99% 的 iOS 開發都不知道的 KVO 崩潰
背景
crash 監控發現有大量的新增崩潰，堆疊如下
0 libsystem_platform.dylib __os_unfair_lock_corruption_abort()
1 libsystem_platform.dylib __os_unfair_lock_lock_slow()
2 Foundation __NSSetBoolValueAndNotify()
分析堆疊
__os_unfair_lock_corruption_abort
log 翻譯：lock 已損壞
_os_unfair_lock_corruption_abort(os_ulock_value_t current)
{
__LIBPLATFORM_CLIENT_CRASH__(current, "os_unfair_lock is corrupt");
}
__os_unfair_lock_lock_slow
在這個方法裡面 __ulock_wait 返回 EOWNERDEAD 呼叫 corruption abort 方法。
int ret = __ulock_wait(UL_UNFAIR_LOCK | ULF_NO_ERRNO | options,
l, current, 0);
if (unlikely(ret < 0)) {
switch (-ret) {
case EINTR:
case EFAULT:
continue;
case EOWNERDEAD:
_os_unfair_lock_corruption_abort(current);
break;
default:
__LIBPLATFORM_INTERNAL_CRASH__(-ret, "ulock_wait failure");
}
}
EOWNERDEAD 的定義
define EOWNERDEAD 105 / Previous owner died /
到這裡猜測是 lock 的 owner 已經野指標了，繼續向下看。
__NSSetBoolValueAndNotify
google 下這個方法是在 KVO 裡面修改屬性的時候呼叫，虛擬碼：
int __NSSetBoolValueAndNotify(int arg0, int arg1, int arg2) {
r31 = r31 - 0x90;
var_30 = r24;
stack[-56] = r23;
var_20 = r22;
stack[-40] = r21;
var_10 = r20;
stack[-24] = r19;
saved_fp = r29;
stack[-8] = r30;
r20 = arg2;
r21 = arg1;
r19 = arg0;
r0 = object_getClass(arg0);
r0 = object_getIndexedIvars(r0); // 理清這個崩潰的關鍵方法，這裡和彙編程式碼不一致，彙編程式碼的入參是 r0 + 0x20
r23 = r0;
os_unfair_recursive_lock_lock_with_options();
CFDictionaryGetValue(*(r23 + 0x18), r21);
r22 = _objc_msgSend$copyWithZone:();
os_unfair_recursive_lock_unlock();
if (*(int8_t *)(r23 + 0x28) != 0x0) {
_objc_msgSend$willChangeValueForKey:();
(class_getMethodImplementation(*r23, r21))(r19, r21, r20);
_objc_msgSend$didChangeValueForKey:();
}
else {
_objc_msgSend$_changeValueForKey:key:key:usingBlock:();
}
var_38 = **qword_9590e8;
r0 = objc_release_x22();
if (**qword_9590e8 != var_38) {
r0 = __stack_chk_fail();
}
return r0;
}
os_unfair_recursive_lock_lock_with_options
崩潰呼叫棧中間還有這一層的內聯呼叫 os_unfair_recursive_lock_lock_with_options。這裡的 lock owner 有個比較賦值的操作，如果 oul_value 等於 OS_LOCK_NO_OWNER 則賦值 self 然後 return。崩潰時這裡繼續向下執行了，那這裡的 oul_value 的取值只能是 lock->oul_value。到這裡猜測崩潰的原因是 lock->oul_value 野指標了。
``` void os_unfair_recursive_lock_lock_with_options(os_unfair_recursive_lock_t lock, os_unfair_lock_options_t options) { os_lock_owner_t cur, self = _os_lock_owner_get_self(); _os_unfair_lock_t l = (_os_unfair_lock_t)&lock->ourl_lock;
if (likely(os_atomic_cmpxchgv2o(l, oul_value,
OS_LOCK_NO_OWNER, self, &cur, acquire))) {
return;
}
if (OS_ULOCK_OWNER(cur) == self) {
lock->ourl_count++;
return;
}
return _os_unfair_lock_lock_slow(l, self, options);
}
