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

Eric Blake
Mon Apr 23 14:54:00 GMT 2012

On 04/23/2012 08:51 AM, Corinna Vinschen wrote:
>> 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.
> Eric, could you create a new package which avoids setting the
> dynamicbase flag for cygreadline and cyghistory?

At the time I created the current cygreadline package, cygwin didn't
have quite as good support for running rebaseall; since things have
improved on that front, I will see about getting a new release of the
readline package this week that disables the ASLR hack I had added way
back when.

Eric Blake    +1-919-301-3266
Libvirt virtualization library

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