Difference between revisions of "Debugging"

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== Generating a backtrace ==
 
== Generating a backtrace ==
Before a backtrace can be initiated, you will first need to have broken into the kernel debugger. See the section [http://www.reactos.org/wiki/index.php/Debugging#Forcing_a_bugcheck forcing a bugcheck] for directions on how to do this.
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Before a backtrace can be initiated, you will first need to have broken into the kernel debugger. See the section [[#Forcing_a_bugcheck|forcing a bugcheck]] for directions on how to do this.
  
 
Once you are in the kernel debugger, the 'kdb:>'  symbol will appear in your debug output. This is a prompt to recieve commands for the debugger.
 
Once you are in the kernel debugger, the 'kdb:>'  symbol will appear in your debug output. This is a prompt to recieve commands for the debugger.

Revision as of 12:43, 18 January 2008

This page describes different methods of debugging ReactOS and the steps necessary to debug ReactOS.

Introduction

To be able to help ReactOS development, whether this be participating in the development of the source code or taking part in crucial testing, you are going to need knowledge of how to generate useful debug logs.

Useful debug logs are essential pieces of information which the developer needs to quickly pinpoint and identify exactly what the operating system is doing. Many people know how to get default debug output from the operating system, but this is generally not particularly useful for locating problems, esspecially bugs.

This article aims to give users knowledge not only on how to generate a debug log, but on how to generate a useful debug log which can be used directly to asses what the operating system is doing.


Available debugging methods

There are various methods to debug ReactOS, some require more knowledge than others. These are listed below.

COM1

<add detail>

Physical serial cable

<add detail>

Debug Output to file

<add detail>

Debug Output to screen

<add detail>

Virtual machines

<add detail>

Named pipe

<add detail>

Riderect to file

<add detail>

kdbg

<add detail>

gdb

<add detail>

Knowing what debug output to generate

<add detail>

Turning on debugging in key modules

<add detail>

Turning on at compile time

<add detail>

Turning on at runtime

<add detail>

Forcing a bugcheck

Bug checks (or system crashes as they are also known) occur when the operating system can no longer operate safely and to avoid corrupting data, it halts operation. This will normally throw up a Blue Screen Of Death, but if you have the kernel debugger activated, it will drop you into a gdb prompt giving you access to explore the system.

There are two reasons for forcing a bugchecks, each reason employing a different method:

Dynamic

If you want to halt the system for any given reason and break into the the kernel debugger, you can force a bugcheck from the keyboard.

If you are not using a DBG build, you first need to add the following value in the registry:

Path:    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\i8042prt
Keyname: CrashOnCtrlScroll
Type:    REG_DWORD
Value:   1

Then, when ReactOS is running, hold down the right CTRL key, and press SCROLL LOCK key twice. A crash can as well be initiated using TAB+B keys while running ReactOS without the need to add anything to the registry.


Static

This is useful when you want to halt the operating system when it hits a particular area of code. This is esspecially useful as you can get an immediate back trace to see where the code flow came before the bugcheck was forced.

For people who are seeing a particular bug, and who have the ability to follow the source code leading up to this, a forced bugcheck can be interted into the code.

This is done in kernel mode by calling the KeBugCheck or KeBugCheckEx, but it is more easily forced by adding: ASSERT(FALSE); into the code.

Again, if you have the kernel debugger activated, you will be dropped into a gdb session whereby you can issue the 'bt' command to get a back trace of the code flow previous to this bugcheck. This is esspecially useful to developers as it helps to pinpoint the buildup to the bugcheck and this can be followed in the source to gain a better understanding of what is happening.


Generating a backtrace

Before a backtrace can be initiated, you will first need to have broken into the kernel debugger. See the section forcing a bugcheck for directions on how to do this.

Once you are in the kernel debugger, the 'kdb:>' symbol will appear in your debug output. This is a prompt to recieve commands for the debugger.

If you are using a virtual machine, ensure that the VM still has keyboard focus as this is the machine in which kdb is running.

Enter 'bt' and you should see the letters appear in your debug output. If so, hit <enter> and you should see something similar to the following:

(drivers\filesystems\vfat\rw.c:809) <\ReactOS\system32\kernel32.dll>
Entered debugger on embedded INT3 at 0x0008:0x800935f2.
kdb:> bt
Eip:
<ntoskrnl.exe:935f3 (lib\rtl\i386\debug_asm.S:31 (DbgBreakPoint@0))>
Frames:
<vfatfs.sys:97de (drivers/filesystems/vfat/misc.c:111 (VfatDispatchRequest))>
<vfatfs.sys:9b25 (drivers/filesystems/vfat/misc.c:167 (VfatBuildRequest))>
<ntoskrnl.exe:3ab23 (ntoskrnl/io/iomgr/irp.c:1088 (IofCallDriver))>
<ntoskrnl.exe:36206 (ntoskrnl/io/iomgr/iofunc.c:686 (IoSynchronousPageWrite))>
<ntoskrnl.exe:59daa (ntoskrnl/mm/section.c:6330 (MmspWriteDataSectionPages))>
<ntoskrnl.exe:244c6 (ntoskrnl/ex/work.c:162 (ExpWorkerThreadEntryPoint))>
<ntoskrnl.exe:70e90 (ntoskrnl/ps/thread.c:134 (PspSystemThreadStartup))>
<ntoskrnl.exe:7b142 (ntoskrnl\ke\i386\ctxswitch.S:258 (KiThreadStartup@156))>
kdb:> 

Examining this in more detail, we can see that the bugcheck occured via the INT3 operation which then dropped us into kdb. The next line shows us Eip which is the instruction pointer, and this points to the last address before the system halted.

Following on from that are the frames. This is the important part of generating our backtrace and it contains all the function addresses in the buildup to the bugcheck. This is the crutial information developers need to understand the codeflow before the bugcheck.

















Debugging preparation

One of the first things you need to decide is how to get the debugging output:

Options for debugging reactos: