[Boost-users] Boost.Context stack growing inwards

Subject: [Boost-users] Boost.Context stack growing inwards
From: Thomas Ziegenhagen (thomas_at_[hidden])
Date: 2016-02-29 09:10:21

Hi there,

I get regular crashes when using Boost.Context after just a few calls to  
make_fcontext(). I built a minimal example as follows:

----
#include <iostream>
#include <vector>
#include <boost/context/all.hpp>
struct Context
{
   boost::context::fcontext_t c_outside, c_inside;
};
Context gContext = { nullptr, nullptr };
void DoIt( intptr_t /*pt*/)
{
   uint8_t test[64];
   memset( test, 0xaf, sizeof( test));
   std::cout << "DoIt(), Point 1" << std::endl;
   boost::context::jump_fcontext( &gContext.c_inside, gContext.c_outside,  
0, true);
   std::cout << "DoIt(), Point 2" << std::endl;
   boost::context::jump_fcontext( &gContext.c_inside, gContext.c_outside,  
0, true);
   std::cout << "DoIt(), Point 3" << std::endl;
   boost::context::jump_fcontext( &gContext.c_inside, gContext.c_outside,  
0, true);
}
int main()
{
   const size_t StackSize = 16384;
   std::vector<uint8_t> stack( StackSize, 0xf1);
   gContext.c_inside = boost::context::make_fcontext( stack.data() + 256,  
stack.size() - 512, DoIt);
   std::cout << "main(), Point 1" << std::endl;
   boost::context::jump_fcontext( &gContext.c_outside, gContext.c_inside,  
0, true);
   std::cout << "main(), Point 2" << std::endl;
   boost::context::jump_fcontext( &gContext.c_outside, gContext.c_inside,  
0, true);
   std::cout << "main(), Point 3" << std::endl;
   boost::context::jump_fcontext( &gContext.c_outside, gContext.c_inside,  
0, true);
   std::cout << "main(), Point 4" << std::endl;
   return 0;
}
------
This notable thing is that I provide a stack with additional 256 bytes of  
safety borders. And it turns out: the provided memory is overwritten way  
beyond the beginning. The actual designated area is not touched at all,  
but instead the area before the beginning is overwritten. The only thing I  
found in the documentation is this:
"sp Member: Pointer to the beginning of the stack (depending of the  
architecture the stack grows downwards or upwards)."
which hints that on some system the stack might grow to negative  
addresses. But up to now I thought that boost.context knows which  
direction is correct, and adjusts the given pointer accordingly. But  
apparently it doesn't. Or the environment underwent a drastic change. Here  
it is Windows 10 64bit, Visual Studio 2015.
For readers remembering my last mails: I have verified this also with an  
"classic" boost build using this command line:
b2.exe -a --reconfigure variant=debug,release link=static  
runtime-link=static --with-context
So I'm pretty sure it's not a build issue.
Question: is this a bug? Probably not, I'm surely not the first to use  
Boost.Context, so I wonder what the correct way is to handle this issue.  
So what do I do about this?
Thanks in advance
Bye, Thomas