Crash with swift_weakLoadStrong in the stack


(Jon Shier) #1

Swift Users:
  I’m currently seeing a crash in my iOS app that has no apparent cause, but a bit of Swift runtime machinery in the stack has me confused.

#0. Crashed: com.apple.main-thread
0 App 0x665ac Controller.handleOtherNotification(Notification) -> () (Controller.swift:92)
1 libswiftCore.dylib 0x131854f swift_unknownWeakLoadStrong + 10
2 App 0x65cfc Controller.handleFinish(Notification) -> () (Controller.swift)
3 App 0x65dd8 @objc Controller.handleNotification(Notification) -> () + 437720

Controller.swift: 92 is a call to a custom UIView subclass method that takes an optional date contained extracted from the notification. Any idea what the core callout would be due to? There are no weak or unknown values being used here. Once the notification observers are called it’s all internal to Controller.

One thing I just noticed is that the line at 3 is the selector for a different notification, which should lead down the path see from 2 onward. It’s redacted and so not easy to see, but in Controller, there’s no path that leads from handleNotification to handleFinish.

Jon


(Greg Parker) #2

That backtrace does look strange. Even if there were some surprising call to swift_unknownWeakLoadStrong() in handleFinish(), there's no way that swift_unknownWeakLoadStrong() would call handleOtherNotification().

(swift_unknownWeakLoadStrong + 10 is the instruction after a call, assuming you're on 64-bit iOS simulator, but that call is to swift::isNativeSwiftWeakReference() which itself doesn't call anything.)

Also, in frame 3, the byte offset from the start of handleNotification() is larger than the address itself. And all of these addresses look too small if you're on 64-bit.

Where did this backtrace come from? Do you have a crash log as generated by the OS?

···

On Feb 13, 2017, at 12:18 PM, Jon Shier via swift-users <swift-users@swift.org> wrote:

Swift Users:
  I’m currently seeing a crash in my iOS app that has no apparent cause, but a bit of Swift runtime machinery in the stack has me confused.

#0. Crashed: com.apple.main-thread
0 App 0x665ac Controller.handleOtherNotification(Notification) -> () (Controller.swift:92)
1 libswiftCore.dylib 0x131854f swift_unknownWeakLoadStrong + 10
2 App 0x65cfc Controller.handleFinish(Notification) -> () (Controller.swift)
3 App 0x65dd8 @objc Controller.handleNotification(Notification) -> () + 437720

Controller.swift: 92 is a call to a custom UIView subclass method that takes an optional date contained extracted from the notification. Any idea what the core callout would be due to? There are no weak or unknown values being used here. Once the notification observers are called it’s all internal to Controller.

One thing I just noticed is that the line at 3 is the selector for a different notification, which should lead down the path see from 2 onward. It’s redacted and so not easy to see, but in Controller, there’s no path that leads from handleNotification to handleFinish.

--
Greg Parker gparker@apple.com <mailto:gparker@apple.com> Runtime Wrangler


(Jon Shier) #3

These crashes are uploaded from device, namely an iPad 4 (running the app in compatibility mode) uploaded through Crashlytics and then downloaded from there. I haven’t been able to reproduce the crash and so I haven’t seen a raw dump. Here’s the full crash stack, redacted, from an iPad 4 running iOS 10.1.1 (this is an iOS 10+ app).

#0. Crashed: com.apple.main-thread
0 App 0x665ac Controller.handleSomeNotification(SomeNotification) -> () (Controller.swift:92)
1 libswiftCore.dylib 0x131854f swift_unknownWeakLoadStrong + 10
2 App 0x65cfc Controller.handleFinishSomeNotification(Notification) -> () (Controller.swift)
3 App 0x65dd8 @objc Controller.handleSomeOtherNotification(Notification) -> () + 437720
4 CoreFoundation 0x1bafa761 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 10
5 CoreFoundation 0x1bafa09d _CFXRegistrationPost + 386
6 CoreFoundation 0x1baf9e81 ___CFXNotificationPost_block_invoke + 40
7 CoreFoundation 0x1bb5581d -[_CFXNotificationRegistrar find:object:observer:enumerator:] + 1188
8 CoreFoundation 0x1ba5a09d _CFXNotificationPost + 540
9 App 0x15b004 specialized specialized NotificationCenter.post<A> (A, forName : NSNotification.Name) -> () (Notifications.swift)
10 App 0x8d6f4 SomeListener.(post(SomeNotification : SomeNotification) -> ()).(closure #1) (SomeNotificationHandler.swift)
11 App 0x96318 partial apply for SomeListener.(post(SomeNotification : SomeNotification) -> ()).(closure #1) (SomeNotificationHandler.swift)
12 libdispatch.dylib 0x1b1f5097 _dispatch_call_block_and_release + 10
13 libdispatch.dylib 0x1b1f5083 _dispatch_client_callout + 22
14 libdispatch.dylib 0x1b1f95fd _dispatch_main_queue_callback_4CF + 890
15 CoreFoundation 0x1bb0aa17 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 8
16 CoreFoundation 0x1bb08cff __CFRunLoopRun + 1422
17 CoreFoundation 0x1ba58073 CFRunLoopRunSpecific + 486
18 CoreFoundation 0x1ba57e81 CFRunLoopRunInMode + 104
19 GraphicsServices 0x1d204bfd GSEventRunModal + 156
20 UIKit 0x20c0c82f -[UIApplication _run] + 574
21 UIKit 0x20c06f61 UIApplicationMain + 150
22 App 0x4bc38 main (AppDelegate.swift:12)
23 libdispatch.dylib 0x1b22250b (Missing)

Essentially I have a listener waiting on a background queue for a push notification to come in, which then repackages the payload into a Notification using a generic convenience method I wrote, and posted onto the main queue. Controller is listening for this two separate notifications, one of which is the SomeNotification. But the @objc method in there is the selector for the other notification, SomeOther. There should be no path between 3 and 2 in the stack, and there are no weak references I can see, except perhaps the implicit ones from NotificationCenter. Badly resymbolicated log? Is that even possible?

Jon

···

On Feb 13, 2017, at 6:52 PM, Greg Parker <gparker@apple.com> wrote:

On Feb 13, 2017, at 12:18 PM, Jon Shier via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

Swift Users:
  I’m currently seeing a crash in my iOS app that has no apparent cause, but a bit of Swift runtime machinery in the stack has me confused.

#0. Crashed: com.apple.main-thread
0 App 0x665ac Controller.handleOtherNotification(Notification) -> () (Controller.swift:92)
1 libswiftCore.dylib 0x131854f swift_unknownWeakLoadStrong + 10
2 App 0x65cfc Controller.handleFinish(Notification) -> () (Controller.swift)
3 App 0x65dd8 @objc Controller.handleNotification(Notification) -> () + 437720

Controller.swift: 92 is a call to a custom UIView subclass method that takes an optional date contained extracted from the notification. Any idea what the core callout would be due to? There are no weak or unknown values being used here. Once the notification observers are called it’s all internal to Controller.

One thing I just noticed is that the line at 3 is the selector for a different notification, which should lead down the path see from 2 onward. It’s redacted and so not easy to see, but in Controller, there’s no path that leads from handleNotification to handleFinish.

That backtrace does look strange. Even if there were some surprising call to swift_unknownWeakLoadStrong() in handleFinish(), there's no way that swift_unknownWeakLoadStrong() would call handleOtherNotification().

(swift_unknownWeakLoadStrong + 10 is the instruction after a call, assuming you're on 64-bit iOS simulator, but that call is to swift::isNativeSwiftWeakReference() which itself doesn't call anything.)

Also, in frame 3, the byte offset from the start of handleNotification() is larger than the address itself. And all of these addresses look too small if you're on 64-bit.

Where did this backtrace come from? Do you have a crash log as generated by the OS?

--
Greg Parker gparker@apple.com <mailto:gparker@apple.com> Runtime Wrangler


(Greg Parker) #4

Ah, it's 32-bit iOS. (I assumed i386 or x86_64 from the unaligned instruction addresses, but forgot that 32-bit ARM can have those too.) The small-looking addresses are okay, then.

That +437720 is still bogus. Ordinarily a symbol with a large offset means that the code was actually in some other function, but there was no symbol available for that other function. In such cases the symbolicator picks the closest symbol available; the large offset means that other symbol was far away. In this case that offset is larger than the address itself, which is impossible. Something in that frame was calculated incorrectly.

You may get better answers from Crashlytics about this. It looks suspiciously like an incorrect backtrace and/or incorrect symbolication of the backtrace.

···

On Feb 13, 2017, at 4:25 PM, Jon Shier <jon@jonshier.com> wrote:

These crashes are uploaded from device, namely an iPad 4 (running the app in compatibility mode) uploaded through Crashlytics and then downloaded from there. I haven’t been able to reproduce the crash and so I haven’t seen a raw dump. Here’s the full crash stack, redacted, from an iPad 4 running iOS 10.1.1 (this is an iOS 10+ app).

