Date: Thu, 16 Jul 2009 17:18:23 -0400
From: Christopher Faylor <cgf-use-the-mailinglist-please AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: perl threads on 2008 R2 64bit = crash ( was: perl 5.10 threads  on 1.5.25 = instant crash )
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On Thu, Jul 16, 2009 at 09:55:52PM +0200, Corinna Vinschen wrote:
>On Jul 16 17:47, Dave Korn wrote:
>> Corinna Vinschen wrote:
>> 
>> > And that's what I get in the Perl testcase:
>> > 
>> >   (gdb) x/xw 0x7efdd000
>> >   0x7efdd000:     0x0088ce68
>> >   (gdb) x/2xw 0x0088ce68
>> >   0x88ce68:       0x0088400c      0x6103ce20   <-- Cygwin exception handler
>> >   (gdb) x/2xw 0x0088400c
>> >   0x88400c:       0x00000000      0x00000001   <-- Huh?
>> > 
>> > This looks wrong, doesn't it?  The question is now, how and why does
>> > that happen?
>> 
>> > Where's the 0x00000000 pointer coming from on 2008?  Is it possible that
>> > the OS overwrote the entry because it appears to be an address in Perl's
>> > stack, so it's a potential security theat?
>> 
>>    The addresses are in the wrong order; SEH registration records should
>> always nest in the same way as stack call frames, i.e. unwinding toward
>> ascending memory addresses, but the second record is at a lower address than
>> the first, so the chain has been mangled somehow.  Perhaps that breaks an
>> integrity check in the kernel?  Where actually is $esp at the time; is the
>> bogus one in an already-released frame below $esp?
>
>Seems so.  $esp is 0x88c8c0.
>
>>   You might want to try again with a watchpoint:
>> 
>> watch *(unsigned int*)0x88ce68
>> 
>> ... and see how and where that head entry gets set up and whether it
>> subsequently gets overwritten somehow.
>
>That was really helpful, Dave.  Thank you!
>
>Here's the result:
>
>(gdb) br pthread_attr_init
>Breakpoint 2 at 0x610f42dc: file /home/corinna/src/cygwin/vanilla/winsup/cygwin/thread.cc, line 1909.
>(gdb) watch *(unsigned int*)0x88ce68
>Hardware watchpoint 3: *(unsigned int *) 8965736
>(gdb) c
>Continuing.
>Hardware watchpoint 3: *(unsigned int *) 8965736
>
>Old value = 8978372
>New value = 8929292
>_cygtls::init_exception_handler (this=0x88ce64,
>    eh=0x6103ce20 <_cygtls::handle_exceptions(_EXCEPTION_RECORD*, _exception_lis
>t*, _CONTEXT*, void*)>)
>    at /home/corinna/src/cygwin/vanilla/winsup/cygwin/cygtls.cc:244
>244       _except_list = &el;
>Current language:  auto; currently c++
>(gdb) p/x 8978372
>$1 = 0x88ffc4
>(gdb) p/x 8929292
>$2 = 0x88400c
>(gdb) p $esp
>$3 = (void *) 0x883e78
>(gdb) bt
>#0  _cygtls::init_exception_handler (this=0x88ce64,
>    eh=0x6103ce20 <_cygtls::handle_exceptions(_EXCEPTION_RECORD*, _exception_lis
>t*, _CONTEXT*, void*)>)
>    at /home/corinna/src/cygwin/vanilla/winsup/cygwin/cygtls.cc:244
>#1  0x61033ff5 in dll_dllcrt0_1 (x=0x883edc)
>    at /home/corinna/src/cygwin/vanilla/winsup/cygwin/dll_init.cc:321
>#2  0x6103414f in dll_dllcrt0 (h=0x6eb70000, p=0x6eb79070)
>    at /home/corinna/src/cygwin/vanilla/winsup/cygwin/dll_init.cc:302
>#3  0x6eb77acf in _cygwin_dll_entry AT 12 ()
>   from /usr/lib/perl5/5.10/i686-cygwin/auto/threads/threads.dll
>^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
>So this exception handler is installed as part of the Perl threads DLL
>initialization.  But appanrelty the address is not valid anymore when
>leaving the DLL initialization.
>
>For testing I disabled the 
>
>  _my_tls.init_exception_handler (_cygtls::handle_exceptions);
>
>call in dll_init.cc:dll_dllcrt0_1() and re-ran the Perl testcase.
>Now it runs fine:
>
>  $ perl ./perlthread.pl
>  Testing threads...
>  I'm a thread!
>  Testing done
>
>Is it possible that we have to remove the exception handler before
>dll_dllcrt0_1 returns?

Are you saying that perl not cleaning up after itself here?  If so, that sounds
like a perl bug.

cgf

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