What it for

Catch crashes in Java & Native(arm/x86), and dump crash information to a file located in sdcard, crash information includes call stack, system version, architecture, and registers state for native crash, etc.

Structure of this project

This is an Android project created via Android Studio, the module 'crashsdk' is an aar library used by the demo app;

The c++ part in the crashsdk is not configured in gradle, so you need build it manually, go to jni directory and type 'ndk-build' to build the target native libs.

Implementation

1. For crash in Java, we use Thread#setDefaultUncaughtExceptionHandler to handle it;

2. For crash in Native, we need a signal handler to get a chance to handle signals such as SIGSEGV; In the handle, we unwind the call stack and dump infomation we interest. Since we are doing these in a signal handler, we should take signal safety into consideration.

Unwind Native Call Stack

When gcc generates code that handles exceptions, it produces tables that describe how to unwind the stack. These tables are found in the .eh_frame section; Read https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Embedded/LSB-Embedded/ehframechpt.html for details of .eh_frame format. The definition of Call Frame Instructions in FDE can be found in DWARF(Debugging With Attributed Record Formats) specification , you can find it here: http://dwarfstd.org

The process of unwinding can be describe as below:

1. Get IP(instruction pointer) where the program crashed(by parsing the third parameter of sa_handler callback);
2. Find the target library in which the crash address located;
3. Parse .eh_frame segment in the library and find the corresponding FDE, restore this call frame with instructions described in CIE&FDE;
4. After restoring call frame we can find out the return address and CFA(call frame address) of previous call frame;
5. With the return address and CFA we can unwind the previous call frame and repeat it until reach the root call frame ( (where the CFA&IP no more change);

In step 1, we may get a wrong crash IP if the crash is caused by wrong instruction pointer. In this case we need try to unwind this frame by calling convention;

Unfortunately, .eh_frame section not always exist, for ARM, a section .ARM.exidx exist and it is similar to .eh_frame. So, if unwind with eh_frame failed we can try .ARM.exidx; For more information of EXIDX , look here: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0038b/IHI0038B_ehabi.pdf