Re: Cygwin dll initialization from win32 application

[Douglas Merrell <doug at merrell dot org>]

On Tue, 11 Mar 2003 15:34:07 -0500, Christopher Faylor wrote:
> On Tue, Mar 11, 2003 at 12:48:15PM -0500, Rich LeGrand wrote:
> > Hi all,
> > This has been discussed before, but I have been unable to find any
> > solutions to this problem.
> >
> > Basically I'm trying to call cygwin functions in the cygwin1.dll
> > from a Windows (msvc) application by calling LoadLibrary() and
> > GetProcAddress().
> >
> > I can see that the cygwin authors have made provision for this
> > (or what looks like a provision) with dll_dllcrt0 (a non-cygwin
> > initialization function.)  But when I call it, it crashes.
>
> This has been broken for some time.  So far no one has stepped
> forward to fix the problem in cygwin, so it has remained broken.

----------------------------------------------------------------
I ran into this same problem and investigated a bit. Windows
appears to be loading cygwin1.dll differently depending on whether
it is:
  1) linked into the application and loaded automatically by
      the Windows loader at runtime (i.e.- compiled with gcc)
or
  2) loaded dynamically by an application call to LoadLibrary
     (i.e.- compiled with MSVC++)

The only difference in the two loaded modules is one double
word at 0x610d2f4c in the .data section (0x610d2f4c memory
address, or 0x000d194c file offset in cygwin1.dll).

In case 1) the value at the location in memory is exactly as
it appears in the raw cygwin1.dll file: 0x00000000.

In case 2) the system appears to have changed the value at
this location during the LoadLibrary call to 0x00000001.

dll_dllcrt0 checks this hardcoded value (see below) as does
the lower level function dll_crt0_FP11per_process. If the
value is 0x00000000, everything works just fine. If the value
is anything other than 0x00000000 (i.e.- in the case where
the library is dynamically loaded using LoadLibrary), the
application crashes with an AccessViolation.

 ? What is this hardcoded value?

 ? Why is the Windows LoadLibrary function changing the value
   at this location?

Test program:
   int main(int argc, char *argv[])
   {
      HMODULE hModule = LoadLibrary("cygwin1.dll");

      /* Use the debugger to inspect the value a 0x610d2f4c */
      DebugBreak();
      /* If this program is linked with gcc, the
         Windows loader *will* automatically load
         cygwin1.dll, so the LoadLibrary call will
         just return the existing module instance.
         The value of the long word at 0x610d2f4c
         will be 0x00000000 as it should be. */
      /* If this program is linked with MSVC++, the
         Windows loader will *not* automatically load
         cygwin1.dll, so the LoadLibrary call will
         load it. LoadLibrary will change the value
         at 0x610d2f4c to 0x0000001.*/

      return 0;
   }

Code at entry point dll_dllcrt0:
6100A270 55                   push        ebp
6100A271 89 E5                mov         ebp,esp
6100A273 83 EC 18             sub         esp,18h
6100A276 89 5D FC             mov         dword ptr [ebp-4],ebx
6100A279 8B 5D 0C             mov         ebx,dword ptr [ebp+0Ch]
6100A27C 85 DB                test        ebx,ebx
6100A27E 0F 84 9C 00 00 00    je          6100A320
6100A284 8B 53 10             mov         edx,dword ptr [ebx+10h]
6100A287 A1 94 B4 0C 61       mov         eax,[610CB494]
6100A28C 89 02                mov         dword ptr [edx],eax
6100A28E A1 4C 2F 0D 61       mov         eax,[610D2F4C] <--*****
6100A293 85 C0                test        eax,eax

-Doug




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