Structured Exception Handling in .NET

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Structured Exception Handling
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An exception is a special condition in a program that makes it immediately jump to a special function called an exception handler. The exception handler then decides how to deal with the exception and can either correct the problem and make the program continue from the same code position or resume execution from another position. An exception handler can also decide to terminate the program if the exception cannot be resolved. There are two basic types of exceptions: hardware exceptions and software exceptions. Hardware exceptions are exceptions generated by the processor, for example when a program accesses an invalid memory page (a page fault) or when a division by zero occurs. A software exception is generated when a program explicitly generates an exception in order to report an error. In C++ for example, an exception can be raised using the throw keyword, which is a commonly used technique for propagating error conditions (as an alternative to returning error codes in function return values). In Windows, the throw keyword is implemented using the RaiseException Win32 API, which goes down into the kernel and follows a similar code path as a hardware exception, eventually returning to user mode to notify the program of the exception. Structured exception handling means that the operating system provides mechanisms for distributing exceptions to applications in an organized manner. Each thread is assigned an exception-handler list, which is a list of routines that can deal with exceptions when they occur. When an exception occurs, the operating system calls each of the registered handlers and the handlers can decide whether they would like to handle the exception or whether the system should keep on looking.
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The exception handler list is stored in the thread information block (TIB) data structure, which is available from user mode and contains the following fields:
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_NT_TIB: +0x000 +0x004 +0x008 +0x00c +0x010 +0x010 +0x014 +0x018 ExceptionList : 0x0012fecc StackBase : 0x00130000 StackLimit : 0x0012e000 SubSystemTib : (null) FiberData : 0x00001e00 Version : 0x1e00 ArbitraryUserPointer : (null) Self : 0x7ffde000
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The TIB is stored in a regular private-allocation user-mode memory. We already know that a single process can have multiple threads, but all threads see the same memory; they all share the same address space. This means that each process can have multiple TIB data structures. How does a thread find its own TIB in runtime On IA-32 processors, Windows uses the FS segment register as a pointer to the currently active thread-specific data structures. The current thread s TIB is always available at FS:[0]. The ExceptionList member is the one of interest; it is the head of the current thread s exception handler list. When an exception is generated, the processor calls the registered handler from the IDT. Let s take a page-fault exception as an example. When an invalid memory address is accessed (an invalid memory address is one that doesn t have a valid page-table entry), the processor generates a page-fault interrupt (interrupt #14), and invokes the interrupt handler from entry 14 at the IDT. In Windows, this entry usually points to the KiTrap0E function in the Windows kernel. KiTrap0E decides which type of page fault has occurred and dispatches it properly. For user-mode page faults that aren t resolved by the memory manager (such as faults caused by an application accessing an invalid memory address), Windows calls into a user-mode exception dispatcher routine called KiUserExceptionDispatcher in NTDLL.DLL. KiUserExceptionDispatcher calls into RtlDispatchException, which is responsible for going through the linked list at ExceptionList and looking for an exception handler that can deal with the exception. The linked list is essentially a chain of _EXCEPTION_REGISTRATION_RECORD data structures, which are defined as follows:
_EXCEPTION_REGISTRATION_RECORD: +0x000 Next : Ptr32 _EXCEPTION_REGISTRATION_RECORD +0x004 Handler : Ptr32
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