Re: Add support for Windows 8, first step

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

Hi Teemu,


I move the discussion over to the cygwin-developers list.


On Oct 12 23:44, Teemu NÃtkinniemi wrote:
> Hello Corinna,
> 
> On 12.10.2011 11:26, Corinna Vinschen wrote:
> 
> >First of all, we need a copyright assignment from you before we can
> >accept non-trivial patches to Cygwin, see http://cygwin.com/contrib.html,
> >the "Before you get started" section.
> 
> I am going to do that just in case I actually get something working
> (see below).

Cool.

> >Windows 8 will very likely support the FAST_CWD stuff, the problem is
> >just to find out how to find the global pointer pointing to the current
> >FAST_CWD structure, and then, if the FAST_CWD structure changed.
> 
> It does do that, the code looks almost the same as in Windows 7.
> There are some differences that I haven't figured out yet. But I
> might actually wait for a beta version of Windows 8 before going
> forward.

Your original mail got me thinking, so I installed the Windows 8 preview
in qemu/kvm, reverted the system to the good old start menu(*), and
installed Cygwin.

First I inspected the FAST_CWD datastructure, and from what I can see,
it resembles closely the *old* structure used before KB 2393802 on Vista
and W7:

  typedef struct _FAST_CWD_8 {
    LONG           ReferenceCount;
    HANDLE         DirectoryHandle;
    ULONG          OldDismountCount;
    UNICODE_STRING Path;
    LONG           FSCharacteristics;
    WCHAR          Buffer[MAX_PATH];
  } FAST_CWD_8, *PFAST_CWD_8;

See?  The only difference is apparently the addition of the
FSCharacteristics member from the current FAST_CWD structure on
Vista/W7.

That makes me a bit suspicious.  Why on earth should the structure
layout be changed back without need?  Maybe it has something to do
with an optimized structure layout on 64 bit?!?

Anyway, I will check in a patch, today or tomorrow, which adds code
to use the new structure on W8.

> >Therefore I don't want to disable this message.  If you're interested
> >to get rid of it, it would be most helpful trying to track down how to
> >find the global FAST_CWD pointer in W8.
> 
> If I understood the code correctly f_cwd_ptr is the location of
> ntdll!RtlpCurDirRef and find_fast_cwd_pointer tries to find that
> location.

Right, that's the idea since RtlpCurDirRef is not exported from ntdll,
unfortunately.

> For some reason I couldn't get breakpoints working when
> debugging cygwin1.dll so I did some disassembling and found the
> correct location for ntdll!RtlpCurDirRef in Windows 8 version of
> ntdll (wow64).
> 
> Any hints for debugging the Cygwin dll itself as find_fast_cwd runs
> once per session so it has been very difficult trying to get a
> working breakpoint.

That's easy.  Start a GDB session for, say /bin/ls.  Before starting the
process, set a breakpoint on 'cwdstuff::override_win32_cwd'.  Then `run'
to the breakpoint.  The only reason that find_fast_cwd() isn't called is
the fact that the global fast_cwd_ptr pointer is not -1.  So just set it
to -1 and then you can step through find_fast_cwd() and
find_fast_cwd_pointer().


Corinna


(*) I really hope Microsoft doesn't remove the old start menu entirely
    from the OS.  The new GUI which is supposed to replace the start
    menu is only useful for tablets and playing, but not for serious
    work, IMO.

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat