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(이 요약에서는 체크가 설명되지 않았으며 일부 경우는 간결함을 위해 생략되었습니다)
주소가 null이면 아무것도 하지 마십시오.
청크가 mmaped된 경우, mummap하고 종료합니다.
_int_free를 호출합니다:
가능하면 청크를 tcache에 추가합니다.
가능하면 청크를 fast bin에 추가합니다.
필요시 청크를 통합하기 위해 _int_free_merge_chunk를 호출하고 정렬되지 않은 목록에 추가합니다.
Free는 __libc_free를 호출합니다.
전달된 주소가 Null(0)인 경우 아무것도 하지 마십시오.
포인터 태그를 확인합니다.
청크가 mmaped인 경우, mummap하고 그게 전부입니다.
그렇지 않으면 색상을 추가하고 그 위에 _int_free를 호출합니다.
```c void __libc_free (void *mem) { mstate ar_ptr; mchunkptr p; /* chunk corresponding to mem */
if (mem == 0) /* free(0) has no effect */ return;
/* Quickly check that the freed pointer matches the tag for the memory. This gives a useful double-free detection. */ if (__glibc_unlikely (mtag_enabled)) *(volatile char *)mem;
int err = errno;
p = mem2chunk (mem);
if (chunk_is_mmapped (p)) /* release mmapped memory. / { / See if the dynamic brk/mmap threshold needs adjusting. Dumped fake mmapped chunks do not affect the threshold. */ if (!mp_.no_dyn_threshold && chunksize_nomask (p) > mp_.mmap_threshold && chunksize_nomask (p) <= DEFAULT_MMAP_THRESHOLD_MAX) { mp_.mmap_threshold = chunksize (p); mp_.trim_threshold = 2 * mp_.mmap_threshold; LIBC_PROBE (memory_mallopt_free_dyn_thresholds, 2, mp_.mmap_threshold, mp_.trim_threshold); } munmap_chunk (p); } else { MAYBE_INIT_TCACHE ();
/* Mark the chunk as belonging to the library again. */ (void)tag_region (chunk2mem (p), memsize (p));
ar_ptr = arena_for_chunk (p); _int_free (ar_ptr, p, 0); }
__set_errno (err); } libc_hidden_def (__libc_free)
</details>
## \_int\_free <a href="#int_free" id="int_free"></a>
### \_int\_free 시작 <a href="#int_free" id="int_free"></a>
다음과 같은 몇 가지 검사를 통해 시작됩니다:
* **포인터**가 **정렬되어** 있는지, 그렇지 않으면 오류 `free(): invalid pointer`가 발생합니다.
* **크기**가 최소 크기보다 작지 않고, **크기**도 **정렬되어** 있는지, 그렇지 않으면 오류: `free(): invalid size`가 발생합니다.
<details>
<summary>_int_free 시작</summary>
```c
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4493C1-L4513C28
#define aligned_OK(m) (((unsigned long) (m) &MALLOC_ALIGN_MASK) == 0)
static void
_int_free (mstate av, mchunkptr p, int have_lock)
{
INTERNAL_SIZE_T size; /* its size */
mfastbinptr *fb; /* associated fastbin */
size = chunksize (p);
/* Little security check which won't hurt performance: the
allocator never wraps around at the end of the address space.
Therefore we can exclude some size values which might appear
here by accident or by "design" from some intruder. */
if (__builtin_expect ((uintptr_t) p > (uintptr_t) -size, 0)
|| __builtin_expect (misaligned_chunk (p), 0))
malloc_printerr ("free(): invalid pointer");
/* We know that each chunk is at least MINSIZE bytes in size or a
multiple of MALLOC_ALIGNMENT. */
if (__glibc_unlikely (size < MINSIZE || !aligned_OK (size)))
malloc_printerr ("free(): invalid size");
check_inuse_chunk(av, p);먼저 관련 tcache에서 이 청크를 할당하려고 시도합니다. 그러나 이전에 몇 가지 검사가 수행됩니다. 해제된 청크와 동일한 인덱스의 tcache의 모든 청크를 반복합니다:
mp_.tcache_count보다 더 많은 항목이 있는 경우: free(): too many chunks detected in tcache
항목이 정렬되지 않은 경우: free(): unaligned chunk detected in tcache 2
해제된 청크가 이미 해제되었고 tcache에 청크로 존재하는 경우: free(): double free detected in tcache 2
모든 것이 잘 진행되면, 청크가 tcache에 추가되고 함수가 반환됩니다.
