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executed again As a result, the system again raises the exception as a firstchance exception, and the cycle continues until the condition disappears This cycle can be seen in action by starting the faulty code under the debugger and instructing it to just notify the user about access violation exceptions instead of waiting for user input:
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c:\>windbgexe -g -G -xn av C:\AWDBIN\WinXPx86chk\02sampleexe
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Unhandled-Breakpoint Exception (STATUS_BREAKPOINT Exception)
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As seen at the beginning of this chapter, this STATUS_BREAKPOINT exception has special meaning for the debugger, and the system behavior is changed slightly when compared to the access-violation exception
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Normal configuration The system exhibits the same behavior as with an access-violation exception Any int 3 processor instruction (executed from within the DebugBreak() or assert() statement) is perceived by the system and user as any other exception Contrary to what we see in the debugger, the code execution does not continue immediately after the int 3 statement KD configuration Because the exception is characteristic of the debugging process, the kernel debugger stops and handles this exception Upon continuation, the execution resumes from the instruction following the int 3 statement KD with SOE configuration Because the STATUS_BREAKPOINT exception is already handled by the kernel mode debugger, the StopOnException flag does not add further changes UM configuration The debugger stops at the breakpoint instruction and handles the exception Upon continuation, the execution resumes from the instruction following the int 3 statement
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Handled Access-Violation Exception The code used in this case is similar to
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what we used to test unhandled-access violations, except that it provides a framebased exception handler for the exception
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Normal configuration As expected, the exception is handled, and the code continues normally after the handler is executed
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KD configuration As expected, the exception is handled, and the code continues normally, without kernel mode notification KD with SOE configuration In this configuration, the exception-handling mechanism forwards the exception to the kernel mode debugger and waits for a continuation disposition Upon continuation (after the g command), the exception is handled in the user mode code, which continues normally UM configuration The debugger stops at the first-chance exception notification according to the debugger default exception-handling settings Upon continuation, the exception handler is handling the exception, and the process execution continues normally
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Handled-Breakpoint Exception What is different when the exception is a debugging-specific exception, such as the STATUS_BREAKPOINT exception or the STATUS_SINGLE_STEP exception All debuggers try to understand and handle such exceptions
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Normal configuration As expected, the exception is handled and the code continues normally KD configuration Because the exception is specially used in debugging, the kernel debugger stops and handles this exception KD with SOE configuration In this configuration, the exception-handling code forwards the exception to the kernel mode debugger and waits for a disposition of it Upon continuation (after the g command), the execution resumes from the instruction following the int 3 statement and the process finishes normally UM configuration The debugger stops at the first-chance exception notification according to the debugger default exception-handling settings Upon continuation, the execution resumes from the instruction following the int 3 statement and the process finishes normally
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After testing all such configurations using different exception codes, several interesting conclusions can be drawn and used in day-to-day work, as follows
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By default, any unhandled exception generates, using Windows Error Reporting (WER), a crash report that can be used for postmortem debugging The customers can centralize such reports at the enterprise level using the Microsoft Corporate Error Reporting or the newer Agentless Exception Monitoring server The customer can also have them uploaded to the WER site to be investigated by Microsoft developers or by the participating software vendors 13 describes how independent software vendors can participate in analyzing WER reports and provide solutions to the commonly reported problems Although users of any software solution don t have a pleasant experience when encountering unhandled exceptions, from the developer perspective, these exceptions provide the necessary feedback loop required to fix all software flaws present in the applications The alternative technique of hiding all exceptions by handling them, irrespective of the types or source, so the user doesn t see them, creates long-term reliability problems that are hard to diagnose and sometimes are never fixed, as there is no visible impact on users In the development and testing phases, the kernel debugger is a very powerful tool and should be used to monitor a percentage of the systems used in product testing if it does not conflict with the application Distributed applications propagating errors from one process to another are usually difficult to debug since the source of the original error is not known in advance If the error was initially an exception raised on any constituent process, it is easy to stop the system execution in that spot using the KD with SOE configuration and the appropriate sx command in the kernel debugger Good developers are usually asserting the state of the process by using various assert techniques Unfortunately, most of the asserts are disabled in the released version of the product, the most likely target of the testing phase, and one big opportunity to make sure that the code works as expected is wasted Really important asserts can be replaced with code that raises a breakpoint and handle intermediately This breakpoint causes the code to stop in the debugger if present or continues the execution with a small performance hit (as the condition asserted should always be true)
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Knowing how the exception is handled by the system in various configurations enables developers to understand why the code stopped where it stopped Developers can use this knowledge to define the error-handling strategy for their product, to rely on an unhandled exception filter to collect crash data, or to handle few exceptions by themselves and collect some information from the process In the development phase, the code can be instrumented and the testing environment can