Two probable basing issues causing fork failures: (1) cygreadline7.dll has ASLR enabled, (2) default base address conflicts with ASLR-relocated/system DLLs

Corinna Vinschen
Mon Apr 23 14:52:00 GMT 2012

On Apr 23 14:23, James Johnston wrote:
> Perhaps I did not make it clear enough, but these issues still exist as far
> as I can tell.  I have clean Windows 7 and Windows XP virtual machines, and
> a clean install of Cygwin that was updated at the time I sent my original
> message.  Both issues I described still exist.  This is why I wrote the
> message.  If the issues weren't existing on an up-to-date Cygwin
> installation, I would not write to this mailing list and waste anyone's time
> - I am usually not that dumb! 
> Just this morning, I turned on my Cygwin installation in the Windows 7 VM.
> This time, cygreadline7.dll decided to relocate to 0x70030000 - different
> from the original location I mentioned in my original e-mail.  This DLL is
> not locating itself in a stable location.  And there are still system DLLs
> located very close to the Cygwin DLLs.
> If having Windows randomly rebase cygreadline7.dll in a child process via
> ASLR is not a problem, I'd simply be interested to know why.  I thought
> *any* Cygwin DLL relocating itself would cause fork to fail.

Yes, it is a problem in the first place if DLLs have the dynamicbase
flag set, because, obviously, it undermines what rebaseall is doing.
It's not a problem if the new address it gets rebased to doesn't collide
with any other used DLL since ASLR on Windows only shuffles ASLR-enabled
DLL addresses when a DLL is loaded by an application for the first time.
Afterwards, it will use the new address for that DLL until reboot.
So, yes, we should make sure that the ASLR flag is not used for Cygwin

Eric, could you create a new package which avoids setting the
dynamicbase flag for cygreadline and cyghistory?

In one of my installations there's also cygperl5_10.dll having the
dynamicbase flag set.  Reini, could you please have a look and make sure
it's not set?

In my installations there's no other DLL with this flag set.

As for the address space, we should stick to using the addresses below
0x70000000, top-down.  The reason is that we also need room for the
application heap.  On 32 bit systems the heap will be placed at 0x20000000
and in case it's too small it will be extended up to the start address
of the Cygwin DLL (minus 3 * 64K).


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

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