#0. Crashed: com.apple.main-thread
0 App 0x665ac Controller.handleSomeNotification(SomeNotification) -> () (Controller.swift:92)
1 libswiftCore.dylib 0x131854f swift_unknownWeakLoadStrong + 10
2 App 0x65cfc Controller.handleFinishSomeNotification(Notification) -> () (Controller.swift)
3 App 0x65dd8 @objc Controller.handleSomeOtherNotification(Notification) -> () + 437720
4 CoreFoundation 0x1bafa761 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 10
5 CoreFoundation 0x1bafa09d _CFXRegistrationPost + 386
6 CoreFoundation 0x1baf9e81 ___CFXNotificationPost_block_invoke + 40
7 CoreFoundation 0x1bb5581d -[_CFXNotificationRegistrar find:object:observer:enumerator:] + 1188
8 CoreFoundation 0x1ba5a09d _CFXNotificationPost + 540
9 App 0x15b004 specialized specialized NotificationCenter.post<A> (A, forName : NSNotification.Name) -> () (Notifications.swift)
10 App 0x8d6f4 SomeListener.(post(SomeNotification : SomeNotification) -> ()).(closure #1) (SomeNotificationHandler.swift)
11 App 0x96318 partial apply for SomeListener.(post(SomeNotification : SomeNotification) -> ()).(closure #1) (SomeNotificationHandler.swift)
12 libdispatch.dylib 0x1b1f5097 _dispatch_call_block_and_release + 10
13 libdispatch.dylib 0x1b1f5083 _dispatch_client_callout + 22
14 libdispatch.dylib 0x1b1f95fd _dispatch_main_queue_callback_4CF + 890
15 CoreFoundation 0x1bb0aa17 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 8
16 CoreFoundation 0x1bb08cff __CFRunLoopRun + 1422
17 CoreFoundation 0x1ba58073 CFRunLoopRunSpecific + 486
18 CoreFoundation 0x1ba57e81 CFRunLoopRunInMode + 104
19 GraphicsServices 0x1d204bfd GSEventRunModal + 156
20 UIKit 0x20c0c82f -[UIApplication _run] + 574
21 UIKit 0x20c06f61 UIApplicationMain + 150
22 App 0x4bc38 main (AppDelegate.swift:12)
23 libdispatch.dylib 0x1b22250b (Missing)

Essentially I have a listener waiting on a background queue for a push notification to come in, which then repackages the payload into a Notification using a generic convenience method I wrote, and posted onto the main queue. Controller is listening for this two separate notifications, one of which is the SomeNotification. But the @objc method in there is the selector for the other notification, SomeOther. There should be no path between 3 and 2 in the stack, and there are no weak references I can see, except perhaps the implicit ones from NotificationCenter. Badly resymbolicated log? Is that even possible?

Jon

On Feb 13, 2017, at 6:52 PM, Greg Parker <gparker@apple.com <mailto:gparker@apple.com>> wrote:

On Feb 13, 2017, at 12:18 PM, Jon Shier via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

Swift Users:
  I’m currently seeing a crash in my iOS app that has no apparent cause, but a bit of Swift runtime machinery in the stack has me confused.

#0. Crashed: com.apple.main-thread
0 App 0x665ac Controller.handleOtherNotification(Notification) -> () (Controller.swift:92)
1 libswiftCore.dylib 0x131854f swift_unknownWeakLoadStrong + 10
2 App 0x65cfc Controller.handleFinish(Notification) -> () (Controller.swift)
3 App 0x65dd8 @objc Controller.handleNotification(Notification) -> () + 437720

Controller.swift: 92 is a call to a custom UIView subclass method that takes an optional date contained extracted from the notification. Any idea what the core callout would be due to? There are no weak or unknown values being used here. Once the notification observers are called it’s all internal to Controller.

One thing I just noticed is that the line at 3 is the selector for a different notification, which should lead down the path see from 2 onward. It’s redacted and so not easy to see, but in Controller, there’s no path that leads from handleNotification to handleFinish.

That backtrace does look strange. Even if there were some surprising call to swift_unknownWeakLoadStrong() in handleFinish(), there's no way that swift_unknownWeakLoadStrong() would call handleOtherNotification().

(swift_unknownWeakLoadStrong + 10 is the instruction after a call, assuming you're on 64-bit iOS simulator, but that call is to swift::isNativeSwiftWeakReference() which itself doesn't call anything.)

Also, in frame 3, the byte offset from the start of handleNotification() is larger than the address itself. And all of these addresses look too small if you're on 64-bit.

Where did this backtrace come from? Do you have a crash log as generated by the OS?

--
Greg Parker gparker@apple.com <mailto:gparker@apple.com> Runtime Wrangler