I'm using Atollic TrueSTUDIO for ARM 5.0.0 Lite for debugging an STM32F3 application via the SWD debug interface. The application receives data via interrupts from a USART.
When I "step over" a relatively long function, the application doesn't pause, i.e. the program does not reach the line after the call. When I then manually pause the application, I find it to be at the entry of the USART ISR, so I concluded that the execution was paused, even though Atollic's debugger didn't recognize it.
The bigger problem is that the same happens when I simply resume: I can't run my application with the debugger attached, as every byte on the USART pauses it.
Is my analysis of the situation correct? Is this the expected behavior, and is there a way to work around it? Non-Atollic specific answers are also very welcome!
To be honest, I couldn't form a clear picture in my mind of what's really going on, but here's a possibility: you're not clearing the proper flags using the USART_ClearITPendingBit() function call from the standard peripheral library, or its equivalent in terms of direct register access. If you don't clear the proper bits, as soon as you return from the ISR, the hardware executes it again, so it looks like you're in an infinite loop inside the ISR.
Related
I've been trying to find the difference between these 2 types of debugging, but couldn't find it anywhere (been googling almost 30 minutes), so I'm asking here: What's the difference between live vs. offline debugging? What do people mean when they say a debugger is "live" vs. "offline"?
Debugging types
There are several ways of debugging that can be distinguished:
live debugging vs. post mortem debugging (what you call "offline" debugging, also called "dump debugging")
kernel debugging vs. user mode debugging
local debugging vs. remote debugging
which give 8 combinations in total.
For live debugging, you can distinguish between invasive debugging vs. noninvasive debugging.
Live debugging vs. offline debugging
In live debugging, the program is running and the debugger is attached to it. This means you can still interact with the program. You can set breakpoints, handle exceptions that would normally cause the program to terminate, modify the memory etc.
The downside of live debugging is its temporal/fluent nature. If you enter a wrong command or step too far, the situation is gone and might not be repeatable.
I mentioned that there are 2 sub-modes for live debugging: invasive and noninvasive debugging: in noninvasive debugging, the debugger does not attach to the target application. It suspends all of the program's threads and has access to the memory, registers, and other such information. However, the debugger cannot control the target.
In post mortem debugging, someone has captured a memory dump of a running program at a certain point in time. In many cases this is done upon a specific event, e.g. an unhandled exception that causes the program to terminate. Since the memory dump is a file on disk, you can analyze it as often as you want and you get the exact same situation.
The downside if post mortem debugging is, of course, that the program is not running, you can't interact with it and it's very hard to find out what happens next.
"Online" debugging is the normal process:
Tell the debugger to tell the program to step forwards;
Look at what the program state is at the moment;
Set a breakpoint for the future;
Tell the debugger to simply run the program;
If the breakpoint 'fires', have a look at the program state now.
There are two ways to "offline" debug:
You can take your source code and manually step through what the processor ought to be doing, watching for unexpected program paths.
Note if you do this, you need to diligently not "know" what the processor is "supposed" to do and just do that: you need to honestly obey the code as though you were the computer. Often you get other people, who don't know the code, to do this instead of you.
You take the result of a run-log, usually captured by a hardware probe, and use the debugger to "post mortem" the run.
The latter usually requires a processor that will transmit what it is doing out a "Trace" port (not all have this), and a hardware device (like a probe) connected to the Trace port to capture the data. That probe then communicates with a debugger, which takes the data and presents it to the programmer. The programmer can work backwards and forwards through this Trace log, and see the execution path that the code actually took, rather than the code the programmer thought it should take.
Some processors not only transmit what instruction they're currently processing, but also what data they read or wrote while doing this. A more sophisticated debugger can take this extra data and provide a 'snapshot' of the system at any time during the run, allowing the programmer to analyse why the code behaved the way it did.
The reason that it is called "offline" is because once the log has been captured, you can disconnect and power down the target, and look at the saved log at any time in the future without still being connected to the probe or processor.
I am programming a pci device with verilog and also writing its driver,
I have probably inserted some bug in the hardware design and when i load the driver with insmod the kernel just gets stuck and doesnt respond. Now Im trying to figure out what's the last driver code line that makes my computer stuck. I have inserted printk in all relevant functions like probe and init but non of them get printed.
