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Date:	Fri, 26 Jun 2015 10:33:59 -0400
From:	Ken Brown <kbrown AT cornell DOT edu>
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To:	cygwin AT cygwin DOT com
Subject:	Re: [ANNOUNCEMENT] TEST RELEASE: Cygwin 2.1.0-0.1
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> <20150626141437 DOT GV31223 AT calimero DOT vinschen DOT de>
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Hi Corinna,

On 6/26/2015 10:14 AM, Corinna Vinschen wrote:
> 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, SS_DISABLE,
>>>>>>    MINSIGSTKSZ, SIGSTKSZ.
>>>>> [...]
>>>> [...]
>>> did you have a chance to test this a bit, in the meantime?
>>
>> 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 script
>> did detect the alternate stack.
>>
>> 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:
>>
>> /* 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;
>>
>>        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 = 16 * 1024 };
>> 	  char *beg, *end, *addr;
>>
>> 	  beg = stack_bottom;
>> 	  end = stack_bottom + stack_direction * rlim.rlim_cur;
>> 	  if (beg > end)
>> 	    addr = beg, beg = end, end = addr;
>> 	  addr = (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);
>> 	}
>>      }
>>
>>    /* Otherwise we can't do anything with this.  */
>>    deliver_fatal_thread_signal (sig);
>> }
>>
>> The code to set up the signal handler on the alternate stack:
>>
>> static bool
>> init_sigsegv (void)
>> {
>>    struct sigaction sa;
>>    stack_t ss;
>>
>>    stack_direction = ((char *) &ss < stack_bottom) ? -1 : 1;
>>
>>    ss.ss_sp = sigsegv_stack;
>>    ss.ss_size = sizeof (sigsegv_stack);
>                   ^^^^^^^^^^^^^^^^^^^^^^^
>
> What's that size in bytes?

SIGSTKSZ

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

Ken

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