```c // From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4515C1-L4554C7 #if USE_TCACHE { size_t tc_idx = csize2tidx (size); if (tcache != NULL && tc_idx < mp_.tcache_bins) { /* Check to see if it's already in the tcache. */ tcache_entry *e = (tcache_entry *) chunk2mem (p);
/* This test succeeds on double free. However, we don't 100% trust it (it also matches random payload data at a 1 in 2^<size_t> chance), so verify it's not an unlikely coincidence before aborting. / if (_glibc_unlikely (e->key == tcache_key)) { tcache_entry *tmp; size_t cnt = 0; LIBC_PROBE (memory_tcache_double_free, 2, e, tc_idx); for (tmp = tcache->entries[tc_idx]; tmp; tmp = REVEAL_PTR (tmp->next), ++cnt) { if (cnt >= mp.tcache_count) malloc_printerr ("free(): too many chunks detected in tcache"); if (__glibc_unlikely (!aligned_OK (tmp))) malloc_printerr ("free(): unaligned chunk detected in tcache 2"); if (tmp == e) malloc_printerr ("free(): double free detected in tcache 2"); / If we get here, it was a coincidence. We've wasted a few cycles, but don't abort. */ } }
if (tcache->counts[tc_idx] < mp_.tcache_count) { tcache_put (p, tc_idx); return; } } } #endif
</details>
### \_int\_free fast bin <a href="#int_free" id="int_free"></a>
먼저 크기가 fast bin에 적합한지 확인하고, 그것을 top chunk에 가깝게 설정할 수 있는지 확인합니다.
그런 다음, 몇 가지 검사를 수행하면서 freed chunk를 fast bin의 맨 위에 추가합니다:
* chunk의 크기가 유효하지 않은 경우(너무 크거나 작음) 트리거: `free(): invalid next size (fast)`
* 추가된 chunk가 이미 fast bin의 맨 위에 있었던 경우: `double free or corruption (fasttop)`
* 맨 위의 chunk의 크기가 우리가 추가하는 chunk의 크기와 다른 경우: `invalid fastbin entry (free)`
<details>
<summary>_int_free Fast Bin</summary>
```c
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4556C2-L4631C4
/*
If eligible, place chunk on a fastbin so it can be found
and used quickly in malloc.
*/
if ((unsigned long)(size) <= (unsigned long)(get_max_fast ())
#if TRIM_FASTBINS
/*
If TRIM_FASTBINS set, don't place chunks
bordering top into fastbins
*/
&& (chunk_at_offset(p, size) != av->top)
#endif
) {
if (__builtin_expect (chunksize_nomask (chunk_at_offset (p, size))
<= CHUNK_HDR_SZ, 0)
|| __builtin_expect (chunksize (chunk_at_offset (p, size))
>= av->system_mem, 0))
{
bool fail = true;
/* We might not have a lock at this point and concurrent modifications
of system_mem might result in a false positive. Redo the test after
getting the lock. */
if (!have_lock)
{
__libc_lock_lock (av->mutex);
fail = (chunksize_nomask (chunk_at_offset (p, size)) <= CHUNK_HDR_SZ
|| chunksize (chunk_at_offset (p, size)) >= av->system_mem);
__libc_lock_unlock (av->mutex);
}
if (fail)
malloc_printerr ("free(): invalid next size (fast)");
}
free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);
atomic_store_relaxed (&av->have_fastchunks, true);
unsigned int idx = fastbin_index(size);
fb = &fastbin (av, idx);
/* Atomically link P to its fastbin: P->FD = *FB; *FB = P; */
mchunkptr old = *fb, old2;
if (SINGLE_THREAD_P)
{
/* Check that the top of the bin is not the record we are going to
add (i.e., double free). */
if (__builtin_expect (old == p, 0))
malloc_printerr ("double free or corruption (fasttop)");
p->fd = PROTECT_PTR (&p->fd, old);
*fb = p;
}
else
do
{
/* Check that the top of the bin is not the record we are going to
add (i.e., double free). */
if (__builtin_expect (old == p, 0))
malloc_printerr ("double free or corruption (fasttop)");
old2 = old;
p->fd = PROTECT_PTR (&p->fd, old);
}
while ((old = catomic_compare_and_exchange_val_rel (fb, p, old2))
!= old2);
/* Check that size of fastbin chunk at the top is the same as
size of the chunk that we are adding. We can dereference OLD
only if we have the lock, otherwise it might have already been
allocated again. */
if (have_lock && old != NULL
&& __builtin_expect (fastbin_index (chunksize (old)) != idx, 0))
malloc_printerr ("invalid fastbin entry (free)");
}청크가 아직 어떤 빈에도 할당되지 않았다면, _int_free_merge_chunk를 호출합니다.
