eCrash: Debugging without Core Dumps
Embedded Linux does a good job of bridging the gap between embedded programming and high-level UNIX programming. It has a full TCP stack, good debugging tools and great library support. This makes for a feature-rich development environment. There is one downside, however. How can you debug problems that occur out in the field?
On full-featured operating systems, it's easy to use a core dump to debug a problem that occurs in the field.
On non-embedded UNIX systems, when a program encounters an exception, it outputs all of its current state to a file on the filesystem. This file is usually called core. This core file contains all the memory the program was using at the time the failure occurred. This allows for post-mortem investigation to diagnose the exception.
Typically, on embedded Linux systems, there is no (or very little) persistent disk storage. On all of the systems on which I have worked, there is more RAM than persistent storage. So, getting a core dump is impossible. This article describes some alternatives to core dumps that will allow you to perform post-mortem debugging.
Programs can fail for reasons other than exceptions. Programs can deadlock, or they can have run-away threads that use up all system resources (memory, CPU or other fixed resources). It also would be beneficial to generate some kind of persistent crash file under these situations.
So, first we need to come up with the information we want to save. Because of memory constraints, saving all of the process' memory is not an option. If it were, you simply could use core dumps! But, there is other very useful information we can save. At the top of the list is the backtrace of the failed thread.
A backtrace is a list of the functions that were called to get to the current position in the program. Even with the absence of system memory and data, a backtrace can shed light onto what was happening at the time of failure.
Many embedded systems also have logs: lists of errors, warnings and metrics to let you know what happened. Having a post-mortem dump of the last few logs before failure is an invaluable asset in finding the root cause of a failure.
In complex, multithreaded systems, you usually have many mutexes. It could be useful, in the case of a deadlock, to show the state of all the processes' mutexes and semaphores.
Showing memory usage statistics also could help diagnose the problem.
Once we have determined the information we want to save, we still need to come up with where to save it. This will vary greatly from system to system. If your system has no persistent storage at all, perhaps you can output the crash information to a serial terminal or display it on an LCD readout. (We have serious space constraints there!) If your system has CompactFlash, you can save it to a filesystem. Or, if it has raw Flash (an MTD device), you can either save it to a jffs2 filesystem, or maybe to a raw sector or two.
If the crash was not too severe, perhaps the crash could be uploaded to a tftp server or sent to a remote syslog facility.
Now that we have a firm grasp on what we want to save, and locations to which we can save it, let's talk about how we are going to do it!
In general, getting a backtrace is not as simple as it sounds. Accessing system registers (like the stack pointer) varies from architecture to architecture. Thankfully, the FSF comes to our rescue in GNU's C Standard Library (see the on-line Resources). Libc has three functions that will aid us in retrieving backtraces: backtrace(), backtrace_symbols() and backtrace_symbols_fd().
The backtrace() function populates an array of pointers with a backtrace of the current thread. This, in general, is enough information for debugging, but it is not very pretty.
The backtrace_symbols() function takes the information populated by backtrace() and returns symbolic names (function names). The only problem with backtrace_symbols is that it is not async-signal safe. backtrace_symbols() uses malloc(). Because malloc() uses spinlocks, it is not safe to be called from a signal handler (it could cause a deadlock).
The backtrace_symbols_fd() function attempts to solve the signal issues associated with malloc and output the symbolic information directly to a file descriptor.
Some functions inside of libc rely on signals themselves: some IO operations, memory allocation and so on. So, we are very limited in what we should do inside of a handler. In our case, we can cheat a little. Because our program already is crashing, a deadlock is not that big of a concern. The code in my examples makes use of several not-allowed functions, such as fwrite(), printf() and sprintf(). But, we can work to avoid some of the functions that are prone to deadlock, such as malloc() and backtrace_symbols().
In my opinion, the biggest loss we have is the loss of backtrace_symbols. But, here is where things get easier. You always can implement your own symbol table and look up the functions from the pointers themselves.
In my examples, I sometimes use backtrace_symbols(). I have not seen a deadlock yet, but it is possible.
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