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 15:59:00 GMT 2012

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.


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

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