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dispatching process and dispatches the exception according to the flag Bit 0 of nt!NTGlobalFlag controls exception-dispatching behavior and is named StopOnException (soe) When the StopOnException flag is set, all exceptions encountered on a process, not attached to a user mode debugger, are first dispatched to the kernel debugger attached to the target system When the flag is not set, the kernel mode debugger does not interfere with exception-dispatching code, unless the exception has special debugging meanings, such as STATUS_BREAPOINT and STATUS_ SINGLE_STEP The best option to use for decoding the flags is the !gflag extension command, which deciphers the contents of nt!NTGlobalFlag, as shown in Listing 320 Listing 320
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kd> dc nt!NtGlobalFlag l1 80540aec 00000001 kd> !gflag Current NtGlobalFlag contents: 0x00000001 soe - Stop On Exception
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This flag, just as all other kernel flags, can be changed from the debugger console The flags can also be changed using the gflagsexe utility installed with Debugging Tools for Windows Listing 321 shows an example of temporary or permanently enabling the StopOnException flag using gflagsexe Listing 321 Changing kernel flags using command line gflagsexe
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c:\> gflags -k +soe Current Running Kernel Settings are: 00000000 soe - Stop On Exception c:\> gflags -r +soe Current Boot Registry Settings are: 00000001 soe - Stop On Exception
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However, for a better interactive experience, the user can start gflagsexe without a parameter and change the kernel flags in the graphical user interface, as shown in Figure 32
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Figure 32 Changing kernel flags using GUI gflagsexe Regardless of how the StopOnException flag is changed, the exception behavior is affected in the same way The next section focuses on the steps taken by the kernel to dispatch an exception, taking into consideration the StopOnException flag as well The logic used to dispatch a user mode exception is described in the following Figure 33 presents this logic in a flow chart format Dispatching a user mode exception can be summarized as follows: 1 When a new exception is raised, the Windows kernel tries to dispatch the exception to the user mode debugger if available If available, the exceptiondispatching flow continues from step 6 When a kernel debugger is attached to the host, the exception dispatching flow continues in step 2; otherwise, it continues from step 4 2 Exceptions that have meaning for the debugger, such as STATUS_ BREAKPOINT or STATUS_SINGLE_STEP, are sent as debugger notification to the kernel debugger When the StopOnException flag is set, all other exceptions are also sent as debugger notifications to the kernel debugger; otherwise, the exception-dispatching flow continues in step 4 The system is frozen, waiting for a reply to the kernel debugger notification
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3 The kernel debugger examines the exception and, depending on the debugger settings, it can handle the exception In this case, the exception is dismissed, and the code execution continues from the exception location when the kernel debugger replies to the debugger notification For unhandled exceptions, the dispatching flow continues from step 4 4 The Windows kernel searches for an exception handler by evaluating all functions from the call stacks for the presence of a frame-based exception handler Exception handler filters found in this phase are called, starting with the most recent function from the stack, until one filter returns EXCEPTION_ EXECUTE_HANDLER Starting with Windows XP and Windows Server 2003, the developer can register additional filters to be called prior to starting the search process using a vectored exception handler mechanism With the exception handler found earlier, the kernel starts to roll back the execution stack to the function owning the handler, executing all the final handlers registered within the functions traversed a process called stack unwinding Finally, the code execution continues with the exception handler in the target function 5 What if the current thread stack contains no handler capable of handling the current exception Each thread guards the procedure code with a built-in filter and handler designed to handle all exceptions not handled by user-provided code This filter, generically called the unhandled exception filter, takes the necessary steps to terminate the process by calling the kernel32!UnhandledExceptionFilter API when an exception is not handled The logic used by unhandled exception filters is described in 13, Postmortem Debugging 6 When a user mode debugger is attached to the process, it receives the exception notification, and it can handle it or not based on the debugger settings (See the previous section Controlling Exceptions and Events from the Debugger regarding exception handling settings) This notification is referred to in the debugger documentation as first chance exception Handling of exceptions unhandled by the debugger continues by searching an exception handler for the exception and unwinding the stack when this is available, as in the process described in step 4 Exceptions handled by the user mode debugger, such as STATUS_BREAKPOINT, continue by executing the code from the location that generated the exception after any adjustment is made by the debugger 7 If the debugger does not handle the exception and no handler is found in step 6, the Windows kernel makes a second attempt to have the exception handled by the debugger, a notification process known as second chance exception If the exception is still not handled by the debugger, the process simply restarts the sequence from step 6 until the exception is handled
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