iOS unrecognized selector crash 自修复技术实现与原理解析

作者介绍:阿里云-移动云-大前端团队。

前言

在开发中 unrecognized selector sent to instance XXXXX 是非常常见的 crash 类型。

例如调用以下一段代码就会产生crash

[[NSNull null] performSelector:@selector(fooDoesNotRecognizeSelector1)];

具体 crash 时的表现见下:

2018-01-11 16:28:04.433573+0800 CYLSwizzleMainDemo[13252:156773356] -[NSNull fooDoesNotRecognizeSelector1]: unrecognized selector sent to instance 0x102870ef0
2018-01-11 16:28:04.440436+0800 CYLSwizzleMainDemo[13252:156773356] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull fooDoesNotRecognizeSelector1]: unrecognized selector sent to instance 0x102870ef0'
*** First throw call stack:
(
   0   CoreFoundation                      0x00000001025a712b __exceptionPreprocess + 171
   1   libobjc.A.dylib                     0x0000000101c3bf41 objc_exception_throw + 48
   2   CoreFoundation                      0x0000000102628024 -[NSObject(NSObject) doesNotRecognizeSelector:] + 132
   3   CoreFoundation                      0x0000000102529f78 ___forwarding___ + 1432
   4   CoreFoundation                      0x0000000102529958 _CF_forwarding_prep_0 + 120
   5   CYLSwizzleMainDemo                  0x0000000101321cef -[AppDelegate application:didFinishLaunchingWithOptions:] + 527
   6   UIKit                               0x0000000103315ac6 -[UIApplication _handleDelegateCallbacksWithOptions:isSuspended:restoreState:] + 299
   7   UIKit                               0x0000000103317544 -[UIApplication _callInitializationDelegatesForMainScene:transitionContext:] + 4113
   8   UIKit                               0x000000010331c9e7 -[UIApplication _runWithMainScene:transitionContext:completion:] + 1720
   9   UIKit                               0x00000001036e5fb0 __111-[__UICanvasLifecycleMonitor_Compatability _scheduleFirstCommitForScene:transition:firstActivation:completion:]_block_invoke + 924
   10  UIKit                               0x0000000103abb998 +[_UICanvas _enqueuePostSettingUpdateTransactionBlock:] + 153
   11  UIKit                               0x00000001036e5ba9 -[__UICanvasLifecycleMonitor_Compatability _scheduleFirstCommitForScene:transition:firstActivation:completion:] + 249
   12  UIKit                               0x00000001036e6423 -[__UICanvasLifecycleMonitor_Compatability activateEventsOnly:withContext:completion:] + 696
   13  UIKit                               0x0000000104063fe9 __82-[_UIApplicationCanvas _transitionLifecycleStateWithTransitionContext:completion:]_block_invoke + 262
   14  UIKit                               0x0000000104063ea2 -[_UIApplicationCanvas _transitionLifecycleStateWithTransitionContext:completion:] + 444
   15  UIKit                               0x0000000103d410a0 __125-[_UICanvasLifecycleSettingsDiffAction performActionsForCanvas:withUpdatedScene:settingsDiff:fromSettings:transitionContext:]_block_invoke + 221
   16  UIKit                               0x0000000103f40126 _performActionsWithDelayForTransitionContext + 100
   17  UIKit                               0x0000000103d40f63 -[_UICanvasLifecycleSettingsDiffAction performActionsForCanvas:withUpdatedScene:settingsDiff:fromSettings:transitionContext:] + 231
   18  UIKit                               0x0000000103abaff5 -[_UICanvas scene:didUpdateWithDiff:transitionContext:completion:] + 392
   19  UIKit                               0x000000010331b266 -[UIApplication workspace:didCreateScene:withTransitionContext:completion:] + 523
   20  UIKit                               0x00000001038f5b97 -[UIApplicationSceneClientAgent scene:didInitializeWithEvent:completion:] + 369
   21  FrontBoardServices                  0x0000000106d74cc0 -[FBSSceneImpl _didCreateWithTransitionContext:completion:] + 338
   22  FrontBoardServices                  0x0000000106d7d7b5 __56-[FBSWorkspace client:handleCreateScene:withCompletion:]_block_invoke_2 + 235
   23  libdispatch.dylib                   0x0000000105fd933d _dispatch_client_callout + 8
   24  libdispatch.dylib                   0x0000000105fde9f3 _dispatch_block_invoke_direct + 592
   25  FrontBoardServices                  0x0000000106da9498 __FBSSERIALQUEUE_IS_CALLING_OUT_TO_A_BLOCK__ + 24
   26  FrontBoardServices                  0x0000000106da914e -[FBSSerialQueue _performNext] + 464
   27  FrontBoardServices                  0x0000000106da96bd -[FBSSerialQueue _performNextFromRunLoopSource] + 45
   28  CoreFoundation                      0x000000010254a101 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
   29  CoreFoundation                      0x00000001025e9f71 __CFRunLoopDoSource0 + 81
   30  CoreFoundation                      0x000000010252ea19 __CFRunLoopDoSources0 + 185
   31  CoreFoundation                      0x000000010252dfff __CFRunLoopRun + 1279
   32  CoreFoundation                      0x000000010252d889 CFRunLoopRunSpecific + 409
   33  GraphicsServices                    0x000000010763b9c6 GSEventRunModal + 62
   34  UIKit                               0x000000010331e4d2 UIApplicationMain + 159
   35  CYLSwizzleMainDemo                  0x00000001013230bf main + 111
   36  libdyld.dylib                       0x0000000106055d81 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb) 

这类 crash 尤其在混合开发,或者 JS 与 native 交互中经常遇到,非常影响用户体验,也降低了 app 的质量与稳定性。

常见的 crash 场景可以总结为:

  • JSON 解析后,空值解析为 NSNULL 对象,造成 crash
  • JS 调用 native 方法,结果由于native底版本,或者 JS 代码编写的问题,找不到方法,导致 app crash。

在研究如何实现 app 自修复该 bug 前,我们可以研究下什么时候会报 unrecognized selector 的异常?

什么时候会报unrecognized selector的异常?

objc是动态语言,每个方法在运行时会被动态转为消息发送,即:objc_msgSend(receiver, selector)。objc在向一个对象发送消息时,runtime库会根据对象的isa指针找到该对象实际所属的类,然后在该类中的方法列表以及其父类方法列表中寻找方法运行,如果,在最顶层的父类中依然找不到相应的方法时,程序在运行时会挂掉并抛出异常unrecognized selector sent to XXX 。但是在这之前,objc的运行时会给出三次拯救程序崩溃的机会:

  1. Method resolution

objc运行时会调用+resolveInstanceMethod:或者 +resolveClassMethod:,让你有机会提供一个函数实现。如果你添加了函数,那运行时系统就会重新启动一次消息发送的过程,否则 ,运行时就会移到下一步,消息转发(Message Forwarding)。

  1. Fast forwarding

如果目标对象实现了-forwardingTargetForSelector:,Runtime 这时就会调用这个方法,给你把这个消息转发给其他对象的机会。
只要这个方法返回的不是nil和self,整个消息发送的过程就会被重启,当然发送的对象会变成你返回的那个对象。否则,就会继续Normal Fowarding。
这里叫Fast,只是为了区别下一步的转发机制。因为这一步不会创建任何新的对象,但下一步转发会创建一个NSInvocation对象,所以相对更快点。

  1. Normal forwarding

这一步是Runtime最后一次给你挽救的机会。首先它会发送-methodSignatureForSelector:消息获得函数的参数和返回值类型。如果-methodSignatureForSelector:返回nil,Runtime则会发出-doesNotRecognizeSelector:消息,程序这时也就挂掉了。如果返回了一个函数签名,Runtime就会创建一个NSInvocation对象并发送-forwardInvocation:消息给目标对象。

拦截调用的整个流程即 Objective-C 的消息转发机制。其具体流程如下图:

iOS unrecognized selector crash 自修复技术实现与原理解析

unrecognized selector sent to instance XXXXX crash 自修复技术实现

原理简单来说:

当调用该对象上某个方法,而该对象上没有实现这个方法的时候,

可以通过“消息转发”进行解决。

可以利用消息转发机制的三个步骤,选择哪一步去改造比较合适呢?

这里我们选择了第二步forwardingTargetForSelector。引用 《大白健康系统--iOS APP运行时Crash自动修复系统》 的分析:

resolveInstanceMethod 需要在类的本身上动态添加它本身不存在的方法,这些方法对于该类本身来说冗余的
forwardInvocation 可以通过 NSInvocation 的形式将消息转发给多个对象,但是其开销较大,需要创建新的 NSInvocation 对象,并且 forwardInvocation 的函数经常被使用者调用,来做多层消息转发选择机制,不适合多次重写
forwardingTargetForSelector 可以将消息转发给一个对象,开销较小,并且被重写的概率较低,适合重写
选择了 forwardingTargetForSelector 之后,可以将 NSObject 的该方法重写,做以下几步的处理:

具体如下:

  • hook forwardingTargetForSelector 方法
  • 添加白名单,限制hook的范围,排除内部类,并自定义需要hook的类
  • 创建桩类 ForwardingTarget
  • 为桩类动态添加对应的selector的imp,指向一个函数,返回 NSNull 对象
  • 将消息转移到该桩类对象 ForwardingTarget 上
  • 将 hook 掉的 crash 信息进行上报,方便发现问题,后期修复掉。

添加白名单,避免出现hook内部方法以及不必要的对象。
内部对象的特征是都以 _ 开头。
其他需要限制的部分,经常会出现在组件化开发、SDK开发中,避免影响到其他模块的正常工作,可以用类的前缀做区分。

