DMARC-Filter:	OpenDMARC Filter v1.4.2 delorie.com 52DBhTlp097264
Authentication-Results:	delorie.com; dmarc=pass (p=none dis=none) header.from=cygwin.com
Authentication-Results:	delorie.com; spf=pass smtp.mailfrom=cygwin.com
DKIM-Filter:	OpenDKIM Filter v2.11.0 delorie.com 52DBhTlp097264
Authentication-Results:	delorie.com;
	dkim=pass (1024-bit key, unprotected) header.d=cygwin.com header.i=@cygwin.com header.a=rsa-sha256 header.s=default header.b=eKRzJfeJ
X-Recipient:	archive-cygwin AT delorie DOT com
DKIM-Filter:	OpenDKIM Filter v2.11.0 sourceware.org 158463858D39
DKIM-Signature:	v=1; a=rsa-sha256; c=relaxed/relaxed; d=cygwin.com;
	s=default; t=1741866208;
	bh=ZeXUDpxEGSka2ukx9FntZHr5B9dEr1axNKCoKByc6z4=;
	h=Date:To:Subject:In-Reply-To:References:List-Id:List-Unsubscribe:
	List-Archive:List-Post:List-Help:List-Subscribe:From:Reply-To:
	From;
	b=eKRzJfeJWlF/LxZp10MR6pJ6HN84VTLoGuY2KHpfz1wW0khDEztBTMat1gvq00AgC
	FiHKBOlmIS6QGKxVbn+ehtJW6ltROu2B25rUrRTVG7rQtZ3KnfU4hcl8GWZvQ5QKql
	W5aeVhO+nr7TPVnq6gNFUQIPIWUtRKiTxXqD6kNA=
X-Original-To:	cygwin AT cygwin DOT com
Delivered-To:	cygwin AT cygwin DOT com
DMARC-Filter:	OpenDMARC Filter v1.4.2 sourceware.org 28ECF3858039
ARC-Filter:	OpenARC Filter v1.0.0 sourceware.org 28ECF3858039
ARC-Seal:	i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1741866177; cv=none;
	b=t/mSQ/HzWQcVhGDOn9OgjsIrDDy1ecLzXQ0Q0QqD+H9VXqk9gY/HDBRYuSpxej/67zWzfrixhK3drS5aiUf9e47HwMuQkRGyA9uC29/rN5Sh9sDjEY0LZZEYeZOKm/g/EwJcaoUNz77k0pRQd4aYbJ7yxIHhzEwz5BkdQyTRddQ=
ARC-Message-Signature:	i=1; a=rsa-sha256; d=sourceware.org; s=key;
	t=1741866177; c=relaxed/simple;
	bh=4lSxNbpAxNR1r8EB9D3U3nI4Jvd0wlx/BjcZZIDsf2o=;
	h=Date:From:To:Subject:Message-Id:Mime-Version:DKIM-Signature;
	b=X4GNODln+pLdSlK4po/NVzDVVn4xXDMCZF5yh828oNE/aXIz9UTKLKaw0jPfkItOF+IMkq306puQw3r4Sir7XqQDkRwoCX5SbOd3W70JwsOsXhjpJSQQSWM9X70AL7qjMaEu9ctfPvcsbVajrmMgPK848vRKCiYbB+qXVKvDlpg=
ARC-Authentication-Results:	i=1; server2.sourceware.org
DKIM-Filter:	OpenDKIM Filter v2.11.0 sourceware.org 28ECF3858039
Date:	Thu, 13 Mar 2025 20:42:52 +0900
To:	cygwin AT cygwin DOT com
Subject:	Re: cygwin 3.6.0: No signals received after swapcontext() is used
Message-Id:	<20250313204252.e340f0de50838f161b0e8323@nifty.ne.jp>
In-Reply-To:	<c0000d72-2b39-2647-648f-9006bed1273e@t-online.de>
References:	<ec6e2050-953f-0d47-c385-cfa598566291 AT t-online DOT de>
	<Z8nxYCxthcsMVqzL AT calimero DOT vinschen DOT de>
	<bf4eb7e1-66e3-e1f9-67e2-c4d4a75ff6c8 AT t-online DOT de>
	<Z864NNIyYwOWk5I3 AT calimero DOT vinschen DOT de>
	<373993a3-9f0f-9750-60a0-950f83b3b0b5 AT t-online DOT de>
	<Z9Gooi9C1UcJBuMW AT calimero DOT vinschen DOT de>
	<Z9Gw6inr56cd4TGe AT calimero DOT vinschen DOT de>
	<Z9G1BBjghen0kWvx AT calimero DOT vinschen DOT de>
	<c0000d72-2b39-2647-648f-9006bed1273e AT t-online DOT de>
X-Mailer:	Sylpheed 3.7.0 (GTK+ 2.24.30; i686-pc-mingw32)
Mime-Version:	1.0
X-BeenThere:	cygwin AT cygwin DOT com
X-Mailman-Version:	2.1.30
List-Id:	General Cygwin discussions and problem reports <cygwin.cygwin.com>
List-Unsubscribe:	<https://cygwin.com/mailman/options/cygwin>,
	<mailto:cygwin-request AT cygwin DOT com?subject=unsubscribe>
List-Archive:	<https://cygwin.com/pipermail/cygwin/>
List-Post:	<mailto:cygwin AT cygwin DOT com>
List-Help:	<mailto:cygwin-request AT cygwin DOT com?subject=help>
List-Subscribe:	<https://cygwin.com/mailman/listinfo/cygwin>,
	<mailto:cygwin-request AT cygwin DOT com?subject=subscribe>
From:	Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
Errors-To:	cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com
Sender:	"Cygwin" <cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com>

