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

James Johnston JamesJ@motionview3d.com
Mon Apr 23 14:24:00 GMT 2012


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.  If having
Cygwin DLLs conflict with system DLLs in the address space is not a
potential problem, I'd also be interested to know why.  Because I am
observing both of these on an *up-to-date Cygwin installation* - not just
the Cygwin 1.7.9 setup.  According to the users guide at
http://cygwin.com/cygwin-ug-net/highlights.html#ov-hi-process - section
"Problems with process creation" - having Cygwin DLLs conflict with system
DLLs is a problem, and having cygreadline7.dll randomly based via ASLR is a
problem - so is my logic faulty in this case or not?

As far as I can tell, I'm up-to-date...  Setup says I have libreadline7
6.1.2-2 installed - the latest according to
http://cygwin.com/packages/libreadline7/.  My file size and date matches
what is shown at that URL.  Yet:

c:\cygwin\bin>dumpbin /headers cygreadline7.dll | more
<snip>
            8040 DLL characteristics
                   Dynamic base
                   Terminal Server Aware

Notice the "Dynamic base" flag set.

-----Original Message-----
Sent: Friday, April 20, 2012 20:50
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

On Fri, Apr 20, 2012 at 05:44:38PM -0000, James Johnston wrote:
>[snip]
>...  Today this issue came
>to a head on one installation of Cygwin 1.7.9,...
>[snip]
>Thoughts, anyone?

Yep.  Update your Cygwin installation.  You're four revisions out of date.

It should come not too great a surprise that we've had extensive discussions
about this issue and have made some changes in recent releases.

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