OS_ALWAYS_INLINE OS_CONST static inline os_lock_owner_t _os_lock_owner_get_self(void) { os_lock_owner_t self; self = (os_lock_owner_t)_os_tsd_get_direct(__TSD_MACH_THREAD_SELF); return self; } ```
object_getIndexedIvars
__NSSetBoolValueAndNotify 裡面的獲取 lock 的方法，這個函式非常關鍵。
``` / * Returns a pointer to any extra bytes allocated with an instance given object. * * @param obj An Objective-C object. * * @return A pointer to any extra bytes allocated with \e obj. If \e obj was * not allocated with any extra bytes, then dereferencing the returned pointer is undefined. * * @note This function returns a pointer to any extra bytes allocated with the instance * (as specified by \c class_createInstance with extraBytes>0). This memory follows the * object's ordinary ivars, but may not be adjacent to the last ivar. * @note The returned pointer is guaranteed to be pointer-size aligned, even if the area following * the object's last ivar is less aligned than that. Alignment greater than pointer-size is never * guaranteed, even if the area following the object's last ivar is more aligned than that. * @note In a garbage-collected environment, the memory is scanned conservatively. / * Returns a pointer immediately after the instance variables declared in an * object. This is a pointer to the storage specified with the extraBytes * parameter given when allocating an object. / void object_getIndexedIvars(id obj) { uint8_t base = (uint8_t )obj;
if (_objc_isTaggedPointerOrNil(obj)) return nil;
if (!obj->isClass()) return base + obj->ISA()->alignedInstanceSize();
Class cls = (Class)obj;
if (!cls->isAnySwift()) return base + sizeof(objc_class);
swift_class_t *swcls = (swift_class_t *)cls;
return base - swcls->classAddressOffset + word_align(swcls->classSize);
} ```
上層呼叫 __NSSetBoolValueAndNotify 裡面:
r0 = object_getClass(arg0)，arg0 是例項物件，r0 是類物件，因為這裡是個 KVO 的呼叫，那正常情況下r0 是 NSKVONotifying_xxx。
對於 KVO 類，object_getIndexedIvars 返回的地址是 (uint8_t *)obj + sizeof(objc_class)。根據函式的註釋，這個地址指向建立類時附在類空間後 extraBytes 大小的一塊記憶體。
debug 除錯
object_getIndexedIvars
__NSSetBoolValueAndNotify 下的呼叫
object_getIndexedIvars 入參是 NSKVONotifying_KVObject，object_getClass 獲取的是 KVO Class。
objc_allocateClassPair
動態建立 KVO 類的方法。
thread #8, queue = 'com.apple.root.default-qos', stop reason = breakpoint 1.1
* frame #0: 0x000000018143a088 libobjc.A.dylib`objc_allocateClassPair
frame #1: 0x000000018259cd94 Foundation`_NSKVONotifyingCreateInfoWithOriginalClass + 152
frame #2: 0x00000001825b8fd0 Foundation`_NSKeyValueContainerClassGetNotifyingInfo + 56
frame #3: 0x000000018254b7dc Foundation`-[NSKeyValueUnnestedProperty _isaForAutonotifying] + 44