Hi Corinna,

On Thu, 13 Mar 2025 10:40:48 +0100
Christian Franke wrote:
> Corinna Vinschen via Cygwin wrote:
> > On Mar 12 17:06, Corinna Vinschen via Cygwin wrote:
> >> On Mar 12 16:30, Corinna Vinschen via Cygwin wrote:
> >>> On Mar 11 12:32, Christian Franke via Cygwin wrote:
> >>>> The attached testcase should test the following use cases of setcontext:
> >>>> - call from regular user space
> >>>> - call from a signal handler interrupting user space
> >>>> - call from a signal handler interrupting a system call
> >>>>
> >>>> It works as expected ... until the signal count reaches 256. Then signals
> >>>> are again only delivered from inside of a system call.
> >>>> [...]
> >>>> Interesting... Hmm... is there some 8-bit counter which overflows and then
> >>>> stucks at 0xff or 0x00?
> >>> It's a kind of stack overflow.  Kind of, because it's not the normal
> >>> thread stack, but a special signal stack in the _cygtls area.
> >>>
> >>> When interrupting a running thread to call a signal handler, the context
> >>> of the thread is changed to restart execution in an assembler function
> >>> called sigdelayed().  The original IP of the thread is pushed on the
> >>> aforementioned signal stack.  Sigdelayed() calls the signal handler.  On
> >>> return it pops the original IP from the signal stack and continues the
> >>> thread.
> >>>
> >>> Now guess what happens if the signal handler bails out with longjmp or
> >>> setcontext/swapcontext.
> >>>
> >>> The signal handler never returns to the sigdelayed() function, the
> >>> original address is never poped from the signal stack, and the signal
> >>> stack has a max. size of 256 address entries...
> >>>
> >>> Theoretically, a small update to sigdelayed() would fix the issue: ather
> >>> then poing the original IP from the signal stack after calling the
> >>> handler, it should pop the IP prior to calling the handler.  That would
> >>> avoid filling up the signal stack when long-jumping out of the signal
> >>> handler.  It should store the IP in one of the callee-saved registers.
> >>> %r13 is unused in sigdelayed so far.
> >>>
> >>> However, even if we do this, there's still the problem that sigdelayed()
> >>> itself takes space on the stack.  If you longjmp/setcontext out of the
> >>> handler, the thread's normal stack will fill up with dead storage of the
> >>> sigdelayed() function, and there's no way out of this trap.  We can't
> >>> restore the stack before the handler returns.
> >>>
> >>> So either way, at one point you get a stack overflow one way or the
> >>> other.
> >>>
> >>> The signal stack overflow is actually rather harmless in comparison
> >>> to a real stack overflow.
> >>>
> >>> If you have any idea how to avoid the real stack overflow, I'd be
> >>> all ears.
> >> Looks like this isn't really a problem with setcontext.  It always
> >> corrects the stack pointer as well.  Apparently I haven't thought
> >> long enough about this.
> >>
> >> I have a patch for sigdelayed() in the loop, stay tuned.
> > Just pushed.  Try cygwin-3.6.0-0.430.ga942476236b5 in a bit.
> 
> Problem does no longer occur. Also tested with 'kill -INT PID && sleep 
> 0.01' in a loop.

After the commit:

commit a942476236b5e39bf30c533d08df7392e326a4c6 (origin/master, origin/main, origin/HEAD)
Author: Corinna Vinschen <corinna AT vinschen DOT de>
Date:   Wed Mar 12 17:17:31 2025 +0100

    Cygwin: sigdelayed: pop return address from signal stack earlier

Christians test case: timersig.c no longer works even with my v3 patches.
I suspect it is because pop(), retaddr() are not working as intended in
call_signal_handler() with this commit.

Could you please have a look?

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

-- 
Problem reports:      https://cygwin.com/problems.html
FAQ:                  https://cygwin.com/faq/
Documentation:        https://cygwin.com/docs.html
Unsubscribe info:     https://cygwin.com/ml/#unsubscribe-simple

- Raw text -