Re: [OT] Wine + Cygwin: `script -e` exit status forwarding randomly return zero for non zero child process

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

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On Sep  7 19:02, Qian Hong wrote:
> Hi Corinna,
> 
> Sorry for delay, our progress on this issue is slow.
> 
> Many thanks for your great information, it does help us became closer
> to the reason.
> 
> On Thu, Sep 3, 2015 at 6:55 PM, Corinna Vinschen
> <corinna-cygwin@cygwin.com> wrote:
> > Comparing the straces, two interesting facts are conspicuous:
> > - After the forked script returned, what happens in the parent is absolutly
> >   identical in both cases, up to a point during exit.  This point is reached
> >   with line 1573 in the "bad" case and line 1580 in the "good" case.  Then
> >   "something" happens:
> >
> 
> Thanks for the analysis, this does help us realized it should be a
> very low level issue, in a level I wasn't doubt about.
> [...]
> >    To me this looks like the "bad" script has been exited forcefully
> >    for some reason.
> >
> 
> Yeah, this is the hard part. Sebastian has some progress on it, I
> quote his note here:
> 
> === quote ===
> 
> Cygwin tries to forcibly kill a thread, the implementation for that is
> available here:
> https://github.com/Alexpux/Cygwin/blob/79511853f788111efd975651f87eabbd4a8cbf6d/winsup/cygwin/cygthread.cc#L296

Guys, no offense meant, but I'd really appreciate if you could peruse
and refer to the original upstream Cygwin repo at

https://sourceware.org/git/?p=newlib-cygwin.git

This is what we're talking about in the first place so I'd like to stick
to this, ok?

> Excerpt from the log with annotations:
> 
> --- snip ---
> 0027:Call KERNEL32.TerminateThread(00000168,00000000) ret=610055ca
> 0027: terminate_thread( handle=0168, exit_code=0 )
> 0027: terminate_thread() = 0 { self=0, last=0 }
> 0027:Ret  KERNEL32.TerminateThread() retval=00000001 ret=610055ca
> // handle 00000168 corresponds to thread 0x0029
> 
> 0027:Call KERNEL32.WaitForSingleObject(00000168,ffffffff) ret=610055e0
> 0027: select( flags=2, cookie=0060ba6c, timeout=infinite,
> prev_apc=0000, result={}, data={WAIT,handles={0168}} )
> 0027: select() = 0 { timeout=infinite, call={APC_NONE}, apc_handle=0000 }
> 0027:Ret  KERNEL32.WaitForSingleObject() retval=00000000 ret=610055e0
> // WaitForSingleObject doesn't block, so cygwin assumes the thread is gone
> 
> [...]
> 
> 0027:Call KERNEL32.VirtualFree(00a10000,00000000,00008000) ret=61005767
> // Cygwin tries to release the thread stack

Ah, ok.  What OS does Wine emulate here?  Have a look at
https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/cygthread.cc;h=e48a73e545e7ca90884fc891f1b188e0ab3bf863;hb=HEAD#l316

The terminate_thread_frees_stack flag is set to false for XP/2003 and to
true for any newer OS.  I guess this is a double-free because Wine's
TerminateThread already freed the stack and Cygwin got the info it's
supposedly running under XP/2003, so it tries to workaround the fact
that TerminateThread on these systems didn't free the stack by themselves.

> As a workaround NtFreeVirtualMemory can be replaced with a no-op
> implementation returning STATUS_SUCCESS.

I don't think this is the right thing to do.  It's a hack covering
the problem that TerminateThread should not free the thread stack
if we're running an XP/2003 emulation.

If my assumption here is incorrect, we need to know what address
ret=61005767 is refering to.  addr2line would help.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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