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executing arbitrary code after a stack overflow has happened, the CLR permits code to probe for sufficient stack before beginning some operation A probe attempts to commit a predetermined amount of stack from the cur rent E S P, and, if it fails, the stack overflow occurs immediately Since this happens entirely before starting the critical operation, you have some assur ance that, so long as the critical code runs in under the probe size worth of stack, a stack overflow will not be triggered The code can still accidentally use more than was probed for, in which case all bets are off Also note that another thread in the system could trigger a stack overflow, leading to the process exiting, so this approach is still not foolproof This probing capability is exposed in a number of ways In its rawest form, you can make a call to the R u n t imeHe l pe r s P r o b e F o rSuff i c i e n t St a c k API, located in the System R u n t ime Comp i l e rS e rv i c e s name space It checks for a hard coded amount of stack space: 1 2 pages of stack (96KB on IA64, 48KB otherwise) For example:
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void C r it i c a l F u n c t ion ( ) { Runt imeHel p e r s P robe ForSuff i c ientSt a c k ( ) j I I We a r e g u a r a nteed 1 2 pages of s t a c k to u s e on t h i s t h read here }
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A call to this API is implicit with any constrained execution region (CER) in the CLR, which is denoted by a try-catch-finally block preceded by a call to R u nt imeH e l pe r s P r e p a reCon st r a i n ed Region s The R u nt imeH e l pe r s E xe c ut eCodeWi t h G u a ra nteedClea n u p API enables you to execute some arbitrary body code and, even if doing so causes a stack overflow, ensures that if the stack is unwound the cleanup code is called, for example in hosted situations like running inside of SQL Server The body code and cleanup code are both represented with delegates passed to the method Note that this does not hold in the unhosted case, because the CLR doesn't unwind the stack normally-it just issues a fail fast Finally, if you need more than 12 pages or would like to probe for a more precise amount, you can simulate this using C#'s stack allocation feature:
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u n safe stat i c void P robe ForSt a c k S p a c e ( int byt e s )
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The P r o b e F o r St a c kS p a c e method takes an integer byt e s representing the number of bytes to probe for and attempts to stack allocate that much data If it fails to do so, a stack overflow will be triggered We'll see later how to rewrite this function to return a bool instead of triggering overflow when there is insufficient space
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I nternal Data Structures (KTH READ ETH READ TEB) A thread's internal state is comprised mainly of three data structures, aside from its user- and kernel-mode stack: the kernel thread (KTHREAD), exec utive thread (ETHREAD), and thread environment block (TEB) You sel dom run into these in everyday programming, but knowing about them
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can come in handy during debugging and even when writing certain classes of programs In fact, the KTHREAD and ETHREAD are in the sys tem address space, not user-mode, and so the only structure you can access from user-mode is the TEB Many Win32 APIs are meant to manipulate fields of these structures without you needing to know that they even exist In this section, we'll briefly review these data structures at a high level, and see some of the debugging commands that allow you to access them The KTHREAD and ETHREAD structures contain a lot of information that is specific to thread management and execution on Windows, for example, thread priority, state, kernel-mode stack addresses, its wait list, owned mutexes, TLS array, and so on You can dump the contents of these data structures from WinDbg using the dt nt ' _kt h r e a d and dt nt ' _et h re a d commands We won' t delve too much into the details of each, since there's quite a bit, and most of it is irrelevant to user-mode (and, in most cases, even kernel-mode!) programming Please refer to Further Read ing, Russinovich and Solomon's Microsoft Windows Internals book for more details on these data structures Because the TEB is available to user-mode code, we'll review it in a bit more detail Related, there is a data structure called the thread information block (TIB) which offers additional information about a thread, but which is, like KTHREAD and ETHREAD only accessible to kernel-mode code The TEB contains things like a pointer to the exception chain, the stack addresses, a pointer to the process environment block (PEB), last error information (from Win32 API calls), and the number of C R I T ICAL_S ECTIONs owned by the thread, among other things
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