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Date:	Mon, 23 Apr 2012 11:44:31 -0400
From:	Christopher Faylor <cgf-use-the-mailinglist-please AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: Two probable basing issues causing fork failures: (1) cygreadline7.dll has ASLR enabled, (2) default base address conflicts with ASLR-relocated/system DLLs
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On Mon, Apr 23, 2012 at 04:51:06PM +0200, Corinna Vinschen wrote:
>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
>DLLs.

Is this something that rebase could turn off when it touches a DLL?

cgf

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