id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
8544,Calling managed DLL from within boost::context may cause a crash,vitaly.blinov@…,olli,"Only Windows platform is affected.

If the code running in the context (coroutine) invokes anything that involves crossing clr.dll (mscorwks.dll) boundary, a crash occurs with about 50% probability.

My investigation showed that call stack of the problem is consistent with clr.dll!Thread::InitThread throwing OutOfMemory exception. With some deep debugging I narrowed the problem down to CommitThreadStack function inside the clr.dll. That method accesses a dword located at FS:[0xE0C] (this is called ""deallocaton stack"" on [http://en.wikipedia.org/wiki/Win32_Thread_Information_Block TIB wiki page]) and compares it with current top of the stack (FS:[0x4]). It appears that exception is thrown if FS:[0xE0C] value is greater than FS:[0x4] (or, perhaps, FS:[0x8]).
That variable is not very well documented, but I believe the pair FS:[0xE0C] - FS:[0x4] defines the maximum stack size. On windows 7, the difference between these is always 0x100000, which gives stack size of 1M. Interestingly, that always the case even if the fiber or thread were created with smaller stack size.

jump_context never touches that variable. As a result, the value in FS:[0xE0C] is defined by the calling thread, and therefore it contains arbitrary value. If it is greater than current top of stack, problem occurs.

clr.dll!CommitThreadStack also appears to be accessing FS:[0xF78], but it's purpose and whether the value stored in it affects the behavior is unknown.

My current workaround of writing current bottom of the stack to FS:[0xE0C] prior to calling managed DLL appears to be working:

(MS VS specific)
{{{#!cpp
DWORD store = __readfsdword(0xE0C);
__writefsdword(0xE0C, __readfsdword(0x8));
call_managed_dll();
__writefsdword(0xE0C, store);
}}}


This bug is very obscure and so far I only managed to observe it on Windows7 and Windows server 2008. 

Suggested fix: Store and restore FS:[0xE0C] in jump_context.",Bugs,closed,To Be Determined,context,Boost 1.53.0,Problem,fixed,"context, coroutine",