```c /* Consolidate other non-mmapped chunks as they arrive. */
else if (!chunk_is_mmapped(p)) {
/* If we're single-threaded, don't lock the arena. */ if (SINGLE_THREAD_P) have_lock = true;
if (!have_lock) __libc_lock_lock (av->mutex);
_int_free_merge_chunk (av, p, size);
if (!have_lock) __libc_lock_unlock (av->mutex); } /* If the chunk was allocated via mmap, release via munmap(). */
else { munmap_chunk (p); } }
</details>
## \_int\_free\_merge\_chunk
이 함수는 SIZE 바이트의 청크 P를 이웃과 병합하려고 시도합니다. 결과 청크를 정렬되지 않은 빈 목록에 넣습니다.
몇 가지 검사가 수행됩니다:
* 청크가 상단 청크인 경우: `double free or corruption (top)`
* 다음 청크가 아레나의 경계를 벗어난 경우: `double free or corruption (out)`
* 청크가 사용 중으로 표시되지 않은 경우 (다음 청크의 `prev_inuse`에서): `double free or corruption (!prev)`
* 다음 청크의 크기가 너무 작거나 너무 큰 경우: `free(): invalid next size (normal)`
* 이전 청크가 사용 중이 아닌 경우, 통합을 시도합니다. 그러나 prev\_size가 이전 청크에 표시된 크기와 다르면: `corrupted size vs. prev_size while consolidating`
<details>
<summary>_int_free_merge_chunk code</summary>
```c
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4660C1-L4702C2
/* Try to merge chunk P of SIZE bytes with its neighbors. Put the
resulting chunk on the appropriate bin list. P must not be on a
bin list yet, and it can be in use. */
static void
_int_free_merge_chunk (mstate av, mchunkptr p, INTERNAL_SIZE_T size)
{
mchunkptr nextchunk = chunk_at_offset(p, size);
/* Lightweight tests: check whether the block is already the
top block. */
if (__glibc_unlikely (p == av->top))
malloc_printerr ("double free or corruption (top)");
/* Or whether the next chunk is beyond the boundaries of the arena. */
if (__builtin_expect (contiguous (av)
&& (char *) nextchunk
>= ((char *) av->top + chunksize(av->top)), 0))
malloc_printerr ("double free or corruption (out)");
/* Or whether the block is actually not marked used. */
if (__glibc_unlikely (!prev_inuse(nextchunk)))
malloc_printerr ("double free or corruption (!prev)");
INTERNAL_SIZE_T nextsize = chunksize(nextchunk);
if (__builtin_expect (chunksize_nomask (nextchunk) <= CHUNK_HDR_SZ, 0)
|| __builtin_expect (nextsize >= av->system_mem, 0))
malloc_printerr ("free(): invalid next size (normal)");
free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);
/* Consolidate backward. */
if (!prev_inuse(p))
{
INTERNAL_SIZE_T prevsize = prev_size (p);
size += prevsize;
p = chunk_at_offset(p, -((long) prevsize));
if (__glibc_unlikely (chunksize(p) != prevsize))
malloc_printerr ("corrupted size vs. prev_size while consolidating");
unlink_chunk (av, p);
}
/* Write the chunk header, maybe after merging with the following chunk. */
size = _int_free_create_chunk (av, p, size, nextchunk, nextsize);
_int_free_maybe_consolidate (av, size);
}구독 계획 확인하기!
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