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Date: Fri, 26 Jun 2015 16:14:37 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: [ANNOUNCEMENT] TEST RELEASE: Cygwin 2.1.0-0.1
Message-ID: <20150626141437.GV31223@calimero.vinschen.de>
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References: <announce DOT 20150620205512 DOT GA28301 AT calimero DOT vinschen DOT de> <558706D5 DOT 1020508 AT cornell DOT edu> <20150622110835 DOT GE28301 AT calimero DOT vinschen DOT de> <20150626111249 DOT GS31223 AT calimero DOT vinschen DOT de> <558D3F4C DOT 6090207 AT cornell DOT edu>
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Hi Ken,

On Jun 26 08:02, Ken Brown wrote:
> On 6/26/2015 7:12 AM, Corinna Vinschen wrote:
> >On Jun 22 13:08, Corinna Vinschen wrote:
> >>On Jun 21 14:47, Ken Brown wrote:
> >>>On 6/20/2015 4:55 PM, Corinna Vinschen wrote:
> >>>>- First cut of an implementation to allow signal handlers running on =
an
> >>>>   alternate signal stack.
> >>>>
> >>>>- New API sigaltstack, plus definitions for SA_ONSTACK, SS_ONSTACK, S=
S_DISABLE,
> >>>>   MINSIGSTKSZ, SIGSTKSZ.
> >>>[...]
> >>[...]
> >did you have a chance to test this a bit, in the meantime?
>=20
> Yes, but I don't have anything definitive to report yet.  I tried to test=
 a
> facility in emacs that uses the alternate stack to recover from stack
> overflow (of the main stack) under some circumstances.  The configure scr=
ipt
> did detect the alternate stack.
>=20
> I then made the stack overflow by defining an elisp function that did an
> infinite recursion.  emacs still crashed, but the "segmentation fault"
> message was printed twice instead of once.  I haven't had a chance yet to
> investigate further and try to see what's going on.  What I hope is that =
the
> alternate stack functioned correctly but the code was still not able to
> recover for some reason.  I've appended below the signal handler in case =
you
> want to see if you think it ought to work on Cygwin.

Thank you.  I'll try to test this in the next couple of days.  One hint
and one question:

> The signal handler:
>=20
> /* Attempt to recover from SIGSEGV caused by C stack overflow.  */
> static void
> handle_sigsegv (int sig, siginfo_t *siginfo, void *arg)
> {
>   /* Hard GC error may lead to stack overflow caused by
>      too nested calls to mark_object.  No way to survive.  */
>   if (!gc_in_progress)
>     {
>       struct rlimit rlim;
>=20
>       if (!getrlimit (RLIMIT_STACK, &rlim))

This getrlimit probably won't work as desired.  I just had a quick look
how this request is handled.  It will return the size of the alternate
stack while running the signal handler, rather than the size of the
initial thread's stack as required by POSIX.  This definitely needs
fixing.

> 	{
> 	  enum { STACK_DANGER_ZONE =3D 16 * 1024 };
> 	  char *beg, *end, *addr;
>=20
> 	  beg =3D stack_bottom;
> 	  end =3D stack_bottom + stack_direction * rlim.rlim_cur;
> 	  if (beg > end)
> 	    addr =3D beg, beg =3D end, end =3D addr;
> 	  addr =3D (char *) siginfo->si_addr;
> 	  /* If we're somewhere on stack and too close to
> 	     one of its boundaries, most likely this is it.  */
> 	  if (beg < addr && addr < end
> 	      && (addr - beg < STACK_DANGER_ZONE
> 		  || end - addr < STACK_DANGER_ZONE))
> 	    siglongjmp (return_to_command_loop, 1);
> 	}
>     }
>=20
>   /* Otherwise we can't do anything with this.  */
>   deliver_fatal_thread_signal (sig);
> }
>=20
> The code to set up the signal handler on the alternate stack:
>=20
> static bool
> init_sigsegv (void)
> {
>   struct sigaction sa;
>   stack_t ss;
>=20
>   stack_direction =3D ((char *) &ss < stack_bottom) ? -1 : 1;
>=20
>   ss.ss_sp =3D sigsegv_stack;
>   ss.ss_size =3D sizeof (sigsegv_stack);
                 ^^^^^^^^^^^^^^^^^^^^^^^

What's that size in bytes?

>   ss.ss_flags =3D 0;
>   if (sigaltstack (&ss, NULL) < 0)
>     return 0;
>=20
>   sigfillset (&sa.sa_mask);
>   sa.sa_sigaction =3D handle_sigsegv;
>   sa.sa_flags =3D SA_SIGINFO | SA_ONSTACK | emacs_sigaction_flags ();
>   return sigaction (SIGSEGV, &sa, NULL) < 0 ? 0 : 1;
> }


Thanks,
Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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