What other code is running when i use insmod before it gets to my init function? (I guess the kernel gets stuck over there)
printks are often not useful debugging such a problem. They are buffered sufficiently that you won't see them in time if the system hangs shortly after printk is called.
It is far more productive to selectively comment out sections of your driver and by process of elimination determine which line is the (first) problem.
Begin by commenting out the entire module's init section leaving only return 0;. Build it and load it. Does it hang? Reboot system, reenable the next few lines (class_create()?) and repeat.
From what you are telling, it is looks like that Linux scheduler is deadlocking by your driver. That's mean that interrupts from the system timer doesn't arrive or have a chance to be handled by kernel. There are two possible reasons:
You hang somewhere in your driver interrupt handler (handler starts its work but never finish it).
Your device creates interrupts storm (Device generates interrupts too frequently as a result your system do the only job -- handling of your device interrupts).
You explicitly disable all interrupts in your driver but doesn't reenable them.
In all other cases system will either crash, either oops or panic with all appropriate outputs or tolerate potential misbehavior of your device.
I guess that printk won't work for such extreme scenario as hang in kernel mode. It is quite heavy weight and due to this unreliable diagnostic tool for scenarios like your.
This trick works only in simpler environments like bootloaders or more simple kernels where system runs in default low-end video mode and there is no need to sync access to the video memory. In such systems tracing via debugging output to the display via direct writing to the video memory can be great and in many times the only tool that can be used for debugging purposes. Linux is not the case.
What techniques can be recommended from the software debugging point of view:
Try to review you driver code devoting special attention to interrupt handler and places where you disable/enable interrupts for synchronization.
Commenting out of all driver logic with gradual uncommenting can help a lot with localization of the issue.
You can try to use remote kernel debugging of your driver. I advice to try to use virtual machine for that purposes, but I'm not aware about do they allow to pass the PCI device in the virtual machine.
You can try the trick with in-memory tracing. The idea is to preallocate the memory chunk with well known virtual and physical addresses and zeroes it. Then modify your driver to write the trace data in this chunk using its virtual address. (For example, assign an unique integer value to each event that you want to trace and write '1' into the appropriate index of bytes array in the preallocated memory cell). Then when your system will hang you can simply force full memory dump generation and then analyze the memory layout packed in the dump using physical address of the memory chunk with traces. I had used this technique with VmWare Workstation VM on Windows. When the system had hanged I just pause a VM instance and looked to the appropriate .vmem file that contains raw memory latout of the physical memory of the VM instance. Not sure that this trick will work easy or even will work at all on Linux, but I would try it.
Finally, you can try to trace the messages on the PCI bus, but I'm not an expert in this field and not sure do it can help in your case or not.
In general kernel debugging is a quite tricky task, where a lot of tricks in use and all they works only for a specific set of cases. :(
I would put a logic analyzer on the bus lines (on FPGA you could use chipscope or similar). You'll then be able to tell which access is in cause (and fix the hardware). It will be useful anyway in order to debug or analyze future issues.
Another way would be to use the kernel crash dump utility which saved me some headaches in the past. But depending your Linux distribution requires installing (available by default in RH). See http://people.redhat.com/anderson/crash_whitepaper/
There isn't really anything that is run before your init. Bus enumeration is done at boot, if that goes by without a hitch the earliest cause for freezing should be something in your driver init AFAIK.
You should be able to see printks as they are printed, they aren't buffered and should not get lost. That's applicable only in situations where you can directly see kernel output, such as on the text console or over a serial line. If there is some other application in the way, like displaying the kernel logs in a terminal in X11 or over ssh, it may not have a chance to read and display the logs before the computer freezes.
If for some other reasons the printks still do not work for you, you can instead have your init function return early. Just test and move the return to later in the init until you find the point where it crashes.
It's hard to say what is causing your freezes, but interrupts is one of those things I would look at first. Make sure the device really doesn't signal interrupts until the driver enables them (that includes clearing interrupt enables on system reset) and enable them in the driver only after all handlers are registered (also, clear interrupt status before enabling interrupts).
