[PATCH] Cygwin: Fixing the math behind rounding down ch.stacklimit to page size

[Tim Adye]

Hi Corinna,

On 29 May, 18:44, Corinna Vinschen wrote:
> Hi Sergejs,
>
> On May 25 17:43, Sergejs Lukanihins wrote:
> > Hello,
> > 
> > Looks like ch.stacklimit wasn't being page-aligned correctly in
> > fork.cc; you need to subtract 1 from page_size to do it correctly (see
> > the attached patch).
> > 
> > As a result, this was causing stack-overflow exceptions whenever the
> > stack needed to grow beyond the stacklimit value. When the stack grows
> > beyond stacklimit value, Windows uses ntdll!_chkstk() function to
> > check the stack and map in additional stack pages. However, it expects
> > stacklimit to be page aligned, and the function does not work
> > correctly if it is not (it triggers STATUS_STACK_OVERFLOW, even if
> > there is enough stack space).
> > 
> > Normally, this was not causing any issues, as the stack never really
> > needs to grow, but it was causing issues when AV software was being
> > injected into the process (specifically, HitmanPro.Alert being
> > injected into git’s sh.exe process). Due to function hooks, it lead to
> > a bigger callstack, and more stack space being required. Making the
> > change specified in the patch actually resolves the issue.
> > 
> > I am providing my patches to the Cygwin sources under the 2-clause BSD license.
>
> Good catch!  Patch pushed.
Thanks for adding this patch, and thanks to Sergejs for providing it!

I think I may have encountered this problem (fork failures for some 
constructs in bash, only when HitmanPro is enabled). Would it be 
possible to make a new cygwin1.dll snapshot so I can test that? I tested 
the current x86_64 snapshot, which was produced just hours before this 
patch. That still exhibits the problem, but seems to work fine otherwise.

Thanks,
Tim.