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


Before I describe the issues I think I've found, I'd like to clarify my
understanding.  In the past, I have periodically run into random issues when
starting new programs from the bash shell.  The errors are always random and
cryptic ("bad address", etc. - that sort of thing).  Today this issue came
to a head on one installation of Cygwin 1.7.9, where simply opening the bash
shell could not happen without a large number of these errors, due to
commands in the /etc/profile failing.  Having been reading on this mailing
list, my understanding is that this is an issue with basing of DLLs - is
that correct?  Specifically, that Cygwin requires that a child process
created by fork() to have its Cygwin DLLs loaded into the child process's
address space in the exact same places as they are loaded in the parent
process, correct?  And it is also correct that Cygwin does not care where
Windows / operating system DLLs are loaded, provided that they do not
conflict with Cygwin DLLs or any heap/stack allocated by the Cygwin parent

I assume the answer to all these questions is yes.  Please correct me if I
am wrong!  So, I then decided to analyze the address space of bash.exe to
see if I could determine the source of the problem, using VMMap at to see where each
DLL is loaded.  (ListDLLs, another SysInternals tool is handy, too).  I have
checked it against a clean Cygwin installation with default packages
installed, which I just installed today in a clean virtual machine.  The
issues still exist in that virtual machine, so I don't feel it is something
specific to my particular configuration.

In both cases, rebaseall was run as part of setup.

An analysis of bash.exe in VMMap on a clean Windows 7 computer shows the
following PE images were loaded in bash's address space (I have cut out most
operating system DLLs at the end of address space):

 * 0x00040000: C:\Windows\System32\apisetschema.dll (ASLR)
 * 0x00400000: C:\cygwin\bin\bash.exe
 * 0x61000000: C:\cygwin\bin\cygwin1.dll
 * 0x6F4A0000: C:\Windows\SysWOW64\winrnr.dll (ASLR)
 * 0x6F500000: C:\cygwin\bin\cygncursesw-10.dll
 * 0x6F710000: C:\cygwin\bin\cygreadline7.dll (ASLR)
 * 0x6F740000: C:\Windows\SysWOW64\pnrpnsp.dll (ASLR)
 * 0x6F760000: C:\Windows\SysWOW64\NapiNSP.dll (ASLR)
 * 0x6F780000: C:\cygwin\bin\cygintl-8.dll
 * 0x6F7E0000: C:\cygwin\bin\cygiconv-2.dll
 * 0x6FA80000: C:\cygwin\bin\cyggcc_s-1.dll
 * 0x73720000: C:\Windows\System32\wow64cpu.dll (ASLR)
 * .... rest of system DLLs ....

Here is the same thing, but on a Windows XP computer (remember, XP does not
support ASLR):

 * 0x00400000: C:\cygwin\bin\bash.exe
 * 0x61000000: C:\cygwin\bin\cygwin1.dll
 * 0x629C0000: C:\WINDOWS\system32\lpk.dll
 * 0x6FB70000: C:\cygwin\bin\cygreadline7.dll
 * 0x6FC20000: C:\cygwin\bin\cygncursesw-10.dll
 * 0x6FDA0000: C:\cygwin\bin\cygintl-8.dll
 * 0x6FDB0000: C:\cygwin\bin\cygiconv-2.dll
 * 0x6FF90000: C:\cygwin\bin\cyggcc_s-1.dll
 * 0x71A50000: C:\WINDOWS\system32\mswsock.dll
 * .... rest of system DLLs ....

Now, right off I see two problems with the Cygwin DLLs; here they are with
some possible solutions:

1.  cygreadline7.dll was compiled with /DYNAMICBASE, as indicated by the
ASLR flag (you can confirm via Visual C++ dumpbin command).  See for more.  What's
really important to note, is that by default this is turned ON in newer
Visual C++ linkers.  I'm not sure why this flag was set, but perhaps the
package maintainer forgot to turn it off?  Anyway, the DYNAMICBASE flag
means that the operating system is free to randomly rebase the DLL anywhere
it feels like: a sure recipe for disaster if fork() requires the DLL to be
loaded in the same place every time!!  In practice this is not usually an
issue because the operating system only rebases once so that the image
memory pages can still be reused across multiple processes.  But I think
that isn't set in stone: it's free to base it in one place for one process,
and then base it somewhere else for a different process (I have observed
this on the problem computer where bash couldn't even run /etc/profile).

It seems to me that the cygreadline7 maintainer needs to be sure this flag
is turned off.  And maybe there are other EXE or DLL files in Cygwin that
have this flag turned on?  I haven't checked.  In that case, perhaps the
rebaseall command could also turn the DYNAMICBASE flag off while it is doing
the rebase?  Additionally, rebaseall could operate on EXE files too, just to
turn off the DYNAMICBASE flag.  That way there would be no concern about
package maintainers who forget to turn off this flag.

2.  Cygwin DLL basing seems to start at 0x61000000, and continues until
almost 0x70000000.  This is too close to the upper end of address space: it
has a high risk of collision with operating system DLLs and ASLR-relocated
images.  In my opinion, starting the rebase at an address like 0x30000000 or
0x40000000 would be a lot safer.  You can see that some system DLLs are
being placed in the same general address range that Cygwin is using.  It's
just by luck there wasn't a collision!  Are there good reasons for not
rebasing at an earlier address?  In general, I have observed ASLR-relocated
DLLs as early as between 0x50000000 and 0x60000000 on my main development
computer, which has a lot of stuff on it.  And also several Windows XP
system DLLs (i.e. no ASLR) being placed in the 0x60000000 range as well
(with an example being shown above).

Thoughts, anyone?

Best regards,

James Johnston

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