Second thing to look at would be bus master transfers, same thing applies: Make sure the device doesn't do anything until it's asked to and let the driver make sure that no busmaster transfers are active before enabling busmastering at the device level.
The fact that the kernel gets stuck as soon as you install your driver module makes me wonder if any other driver (built in to kernel?) is already driving the device. I made this mistake once which is why i am asking. I'd look for the string "kernel driver in use" in the output of 'lspci' before installing the module. In any case, your printk's should be visible in dmesg output.
in addition to Claudio's suggestion, couple more debug ideas:
1. try kgdb (https://www.kernel.org/doc/htmldocs/kgdb/EnableKGDB.html)
2. use JTAG interfaces to connect to debug tools (these i think vary between devices, vendors so you'll have to figure out which debug tools you need to the particular hardware)
I'm just getting started learning FreeScale DSCs (MC56F800x series). I've done some work with AVRs using both AVR Studio on Windows and Eclipse and avr-gcc on Linux. CodeWarrior is just not as intuitive.
Right now I'm stuck trying to debug a simple program. I start the debugger using the built-in simulator, but it never reaches the first line of main(). Instead it seems to get stuck in some initialization code (MC56F8006_init.asm), specifically this line:
;; Loop until OCCS_STAT[LCK0] = 1
wait_for_lock:
brclr #OCCS_STAT_LCK0,x:>OCCS_STAT,wait_for_lock
I've let it run for quite a while and it never gets past this. It's obviously waiting for something, but what? You would think the simulator would just work... argh. Maybe there's some options I can change to make it pass this step?
I'm going to keep digging and will post an answer here if I find it first.
Updates:
Here's what I've found:
OCCS
On Chip Clock Synthesis
brclr
Branch if Bits Clear
The instruction loops until OCCS_STAT LCK0 is set. This register means the on-chip oscillator's PLL has locked (waits for clock stabilization).
I'm still not sure why the simulator spins forever on this line, and how I can solve this without resorting to hacking up the init code (which is part of the code library and not within my project).
I am not familiar with the part or the simulator, but it seems likely that the simulator is instruction-set-only and does not simulate the PLL hardware.
In most embedded development systems, the run-time startup code is provided as source and you could modify it (or rather make a local copy in your project and assemble and link that to override the default start-up). Alternately you could simply place a breakpoint in this loop, and advance the program-counter register to get it out of the loop. In many debuggers it is possible to attach a script to a breakpoint to do this automatically.
Just curious here: is it possible to invoke a Windows Blue Screen of Death using .net managed code under Windows XP/Vista? And if it is possible, what could the example code be?
Just for the record, this is not for any malicious purpose, I am just wondering what kind of code it would take to actually kill the operating system as specified.
The keyboard thing is probably a good option, but if you need to do it by code, continue reading...
You don't really need anything to barf, per se, all you need to do is find the KeBugCheck(Ex) function and invoke that.
http://msdn.microsoft.com/en-us/library/ms801640.aspx
http://msdn.microsoft.com/en-us/library/ms801645.aspx
For manually initiated crashes, you want to used 0xE2 (MANUALLY_INITIATED_CRASH) or 0xDEADDEAD (MANUALLY_INITIATED_CRASH1) as the bug check code. They are reserved explicitly for that use.
However, finding the function may prove to be a bit tricky. The Windows DDK may help (check Ntddk.h) - I don't have it available at the moment, and I can't seem to find decisive info right now - I think it's in ntoskrnl.exe or ntkrnlpa.exe, but I'm not sure, and don't currently have the tools to verify it.
You might find it easier to just write a simple C++ app or something that calls the function, and then just running that.
Mind you, I'm assuming that Windows doesn't block you from accessing the function from user-space (.NET might have some special provisions). I have not tested it myself.
I do not know if it really works and I am sure you need Admin rights, but you could set the CrashOnCtrlScroll Registry Key and then use a SendKeys to send CTRL+Scroll Lock+Scroll Lock.