frame #4: 0x000000018254b504 Foundation`-[NSKeyValueUnnestedProperty isaForAutonotifying] + 88
frame #5: 0x000000018254b32c Foundation`-[NSObject(NSKeyValueObserverRegistration) _addObserver:forProperty:options:context:] + 404
frame #6: 0x000000018254b054 Foundation`-[NSObject(NSKeyValueObserverRegistration) addObserver:forKeyPath:options:context:] + 136
frame #7: 0x00000001040d1860 Test`__29-[ViewController viewDidLoad]_block_invoke(.block_descriptor=0x0000000282a55170) at ViewController.m:28:13
frame #8: 0x00000001043d05a8 libdispatch.dylib`_dispatch_call_block_and_release + 32
frame #9: 0x00000001043d205c libdispatch.dylib`_dispatch_client_callout + 20
frame #10: 0x00000001043d4b94 libdispatch.dylib`_dispatch_queue_override_invoke + 1052
frame #11: 0x00000001043e6478 libdispatch.dylib`_dispatch_root_queue_drain + 408
frame #12: 0x00000001043e6e74 libdispatch.dylib`_dispatch_worker_thread2 + 196
frame #13: 0x00000001d515fdbc libsystem_pthread.dylib`_pthread_wqthread + 228
_NSKVONotifyingCreateInfoWithOriginalClass
objc_allocateClassPair 的上層呼叫。 allocate 之前的 context w2 是個固定值 0x30，即建立 KVO Class 入參 extraBytes 的大小是 0x30
0x18259cd78 <+124>: mov x1, x21
0x18259cd7c <+128>: mov x2, x22
0x18259cd80 <+132>: bl 0x188097080
0x18259cd84 <+136>: mov x0, x20
0x18259cd88 <+140>: mov x1, x19
0x18259cd8c <+144>: mov w2, #0x30
0x18259cd90 <+148>: bl 0x1880961f0 // objc_allocateClassPair
0x18259cd94 <+152>: cbz x0, 0x18259ce24 ; <+296>
0x18259cd98 <+156>: mov x21, x0
0x18259cd9c <+160>: bl 0x188096410 // objc_registerClassPair
0x18259cda0 <+164>: mov x0, x19
0x18259cda4 <+168>: bl 0x182b45f44 ; symbol stub for: free
0x18259cda8 <+172>: mov x0, x21
0x18259cdac <+176>: bl 0x1880967e0 // object_getIndexedIvars
0x18259cdb0 <+180>: mov x19, x0
0x18259cdb4 <+184>: stp x20, x21, [x0]
_NSKVONotifyingCreateInfoWithOriginalClass+184 處將 x20 和 x21 寫入 [x0]，此時 x0 指向的是大小為 extraBytes 的記憶體，列印 x20 和 x21 的值
x20 = 0x00000001117caa10 (void *)0x00000001117caa38: KVObject（向上回溯這個值取自 _NSKVONotifyingCreateInfoWithOriginalClass 的入參 x0）
x21 NSKVONotifying_KVObject
根據這裡可以看出 object_getIndexedIvars 返回的地址，依次儲存了 KVObject(origin Class) 和 NSKVONotifying_KVObject(KVO Class)。
檢視 _NSKVONotifyingCreateInfoWithOriginalClass 的虛擬碼，對 [x0] 有 5 次寫入的操作，並且最終這個方法返回的是 x0 的地址。
function __NSKVONotifyingCreateInfoWithOriginalClass {
r31 = r31 - 0x50;
stack[32] = r22;
stack[40] = r21;
stack[48] = r20;
stack[56] = r19;
stack[64] = r29;
stack[72] = r30;
r20 = r0;
if (*(int8_t *)0x993e78 != 0x0) {
os_unfair_lock_assert_owner(0x993e7c);
}
r0 = class_getName(r20);
r22 = strlen(r0) + 0x10;
r0 = malloc(r22);
r19 = r0;
strlcpy(r0, "NSKVONotifying_", r22);
