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Date: Mon, 23 Apr 2012 12:20:37 -0400
From: Ken Brown <kbrown AT cornell DOT edu>
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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 4/23/2012 11:58 AM, Corinna Vinschen wrote:
> On Apr 23 11:44, Christopher Faylor wrote:
>> 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?
>
> In theory that's the job of peflags, not of rebase.  And somebody could
> want the ASLR flag to be set on certain DLLs.  But probably we can safely
> assume that the Cygwin distro DLLs should not have set the dynamicbase
> flag and the rebaseall script could call rebase with an extra flag which
> automatically removes the dynamicbase flag from all rebased DLLs.

Maybe it would also be a good idea to modify peflagsall so that by 
default it removes the dynamicbase flag rather than setting it.

Ken


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