But I believe that this HAS to come from the Keyboard Driver, so I guess a simple SendKeys is not good enough and you would either need to somehow hook into the Keyboard Driver (sounds really messy) or check of that CrashDump has an API that can be called with P/Invoke.
http://support.microsoft.com/kb/244139
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\i8042prt\Parameters
Name: CrashOnCtrlScroll
Data Type: REG_DWORD
Value: 1
Restart
I would have to say no. You'd have to p/invoke and interact with a driver or other code that lives in kernel space. .NET code lives far removed from this area, although there has been some talk about managed drivers in future versions of Windows. Just wait a few more years and you can crash away just like our unmanaged friends.
As far as I know a real BSOD requires failure in kernel mode code. Vista still has BSOD's but they're less frequent because the new driver model has less drivers in kernel mode. Any user-mode failures will just result in your application being killed.
You can't run managed code in kernel mode. So if you want to BSOD you need to use PInvoke. But even this is quite difficult. You need to do some really fancy PInvokes to get something in kernel mode to barf.
But among the thousands of SO users there is probably someone who has done this :-)
You could use OSR Online's tool that triggers a kernel crash. I've never tried it myself but I imagine you could just run it via the standard .net Process class:
http://www.osronline.com/article.cfm?article=153
I once managed to generate a BSOD on Windows XP using System.Net.Sockets in .NET 1.1 irresponsibly. I could repeat it fairly regularly, but unfortunately that was a couple of years ago and I don't remember exactly how I triggered it, or have the source code around anymore.
Try live videoinput using directshow in directx8 or directx9, most of the calls go to kernel mode video drivers. I succeded in lots of blue screens when running a callback procedure from live videocaptureing source, particulary if your callback takes a long time, can halt the entire Kernel driver.
It's possible for managed code to cause a bugcheck when it has access to faulty kernel drivers. However, it would be the kernel driver that directly causes the BSOD (for example, uffe's DirectShow BSODs, Terence Lewis's socket BSODs, or BSODs seen when using BitTorrent with certain network adapters).
Direct user-mode access to privileged low-level resources may cause a bugcheck (for example, scribbling on Device\PhysicalMemory, if it doesn't corrupt your hard disk first; Vista doesn't allow user-mode access to physical memory).
If you just want a dump file, Mendelt's suggestion of using WinDbg is a much better idea than exploiting a bug in a kernel driver. Unfortunately, the .dump command is not supported for local kernel debugging, so you would need a second PC connected over serial or 1394, or a VM connected over a virtual serial port. LiveKd may be a single-PC option, if you don't need the state of the memory dump to be completely self-consistent.
This one doesn't need any kernel-mode drivers, just a SeDebugPrivilege. You can set your process critical by NtSetInformationProcess, or RtlSetProcessIsCritical and just kill your process. You will see same bugcheck code as you kill csrss.exe, because you set same "critical" flag on your process.
Unfortunately, I know how to do this as a .NET service on our server was causing a blue screen. (Note: Windows Server 2008 R2, not XP/Vista).
I could hardly believe a .NET program was the culprit, but it was. Furthermore, I've just replicated the BSOD in a virtual machine.
The offending code, causes a 0x00000f4:
string name = string.Empty; // This is the cause of the problem, should check for IsNullOrWhiteSpace
foreach (Process process in Process.GetProcesses().Where(p => p.ProcessName.StartsWith(name, StringComparison.OrdinalIgnoreCase)))
{
Check.Logging.Write("FindAndKillProcess THIS SHOULD BLUE SCREEN " + process.ProcessName);
process.Kill();
r = true;
}
If anyone's wondering why I'd want to replicate the blue screen, it's nothing malicious. I've modified our logging class to take an argument telling it to write direct to disk as the actions prior to the BSOD weren't appearing in the log despite .Flush() being called. I replicated the server crash to test the logging change. The VM duly crashed but the logging worked.
EDIT: Killing csrss.exe appears to be what causes the blue screen. As per comments, this is likely happening in kernel code.
I found that if you run taskkill /F /IM svchost.exe as an Administrator, it tries to kill just about every service host at once.
I keep wondering how does a debugger work? Particulary the one that can be 'attached' to already running executable. I understand that compiler translates code to machine language, but then how does debugger 'know' what it is being attached to?
The details of how a debugger works will depend on what you are debugging, and what the OS is. For native debugging on Windows you can find some details on MSDN: Win32 Debugging API.
The user tells the debugger which process to attach to, either by name or by process ID. If it is a name then the debugger will look up the process ID, and initiate the debug session via a system call; under Windows this would be DebugActiveProcess.
