Yesterday, I listed the set of kernel mode to user mode callback entrypoints (as of Windows Server 2008). Although some of the callbacks share certain similarities in their modes of operation, there remain significant differences between each of them, in terms of both calling convention and what functionality they perform.
KiUserExceptionDispatcher is the routine responsible for calling the user mode portion of the SEH dispatcher. When an exception occurs, and it is an exception that would generate an SEH event, the kernel checks to see whether the exception occurred while running user mode code. If so, then the kernel alters the trap frame on the stack, such that when the kernel returns from the interrupt or exception, execution resumes at KiUserExceptionDispatcher instead of the instruction that raised the fault. The kernel also arranges for several parameters (a PCONTEXT and a PEXCEPTION_RECORD) that describe the state of the machine when the exception occurred to be passed to KiUserExceptionDispatcher upon the return to user mode. (This model of changing the return address for a return from kernel mode to user mode is a common idiom in the Windows kernel for several user mode event notification mechanisms.)
Once the kernel mode stack unwinds and control is transferred to KiUserExceptionDispatcher in user mode, the exception is processed locally via a call to RtlDispatchException, which is the core of the user mode exception dispatcher logic. If the exception was successfully dispatched (that is, an exception handler handled it), the final user mode context is realized with a call to RtlRestoreContext, which simply loads the registers in the given context into the processor’s architectural execution state.
Otherwise, the exception is “re-thrown” to kernel mode for last chance processing via a call to NtRaiseException. This gives the user mode debugger (if any) a final shot at handling the exception, before the kernel terminates the process. (The kernel internally provides the user mode debugger and kernel debugger a first chance shot at such exceptions before arranging for KiUserExceptionDispatcher to be run.)
I have posted a pseudo-C representation of KiUserExceptionDispatcher for the curious. Note that it uses a specialized custom calling convention, and by virtue of this, is actually an assembler function. However, a C representation is often easier to understand.
Not all user mode exceptions originate from kernel mode in this fashion; in many cases (such as with the RaiseException API), the exception dispatching process is originated entirely from user mode and KiUserExceptionDispatcher is not involved.
Incidentally, if one has been following along with some of the recent postings, the reason why the invalid parameter reporting mechanism of the Visual Studio 2005 CRT doesn’t properly break into into an already attached debugger should start to become clear now, given some additional knowledge of how the exception dispatching process works.
Because the VS2005 CRT simulates an exception by building a context and exception record, and passing these to UnhandledExceptionFilter, the normal exception dispatcher logic is not involved. This means that nobody makes a call to the NtRaiseException system service (as would normally be the case if UnhandledExceptionFilter were called as a part of normal exception dispatching), and thus there is no notification sent to the user mode debugger asking it to pre-process the simulated STATUS_INVALID_PARAMETER exception.
Update: Posted representation of KiUserExceptionDispatcher.
Next up: Taking a look at the user mode APC dispatcher (KiUserApcDispatcher).