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From:	"Dave Korn" <dave DOT korn AT artimi DOT com>
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Subject:	RE: cygwin bash crashes on Win Serv 2008
Date:	Thu, 23 Oct 2008 17:52:10 +0100
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Corinna Vinschen wrote on 23 October 2008 17:21:

>>   Only thing I can think of is "Not if %ss has been mucked around with it
>> isn't".
> 
> Yeah, I heard about that.  But what is %ss doing in Windows 

  Same as usual.  Pointing to the stack segment.  It just /happens/ that the
SS is a full 32-bit flat mapping of the same virtual memory space as %cs, %ds,
etc,. but it doesn't (in theory) have to be.

> and why should it be messed up with TS?!?  

  Extra security measures turned on by default on multi-user servers that are
turned off by default usually?

> And why are only Cygwin processes affected, and then only some?

  Just lucky, I guess...


>>   I'd use windbg on this, take a look at the exception record and
>> selectors and stuff.
> 
> The exception record was a good hint (I don't know what you mean by
> "selectors", sorry).  

  See the "dg" command, e.g. "dg %cs", "dg %ds", "dg %ss".

> Unfortunately it puzzles me even more:
> 
>     ExceptionAddress: 00419d97 (image00400000+0x00019d97)
>        ExceptionCode: c0000005 (Access violation)
>       ExceptionFlags: 00000000
>     NumberParameters: 2
>        Parameter[0]: 00000008
>        Parameter[1]: 00419d97
>     Attempt to execute non-executable address 00419d97
> 
> Huh?  Why should this address (this application function) be
> "non-executable", while it's executable when TS is not installed?

  DEP?  ASLR?  SafeSEH?  As well as "dg" there are some other commands in
windbg that'll show you memory types and attributes.

> Could this have something to do with the executbale header gcc creates?

  Dunno - which executable header?  Seems unlikely since we're in a completely
different memory page and well beyond the header area into the .text segment.

  (Actually, are we in the .text segment, or is there a thunk of some kind in
.rdata?  And is the difference perhaps related to the use-or-not, or the
need-for-or-not, of ld's --{en,dis}able-runtime-pseudo-reloc options?)

    cheers,
      DaveK
-- 
Can't think of a witty .sigline today....


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