From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: Cygwin bash's built-in test command crashes on Windows 2008 Server
 64bit under KVM
Date: Sat, 14 Feb 2009 00:06:26 +0200
Message-ID: <4995EEE2.5090600@redhat.com>
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To: Matteo Frigo <athena@fftw.org>
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Matteo Frigo wrote:
> Matteo Frigo <athena@fftw.org> writes:
>
>   
>> Avi Kivity <avi@redhat.com> writes:
>>
>>     
>>> Can you run the slightly modified gs.c (attached) and rerun on AMD?
>>> The is to see if the runtime somehow restores gs.
>>>       
>> Crashes as follows:
>>
>> w2k3-64:~$ ./a.exe 
>> gs: 2b
>> gs:0x30: 7efdb000
>> Segmentation fault (core dumped)
>>     
>
> A little bit more information:
>
> w2k3-64:~$ gdb a.exe
> GNU gdb 6.8.0.20080328-cvs (cygwin-special)
> Copyright (C) 2008 Free Software Foundation, Inc.
> License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
> This is free software: you are free to change and redistribute it.
> There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
> and "show warranty" for details.
> This GDB was configured as "i686-pc-cygwin"...
> (no debugging symbols found)
> (gdb) r
> Starting program: /home/athena/a.exe 
> [New thread 1620.0x6dc]
> Error: dll starting at 0x77d40000 not found.
> Error: dll starting at 0x77d40000 not found.
> Error: dll starting at 0x77c20000 not found.
> Error while mapping shared library sections:
> /cygdrive/c/WINDOWS/SysWOW64/ntdll32.dll: No such file or directory.
> (no debugging symbols found)
> (no debugging symbols found)
> (no debugging symbols found)
> (no debugging symbols found)
> (no debugging symbols found)
> [New thread 1620.0x74c]
> (no debugging symbols found)
> (no debugging symbols found)
> (no debugging symbols found)
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x0040109d in main ()
> (gdb) x/i $pc
> 0x40109d <main+77>:	mov    %gs:0x30,%esi
> (gdb) p/x $gs
> $1 = 0x2b
> (gdb) 
>   

Okay, at least this makes some little bit of sense.  On both Intel and 
AMD, 'mov gs' clobbers gs.base as expected.  On AMD, something further 
down the line (some syscall likely) restores gs.base, but on Intel it 
doesn't.  When we avoid the syscall, we get a crash on AMD as well.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.