其中动态创建的方法,返回值为什么返回一个 NSNull,而不是其他的值。

这样做的好处在于,在设置白名单的时候,只需要将 NSNull 设置进白名单,就可以解决方法返回值调用方法造成的crash。返回其他类型,就需要在白名单中多设置一种类型。

可以解决如下问题:

 id foo = [[NSNull null] performSelector:@selector(fooDoesNotRecognizeSelector1)];
 [foo performSelector:@selector(fooDoesNotRecognizeSelector2)];

hook时注意如果对象的类本事如果重写了forwardInvocation方法的话,就不应该对forwardingTargetForSelector进行重写了,否则会影响到该类型的对象原本的消息转发流程。

通过重写NSObjectforwardingTargetForSelector方法,我们就可以将无法识别的方法进行拦截并且将消息转发到安全的桩类对象中,从而可以使app继续正常运行。

具体的实现代码如下:

//
//  ForwardingTarge.h
//  
//
//  Created by ChenYilong on 18/01/10.
//  Copyright © 2018年  All rights reserved.
//

#import <Foundation/Foundation.h>

@interface ForwardingTarget : NSObject

@end
//
//  ForwardingTarge.m
//  
//
//  Created by ChenYilong on 18/01/10.
//  Copyright © 2018年  All rights reserved.
//

#import "ForwardingTarget.h"
#import <objc/runtime.h>

@implementation ForwardingTarget

id ForwardingTarget_dynamicMethod(id self, SEL _cmd) {
   return [NSNull null];
}

+ (BOOL)resolveInstanceMethod:(SEL)sel {
   class_addMethod(self.class, sel, (IMP)ForwardingTarget_dynamicMethod, "@@:");
   [super resolveInstanceMethod:sel];
   return YES;
}

- (id)forwardingTargetForSelector:(SEL)aSelector {
   id result = [super forwardingTargetForSelector:aSelector];
   return result;
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
   id result = [super methodSignatureForSelector:aSelector];
   return result;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation {
   [super forwardInvocation:anInvocation];
}

- (void)doesNotRecognizeSelector:(SEL)aSelector {
   [super doesNotRecognizeSelector:aSelector]; // crash
}

@end
//
//  NSObject+DoesNotRecognizeSelectorExtension.h
//  
//
//  Created by ChenYilong on 18/01/10.
//  Copyright © 2018年  All rights reserved.
//

#import <Foundation/Foundation.h>

@interface NSObject (DoesNotRecognizeSelectorExtension)

@end
//
//  NSObject+DoesNotRecognizeSelectorExtension.m
//  
//
//  Created by ChenYilong on 18/01/10.
//  Copyright © 2018年 All rights reserved.
//

#import "NSObject+DoesNotRecognizeSelectorExtension.h"
#import <objc/runtime.h>
#import "ForwardingTarget.h"

static ForwardingTarget *_target = nil;

@implementation NSObject (DoesNotRecognizeSelectorExtension)

+ (void)load {
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
       _target = [ForwardingTarget new];;
       not_recognize_selector_classMethodSwizzle([self class], @selector(forwardingTargetForSelector:), @selector(doesnot_recognize_selector_swizzleForwardingTargetForSelector:));
   });
}

+ (BOOL)isWhiteListClass:(Class)class {
   NSString *classString = NSStringFromClass(class);
   BOOL isInternal = [classString hasPrefix:@"_"];
   if (isInternal) {
       return NO;
   }
   BOOL isNull =  [classString isEqualToString:NSStringFromClass([NSNull class])];
   
   BOOL isMyClass  = [classString ...];
   return isNull || isMyClass;
}

- (id)doesnot_recognize_selector_swizzleForwardingTargetForSelector:(SEL)aSelector {
   id result = [self doesnot_recognize_selector_swizzleForwardingTargetForSelector:aSelector];
   if (result) {
       return result;
   }
   BOOL isWhiteListClass = [[self class] isWhiteListClass:[self class]];
   if (!isWhiteListClass) {
       return nil;
   }
   
   if (!result) {
       result = _target;
   }
   return result;
}

#pragma mark - private method

BOOL not_recognize_selector_classMethodSwizzle(Class aClass, SEL originalSelector, SEL swizzleSelector) {
   Method originalMethod = class_getInstanceMethod(aClass, originalSelector);
   Method swizzleMethod = class_getInstanceMethod(aClass, swizzleSelector);
   BOOL didAddMethod =
   class_addMethod(aClass,
                   originalSelector,
                   method_getImplementation(swizzleMethod),
                   method_getTypeEncoding(swizzleMethod));
   if (didAddMethod) {
       class_replaceMethod(aClass,
                           swizzleSelector,
                           method_getImplementation(originalMethod),
                           method_getTypeEncoding(originalMethod));
   } else {
       method_exchangeImplementations(originalMethod, swizzleMethod);
   }
   return YES;
}

@end

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