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Date: Wed, 12 Mar 2025 17:23:32 +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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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.


Thanks,
Corinna

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