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Message-ID: <3BA80460.3010208@ece.gatech.edu>
Date: Tue, 18 Sep 2001 22:35:12 -0400
From: Charles Wilson <cwilson AT ece DOT gatech DOT edu>
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To: Robert Collins <robert DOT collins AT itdomain DOT com DOT au>
CC: cygwin AT cygwin DOT com
Subject: Re: [BUG] cygwin-1.3.3-2 -- making auto-import dlls
References: <EA18B9FA0FE4194AA2B4CDB91F73C0EF08F197 AT itdomain002 DOT itdomain DOT net DOT au>
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Robert Collins wrote:

> I went round this one with Paul's original patch.
> 
> Auto image base is bad (for cygwin).
> 
> Very bad.
> 
> Ok, now the scare is over, here is why:
> 
> Lemma: Cygwin1.dll is only relocatable if it is consistently relocated
> to the same address in _all_ process's that load cygwin1.dll. Proof:
> cygwin copies the process address space to fork, if references to
> cygwin1.dll are not valid in the hild, the child will die badly.
> 
> Auto image base will conflict with cygwin1.dll semi-randomly. Because
> other cygwin linked .dll's come before cygwin1.dll in the PE header
> (don't know if this is due to link order, sorting of the file or
> whathaveyou) cygwin1.dll is the last .dll. Thus when there is a
> conflict, cygwin1.dll gets relocated.
> 
> The problem: bash does not depend on these conflicting .dll's, so every
> process started that does depend on them will fail to initialise.
> 
> Solution: don't use auto-image-base. The overhead from relocation is
> quite low, why not let windows just do its thing.


Okay, suppose I have 5 dll's that are all based at the default, 
0x10000000. I start an exe that depends on those 5,  so at startup time 
they all get relocated to (random) address in my exe's address space. 
Then cygwin1.dll is loaded. But the random addresses chosen for the 
relocation for the first 5 dlls MAY or may NOT conflict with 
cygwin1.dll's *fixed* location.

Boom (sometimes).

How is this better than the current "boom (sometimes)" behavior?  What 
am I missing?

--Chuck




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