strlcat(r19, r21, r22);
r0 = objc_allocateClassPair(r20, r19, 0x30);
if (r0 != 0x0) {
objc_registerClassPair(r0);
free(r19);
r0 = object_getIndexedIvars(r21);
r19 = r0;
*(int128_t *)r0 = r20; // 第一次寫入 Class
*(int128_t *)(r0 + 0x8) = r21; // 第二次寫入 Class
*(r19 + 0x10) = CFSetCreateMutable(0x0, 0x0, *qword_9592d8); // 第三次寫入 CFSet
*(int128_t *)(r19 + 0x18) = CFDictionaryCreateMutable(0x0, 0x0, 0x0, *qword_959598); // 第四次寫入 CFDictionary
*(int128_t *)(r19 + 0x20) = 0x0; // 第五次寫入空值
if (*qword_9fc560 != -0x1) {
dispatch_once(0x9fc560, 0x8eaf98);
}
if (class_getMethodImplementation(*r19, @selector(willChangeValueForKey:)) != *qword_9fc568) {
r8 = 0x1;
}
else {
r0 = *r19;
r0 = class_getMethodImplementation(r0, @selector(didChangeValueForKey:));
r8 = *qword_9fc570;
if (r0 != r8) {
r8 = *qword_9fc570;
if (CPU_FLAGS & NE) {
r8 = 0x1;
}
}
}
*(int8_t *)(r19 + 0x28) = r8;
_NSKVONotifyingSetMethodImplementation(r19, @selector(_isKVOA), 0x44fab4, 0x0);
_NSKVONotifyingSetMethodImplementation(r19, @selector(dealloc), 0x44fabc, 0x0);
_NSKVONotifyingSetMethodImplementation(r19, @selector(class), 0x44fd2c, 0x0);
}
else {
if (*qword_9fc558 != -0x1) {
dispatch_once(0x9fc558, 0x8eaf78);
}
if (os_log_type_enabled(*0x9fc550, 0x10) != 0x0) {
_os_log_error_impl(0x0, *0x9fc550, 0x10, "KVO failed to allocate class pair for name %s, automatic key-value observing will not work for this class", &stack[0], 0xc);
}
free(r19);
r19 = 0x0;
}
if (**qword_9590e8 == **qword_9590e8) {
r0 = r19;
}
else {
r0 = __stack_chk_fail();
}
return r0;
}
_NSKVONotifyingCreateInfoWithOriginalClass 的上層呼叫，入參是 [x19, #0x8]，返回的引數寫入 [x19, #0x28]
0x1825b8fc0 <+40>: ldr x0, [x19, #0x28]
0x1825b8fc4 <+44>: b 0x1825b8fd4 ; <+60>
0x1825b8fc8 <+48>: ldr x0, [x19, #0x8]
-> 0x1825b8fcc <+52>: bl 0x18259ccfc ; _NSKVONotifyingCreateInfoWithOriginalClass
0x1825b8fd0 <+56>: str x0, [x19, #0x28]
0x1825b8fd4 <+60>: ldp x29, x30, [sp, #0x10]
0x1825b8fd8 <+64>: ldp x20, x19, [sp], #0x20
列印 x19 是一個 NSKeyValueContainerClass 型別的例項物件，這個物件類的 ivars layout
ivars 0x99f3c0 __OBJC_$_INSTANCE_VARIABLES_NSKeyValueContainerClass
entsize 32
count 5
offset 0x9e6048 _OBJC_IVAR_$_NSKeyValueContainerClass._originalClass 8
name 0x90bd27 _originalClass
type 0x929ae6 #
alignment 3
size 8
offset 0x9e6050 _OBJC_IVAR_$_NSKeyValueContainerClass._cachedObservationInfoImplementation 16
name 0x90bd36 _cachedObservationInfoImplementation
type 0x92bb88 ^?
alignment 3
size 8
offset 0x9e6058 _OBJC_IVAR_$_NSKeyValueContainerClass._cachedSetObservationInfoImplementation 24
name 0x90bd5b _cachedSetObservationInfoImplementation
type 0x92bb88 ^?
