inline void *
tag_new_usable (void *ptr)
{
if (__glibc_unlikely (mtag_enabled) && ptr)
{
mchunkptr cp = mem2chunk(ptr);
ptr = __libc_mtag_tag_region (__libc_mtag_new_tag (ptr), memsize (cp));
}
return ptr;
}
```
## 示例
### 快速堆示例
来自 [https://guyinatuxedo.github.io/25-heap/index.html](https://guyinatuxedo.github.io/25-heap/index.html) 的快速堆示例，但在 arm64：
```c
#include
#include
#include
void main(void)
{
char *ptr;
ptr = malloc(0x10);
strcpy(ptr, "panda");
}
```
在主函数的末尾设置一个断点，让我们找出信息存储的位置：
可以看到字符串 panda 存储在 `0xaaaaaaac12a0`（这是 malloc 在 `x0` 中给出的地址）。检查 0x10 字节之前，可以看到 `0x0` 表示 **前一个块未使用**（长度为 0），而这个块的长度为 `0x21`。
保留的额外空间（0x21-0x10=0x11）来自 **添加的头部**（0x10），而 0x1 并不意味着它被保留为 0x21B，而是当前头部长度的最后 3 位具有一些特殊含义。由于长度始终是 16 字节对齐的（在 64 位机器上），这些位实际上永远不会被长度数字使用。
```
0x1: Previous in Use - Specifies that the chunk before it in memory is in use
0x2: Is MMAPPED - Specifies that the chunk was obtained with mmap()
0x4: Non Main Arena - Specifies that the chunk was obtained from outside of the main arena
```
### 多线程示例
多线程
```c
#include
#include
#include
#include
#include
void* threadFuncMalloc(void* arg) {
printf("Hello from thread 1\n");
char* addr = (char*) malloc(1000);
printf("After malloc and before free in thread 1\n");
free(addr);
printf("After free in thread 1\n");
}
void* threadFuncNoMalloc(void* arg) {
printf("Hello from thread 2\n");
}
int main() {
pthread_t t1;
void* s;
int ret;
char* addr;
printf("Before creating thread 1\n");
getchar();
ret = pthread_create(&t1, NULL, threadFuncMalloc, NULL);
getchar();
printf("Before creating thread 2\n");
ret = pthread_create(&t1, NULL, threadFuncNoMalloc, NULL);
printf("Before exit\n");
getchar();
return 0;
}
```
调试前面的示例可以看到，开始时只有 1 个 arena：
然后，在调用第一个线程，即调用 malloc 的线程后，创建了一个新的 arena：
在其中可以找到一些 chunks：
## Bins & Memory Allocations/Frees
检查 bins 是什么，它们是如何组织的，以及内存是如何分配和释放的：
{{#ref}}
bins-and-memory-allocations.md
{{#endref}}
## Heap Functions Security Checks
涉及堆的函数在执行其操作之前会进行某些检查，以确保堆没有被破坏：
{{#ref}}
heap-memory-functions/heap-functions-security-checks.md
{{#endref}}
## References
- [https://azeria-labs.com/heap-exploitation-part-1-understanding-the-glibc-heap-implementation/](https://azeria-labs.com/heap-exploitation-part-1-understanding-the-glibc-heap-implementation/)
- [https://azeria-labs.com/heap-exploitation-part-2-glibc-heap-free-bins/](https://azeria-labs.com/heap-exploitation-part-2-glibc-heap-free-bins/)
.png)
.png)
.png)
.png)
 (1) (1) (1) (1) (1) (1) (1) (1).png)
 (1) (1) (1) (1) (1) (1) (1) (1) (1).png)
 (1) (1) (1) (1) (1).png)
