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Date: Fri, 14 Mar 2025 20:35:48 +0900
To: cygwin AT cygwin DOT com
Subject: Re: cygwin 3.6.0: No signals received after swapcontext() is used
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From: Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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On Fri, 14 Mar 2025 11:01:25 +0100
Corinna Vinschen wrote:
> On Mar 14 12:56, Takashi Yano via Cygwin wrote:
> > On Fri, 14 Mar 2025 08:12:36 +0900
> > Takashi Yano wrote:
> > > On Thu, 13 Mar 2025 23:46:49 +0100
> > > Corinna Vinschen wrote:
> > > > I have a slighty changed version. This one treats anything other
> > > > than 0, 1 or 2 new addresses on the stack as bug.  I really made
> > > > an effort trying to come up with a situation where the signal
> > > > stack underflows, but I just couldn't.  If I'm missing something,
> > > > please explain how this may happen.
> > > > 
> > > > Apart from that, I attached my patch proposal.
> > > 
> > > I think the following is the right thing. This version pulls return
> > > addresses completely (not only one) before calling signal handler.
> > > I think, stackptr - orig_stackptr can be larger than 2 when
> > > user code
> > >   signal handler 1
> > >     signal handler 2
> > >       signal handler 3
> > >         signal handler 4
> > >         ret
> > >       ret
> > >     ret
> > >   HERE <= stackptr - orig_stackptr == 3
> > >   ret
> > > Is this right?
> > 
> > No, I was wrong. Every time when call_signal_handler() is
> > called, the _cygtls::stack is pulled, so, it always becomes
> > empty. Therefore, stackptr - orig_stackptr is never more
> > than two.
> > 
> > So, _cygtls::stack needs only two spaces maximum. Please
> > look attached v2 patch. Do I overlook something?
> 
> I don't think so.  I was mulling in circles over this tonight
> (don't ask me how I slept!) and came to the same conclusion.
> But here's the problem:
> 
> I'm simply not 100% sure.
> 
> What concerns me is that stackptr points beyond stack if the stack
> is full (i.e., sigdelayed + return address).
> 
> That was what happened before I applied a942476236b5: stackptr was
> incremented until it pointed at _cygtls::initialized, and eventually it
> overwrote it.  Fortunately, that stopped further incrementing due to the
> isinitialized() test.
> 
> So, if there *is* a twisted situation which results in pushing another
> return address onto the stack, a stack size of 2 would again result in
> initialized being overwritten.  So I wonder if we should keep kind of
> an airbag for an unusual situation.  Plus trying to keep stackptr inside
> stack even if it's full.  So that stackptr never grows into initialized:
> 
>   #define TLS_STACK_SIZE 5
> 
> and
> 
>     void push (__tlsstack_t addr)
>     {
>       if (stackptr < (__tlsstack_t *) &initialized)
> 	*stackptr++ = (__tlsstack_t) addr;
>     }
> 
> What do you think?

Yeah. We do not have to minimize the stack space at the cost of
taking risks.

One more thing. I am also concerned that pop() lacks a guard.
If pop() calls when stack is empty, then push() destroys the
stackptr pointer value.

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

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