alignment 3
size 8
offset 0x9e6060 _OBJC_IVAR_$_NSKeyValueContainerClass._cachedSetObservationInfoTakesAnObject 32
name 0x90bd83 _cachedSetObservationInfoTakesAnObject
type 0x92a01a B
alignment 0
size 1
offset 0x9e6068 _OBJC_IVAR_$_NSKeyValueContainerClass._notifyingInfo 40
name 0x90bdaa _notifyingInfo
type 0x92bdd7 ^{?=##^{__CFSet}^{__CFDictionary}{os_unfair_recursive_lock_s={os_unfair_lock_s=I}I}B}
alignment 3
size 8
offset 0x8 name：_originalClass type：Class
offset 0x28 name：_notifyingInfo type：struct
_notifyingInfo 結構體
{
Class,
Class,
__CFSet,
__CFDictionary,
os_unfair_recursive_lock_s
}
type encoding：
從 context 可以看出_NSKVONotifyingCreateInfoWithOriginalClass 這個方法入參是 _OBJC_IVAR__NSKeyValueContainerClass._originalClass。返回值 x0 是 _OBJC_IVAR__NSKeyValueContainerClass._notifyingInfo。5 次對 [x0] 的寫入是在初始化 _notifyingInfo。
崩潰時的 context:
0x1825231f0 <+56>: bl 0x1880967c0 // object_getClass
0x1825231f4 <+60>: bl 0x1880967e0 // object_getIndexedIvars
0x1825231f8 <+64>: mov x23, x0 // x0 == _notifyingInfo
0x1825231fc <+68>: add x24, x0, #0x20 // x24 == os_unfair_recursive_lock_s
0x182523200 <+72>: mov x0, x24
0x182523204 <+76>: mov w1, #0x0
0x182523208 <+80>: bl 0x188096910 // os_unfair_recursive_lock_lock_with_options crash 呼叫棧
呼叫 object_getClass 獲取 Class，呼叫 object_getIndexedIvars 獲取到 _notifyingInfo，_notifyingInfo + 偏移量 0x20 獲取 os_unfair_recursive_lock_s，崩潰的原因是這把鎖的 owner 損壞了，lock 也是一個結構體，ower 也是根據 offset 獲取的。
結論
從崩潰的上下文來看，最可能出問題的是獲取 _notifyingInfo，因為只有 KVO Class 才能獲取到 _notifyingInfo 這個結構體，如果在呼叫 __NSSetBoolValueAndNotify 的過程中，在其它執行緒監聽被移除，此時 object_getClass 取到的不是 KVO Class 那後續再根據 offset 去取 lock，這個時候就有可能發生上述崩潰。
線下暴力復現驗證了上述猜測。
``` - （void)start { __block KVObject *obj = [KVObject new]; dispatch_async(dispatch_get_global_queue(0, 0x0), ^{ for (int i = 0; i < 100000; i++) { [obj addObserver:self forKeyPath:@"value" options:0x7 context:nil]; [obj removeObserver:self forKeyPath:@"value"]; } });
dispatch_async(dispatch_get_global_queue(0, 0x0), ^{
for (int i = 0; i < 100000; i++) {
obj.value = YES;
obj.value = NO;
}
});
}
- (void)observeValueForKeyPath:(NSString )keyPath ofObject:(id)object change:(NSDictionary
解決這個問題的思路就是保證執行緒安全，我們在線上斷點找到了 removeObserver 的程式碼，將 removeObserver 和觸發監聽的程式碼放在了同一個序列佇列。當然如果 removeObserver 在 dealloc 裡面，理論上也不會出現這類問題。
__NSSetxxxValueAndNotify 系列方法都有可能會觸發這個崩潰，類似的問題可以按照相同的思路解決。
00000000004e05cd t __NSSetBoolValueAndNotify
00000000004e0707 t __NSSetCharValueAndNotify
00000000004e097b t __NSSetDoubleValueAndNotify
00000000004e0abc t __NSSetFloatValueAndNotify
00000000004e0bfd t __NSSetIntValueAndNotify
00000000004e10e7 t __NSSetLongLongValueAndNotify
00000000004e0e6f t __NSSetLongValueAndNotify
00000000004e0491 t __NSSetObjectValueAndNotify
00000000004e15d5 t __NSSetPointValueAndNotify
00000000004e1734 t __NSSetRangeValueAndNotify
00000000004e188a t __NSSetRectValueAndNotify
00000000004e135f t __NSSetShortValueAndNotify
00000000004e19e8 t __NSSetSizeValueAndNotify
00000000004e0841 t __NSSetUnsignedCharValueAndNotify
00000000004e0d36 t __NSSetUnsignedIntValueAndNotify
00000000004e1223 t __NSSetUnsignedLongLongValueAndNotify
00000000004e0fab t __NSSetUnsignedLongValueAndNotify
00000000004e149a t __NSSetUnsignedShortValueAndNotify
00000000004de834 t __NSSetValueAndNotifyForKeyInIvar