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Date:	Sun, 28 Feb 2021 20:16:56 -0800 (PST)
X-X-Sender:	jeremyd AT resin DOT csoft DOT net
To:	cygwin AT cygwin DOT com
Subject:	Re: cygwin + binutils 2.36 + ASLR/dynamicbase defaults
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From:	Jeremy Drake via Cygwin <cygwin AT cygwin DOT com>
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Cc:	ASSI <Stromeko AT nexgo DOT de>
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On Sun, 28 Feb 2021, ASSI wrote:

> > Is this still problematic for cygwin?
>
> Yes it is and I'm currently figuring out how to best get rid of it in
> order to be able to update binutils (why this was ever allowed in
> without an accompanying configure option is a mystery to me).

Well, Microsoft's LINK.EXE and LLVM's LLD have already been using these
new defaults for some time.  But I was surprised how quickly my patch was
accepted/merged.

> I've
> already nixed it for Cygwin, but I'm not yet sure what to do for the
> cross compilation toolchain.  While it should in principle work there,
> I'm pretty sure that there will be problems when it comes to the nitty
> gritty details.  It's already transpired that some of the linker scripts
> can't deal with the larger base addresses this change does generate
> eventually.

To clarify, default base addresses should not have changed for cygwin
targets, they were already above 4GB.

I have a prelimiary patch that I plan to send upstream once I get some
testing done on it, which reverts the default dll characteristics for
cygwin targets.  I don't know if what you've done to 'nix it' for Cygwin
was similar.

I have not seen anything one way or the other on the NXCOMPAT flag.  Does
that also needs to be reverted for Cygwin?

> > The reason I'm asking is because we updated to 2.36 in MSYS2 and are
> > wondering if we need to patch this out (or change the defaults) It
> > seems to work as is right now, but maybe we are just lucky(?).
>
> You are just lucky and need to test more. :-)

I have seen the issues you described on 32-bit, but my understanding of
how ASLR works suggested that it should be very rare on 64-bit.

> Note that the change does not only affect DLL as the commit message
> would want you to believe and you will eventually end up with a
> situation where ASLR tries moves the stack of an executable, at which
> point you can no longer fork.

... but now that you mention the stack moving, yes, I could see that
being an issue.

Yes, the specific field where these flags are stored is called "DLL
Characteristics" so that is how it was referred to, but they do not
exclusively apply to DLLs.

https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#dll-characteristics
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