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Date:	Wed, 12 Mar 2025 17:06:02 +0100
To:	cygwin AT cygwin DOT com
Subject:	Re: cygwin 3.6.0: No signals received after swapcontext() is used
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From:	Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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Cc:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
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On Mar 12 16:30, Corinna Vinschen via Cygwin wrote:
> On Mar 11 12:32, Christian Franke via Cygwin wrote:
> > Corinna Vinschen via Cygwin wrote:
> > > It's not quite clear to me why signal handling should be broken if
> > > setcontext is used inside a signal handler.  The incyg flag is false
> > > when running the signal handler and that's correct.  Theoretically a
> > > running signal handler is not different from other process code.
> > > 
> > > Do you have an STC, by any chance?
> > 
> > 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.


Corinna

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