Once attached, the debugger will enter an event loop much like for any UI, but instead of events coming from the windowing system, the OS will generate events based on what happens in the process being debugged – for example an exception occurring. See WaitForDebugEvent.
The debugger is able to read and write the target process' virtual memory, and even adjust its register values through APIs provided by the OS. See the list of debugging functions for Windows.
The debugger is able to use information from symbol files to translate from addresses to variable names and locations in the source code. The symbol file information is a separate set of APIs and isn't a core part of the OS as such. On Windows this is through the Debug Interface Access SDK.
If you are debugging a managed environment (.NET, Java, etc.) the process will typically look similar, but the details are different, as the virtual machine environment provides the debug API rather than the underlying OS.
As I understand it:
For software breakpoints on x86, the debugger replaces the first byte of the instruction with CC (int3). This is done with WriteProcessMemory on Windows. When the CPU gets to that instruction, and executes the int3, this causes the CPU to generate a debug exception. The OS receives this interrupt, realizes the process is being debugged, and notifies the debugger process that the breakpoint was hit.
After the breakpoint is hit and the process is stopped, the debugger looks in its list of breakpoints, and replaces the CC with the byte that was there originally. The debugger sets TF, the Trap Flag in EFLAGS (by modifying the CONTEXT), and continues the process. The Trap Flag causes the CPU to automatically generate a single-step exception (INT 1) on the next instruction.
When the process being debugged stops the next time, the debugger again replaces the first byte of the breakpoint instruction with CC, and the process continues.
I'm not sure if this is exactly how it's implemented by all debuggers, but I've written a Win32 program that manages to debug itself using this mechanism. Completely useless, but educational.
In Linux, debugging a process begins with the ptrace(2) system call. This article has a great tutorial on how to use ptrace to implement some simple debugging constructs.
If you're on a Windows OS, a great resource for this would be "Debugging Applications for Microsoft .NET and Microsoft Windows" by John Robbins:
http://www.amazon.com/dp/0735615365
(or even the older edition: "Debugging Applications")
The book has has a chapter on how a debugger works that includes code for a couple of simple (but working) debuggers.
Since I'm not familiar with details of Unix/Linux debugging, this stuff may not apply at all to other OS's. But I'd guess that as an introduction to a very complex subject the concepts - if not the details and APIs - should 'port' to most any OS.
I think there are two main questions to answer here:
1. How the debugger knows that an exception occurred?
When an exception occurs in a process that’s being debugged, the debugger gets notified by the OS before any user exception handlers defined in the target process are given a chance to respond to the exception. If the debugger chooses not to handle this (first-chance) exception notification, the exception dispatching sequence proceeds further and the target thread is then given a chance to handle the exception if it wants to do so. If the SEH exception is not handled by the target process, the debugger is then sent another debug event, called a second-chance notification, to inform it that an unhandled exception occurred in the target process. Source
2. How the debugger knows how to stop on a breakpoint?
The simplified answer is: When you put a break-point into the program, the debugger replaces your code at that point with a int3 instruction which is a software interrupt. As an effect the program is suspended and the debugger is called.
Another valuable source to understand debugging is Intel CPU manual (Intel® 64 and IA-32 Architectures
Software Developer’s Manual). In the volume 3A, chapter 16, it introduced the hardware support of debugging, such as special exceptions and hardware debugging registers. Following is from that chapter:
T (trap) flag, TSS — Generates a debug exception (#DB) when an attempt is
made to switch to a task with the T flag set in its TSS.
I am not sure whether Window or Linux use this flag or not, but it is very interesting to read that chapter.
Hope this helps someone.
My understanding is that when you compile an application or DLL file, whatever it compiles to contains symbols representing the functions and the variables.
When you have a debug build, these symbols are far more detailed than when it's a release build, thus allowing the debugger to give you more information. When you attach the debugger to a process, it looks at which functions are currently being accessed and resolves all the available debugging symbols from here (since it knows what the internals of the compiled file looks like, it can acertain what might be in the memory, with contents of ints, floats, strings, etc.). Like the first poster said, this information and how these symbols work greatly depends on the environment and the language.