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Date: Fri, 14 Mar 2025 08:12:36 +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 Thu, 13 Mar 2025 23:46:49 +0100
Corinna Vinschen wrote:
> On Mar 13 17:30, Corinna Vinschen via Cygwin wrote:
> > On Mar 13 21:31, Takashi Yano via Cygwin wrote:
> > > What about following patch instead of your sigdelayed patch?
> > > [...]
> > > @@ -1834,6 +1841,26 @@ _cygtls::call_signal_handler ()
> > >  	   signal handler. */
> > >  	thisfunc (thissig, &thissi, thiscontext);
> > >  
> > > +      lock ();
> > > +      if (stackptr == ptr)
> > > +	push (retaddr1);
> > > +      else if (stackptr == ptr + 1)
> > > +	{
> > > +	  DWORD64 retaddr3 = pop();
> > > +	  push (retaddr1);
> > > +	  push (retaddr3);
> > > +	}
> > > +      else if (stackptr == ptr - 1)
> > > +	{
> > > +	  if (retaddr2)
> > > +	    push (retaddr2);
> > > +	  else
> > > +	    stackptr++;
> > > +	}
> > > +      else
> > > +	api_fatal ("Signal stack corrupted?.");
> > > +      unlock ();
> > > +
> > 
> > This... looks confusing and desperately needs comments (or at least
> > I need comments).
> > 
> > stackptr == ptr + 1 occurs if another signal arrived while the handler
> > was running, but isn't there a chance that sigdelayed has been pushed
> > as well, i.e., stackptr == ptr + 2?
> > 
> > I have no idea how the stackptr == ptr - 1 situation is supposed to
> > happen, though.  `else stackptr++;' looks weird.  If you don't push a
> > known address, what do you expect retaddr() pointing to, afterwards?
> 
> 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?

diff --git a/winsup/cygwin/exceptions.cc b/winsup/cygwin/exceptions.cc
index c9fe6a386..453f9fea3 100644
--- a/winsup/cygwin/exceptions.cc
+++ b/winsup/cygwin/exceptions.cc
@@ -1758,6 +1758,14 @@ _cygtls::call_signal_handler ()
       reset_signal_arrived ();
       incyg = false;
       current_sig = 0;	/* Flag that we can accept another signal */
+
+      /* We have to fetch the original return address from the signal stack
+	 prior to calling the signal handler.  This avoids filling up the
+	 signal stack if the signal handler longjumps (longjmp/setcontext). */
+      __tlsstack_t retaddrs[TLS_STACK_SIZE];
+      int n1 = stackptr - stack;
+      memcpy (retaddrs, stack, n1 * sizeof (__tlsstack_t));
+      stackptr = stack;
       unlock ();	/* unlock signal stack */
 
       /* Alternate signal stack requested for this signal and alternate signal
@@ -1834,6 +1842,15 @@ _cygtls::call_signal_handler ()
 	   signal handler. */
 	thisfunc (thissig, &thissi, thiscontext);
 
+      lock ();
+      int n2 = stackptr - stack;
+      if (n1 + n2 > TLS_STACK_SIZE)
+	api_fatal ("Signal stack exhausted.");
+      memcpy (retaddrs + n1, stack, n2 * sizeof (__tlsstack_t));
+      memcpy (stack, retaddrs, (n1 + n2) * sizeof (__tlsstack_t));
+      stackptr += n1;
+      unlock ();
+
       incyg = true;
 
       set_signal_mask (_my_tls.sigmask, (this_sa_flags & SA_SIGINFO)


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

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