Re: Fw: Re: [PATCH 1/3] signal(i386): alternative signal stack wraparound occurs

Re: Fw: Re: [PATCH 1/3] signal(i386): alternative signal stack wraparound occurs

[Roland McGrath]

cf http://lkml.org/lkml/2007/10/3/41

To summarize: on Linux, SA_ONSTACK decides whether you are already on the
signal stack based on the value of the SP at the time of a signal.  If
you are not already inside the range, you are not "on the signal stack"
and so the new signal handler frame starts over at the base of the signal
stack.

sigaltstack (and sigstack before it) was invented in BSD.  There, the
SA_ONSTACK behavior has always been different.  It uses a kernel state
flag to decide, rather than the SP value.  When you first take an
SA_ONSTACK signal and switch to the alternate signal stack, it sets the
SS_ONSTACK flag in the thread's sigaltstack state in the kernel.
Thereafter you are "on the signal stack" and don't switch SP before
pushing a handler frame no matter what the SP value is.  Only when you
sigreturn from the original handler context do you clear the SS_ONSTACK
flag so that a new handler frame will start over at the base of the
alternate signal stack.

The undesireable effect of the Linux behavior is that an overflow of the
alternate signal stack can not only go undetected, but lead to a ring
buffer effect of clobbering the original handler frame at the base of the
signal stack for each successive signal that comes just after the
overflow.  This is what Shi Weihua's test case demonstrates.  Normally
this does not come up because of the signal mask, but the test case uses
SA_NODEFER for its SIGSEGV handler.

The other subtle part of the existing Linux semantics is that a simple
longjmp out of a signal handler serves to take you off the signal stack
in a safe and reliable fashion without having used sigreturn (nor having
just returned from the handler normally, which means the same).  After
the longjmp (or even informal stack switching not via any proper libc or
kernel interface), the alternate signal stack stands ready to be used
again.

A paranoid program would allocate a PROT_NONE red zone around its
alternate signal stack.  Then a small overflow would trigger a SIGSEGV in
handler setup, and be fatal (core dump) whether or not SIGSEGV is
blocked.  As with thread stack red zones, that cannot catch all overflows
(or underflows).  e.g., a local array as large as page size allocated in
a function called from a handler, but not actually touched before more
calls push more stack, could cause an overflow that silently pushes into
some unrelated allocated pages.

The BSD behavior does not do anything in particular about overflow.  But
it does at least avoid the wraparound or "ring buffer effect", so you'll
just get a straightforward all-out overflow down your address space past
the low end of the alternate signal stack.  I don't know what the BSD
behavior is for longjmp out of an SA_ONSTACK handler.

The POSIX wording relating to sigaltstack is pretty minimal.  I don't
think it speaks to this issue one way or another.  (The program that
overflows its stack is clearly in undefined behavior territory of one
sort or another anyhow.)

Given the longjmp issue and the potential for highly subtle complications
in existing programs relying on this in arcane ways deep in their code, I
am very dubious about changing the behavior to the BSD style persistent
flag.  I think Shi Weihua's patches have a similar effect by tracking the
SP used in the last handler setup.

I think it would be sensible for the signal handler setup code to detect
when it would itself be causing a stack overflow.  Maybe something like
the following patch (untested).  This issue exists in the same way on all
machines, so ideally they would all do a similar check.  

When it's the handler function itself or its callees that cause the
overflow, rather than the signal handler frame setup alone crossing the
boundary, this still won't help.  But I don't see any way to distinguish
that from the valid longjmp case.


Thanks,
Roland

---

diff --git a/arch/x86/kernel/signal_32.c b/arch/x86/kernel/signal_32.c
index d58d455..0000000 100644  
--- a/arch/x86/kernel/signal_32.c
+++ b/arch/x86/kernel/signal_32.c
@@ -295,6 +295,13 @@ get_sigframe(struct k_sigaction *ka, str
 	/* Default to using normal stack */
 	esp = regs->esp;
 
+	/*
+	 * If we are on the alternate signal stack and would overflow it, don't.
+	 * Return an always-bogus address instead so we will die with SIGSEGV.
+	 */
+	if (on_sig_stack(esp) && !likely(on_sig_stack(esp - frame_size)))
+		return (void __user *) -1L;
+
 	/* This is the X/Open sanctioned signal stack switching.  */
 	if (ka->sa.sa_flags & SA_ONSTACK) {
 		if (sas_ss_flags(esp) == 0)

Re: Fw: Re: [PATCH 1/3] signal(i386): alternative signal stack wraparound occurs

[Ingo Molnar]

* Roland McGrath <roland@redhat.com> wrote:

[...]
> I think it would be sensible for the signal handler setup code to 
> detect when it would itself be causing a stack overflow.  Maybe 
> something like the following patch (untested).  This issue exists in 
> the same way on all machines, so ideally they would all do a similar 
> check.
> 
> When it's the handler function itself or its callees that cause the 
> overflow, rather than the signal handler frame setup alone crossing 
> the boundary, this still won't help.  But I don't see any way to 
> distinguish that from the valid longjmp case.

thanks Roland for the detailed analysis. I've queued up the patch below 
in the x86 tree. I suspect we can wait with this for v2.6.25, due to 
this being long-standing behavior of Linux? Thus we could observe the 
effects of this patch for a longer time.

I'm wondering how widespread sigaltstack use is in general. Googling 
around gives:

  http://google.com/codesearch?q=sigaltstack

~400 matches. A more common signal API in comparison:

  http://google.com/codesearch?q=sigaction

gives ~139,000 matches. So i guess the right approach would be to also 
extend LTP's sigaltstack tests to cover whatever new desired behavior - 
so that we at least have a minimal chance of catching obvious breakages 
here.

	Ingo

--------------->
Subject: x86: protect against sigaltstack wraparound
From: Roland McGrath <roland@redhat.com>

cf http://lkml.org/lkml/2007/10/3/41

To summarize: on Linux, SA_ONSTACK decides whether you are already on the
signal stack based on the value of the SP at the time of a signal.  If
you are not already inside the range, you are not "on the signal stack"
and so the new signal handler frame starts over at the base of the signal
stack.

sigaltstack (and sigstack before it) was invented in BSD.  There, the
SA_ONSTACK behavior has always been different.  It uses a kernel state
flag to decide, rather than the SP value.  When you first take an
SA_ONSTACK signal and switch to the alternate signal stack, it sets the
SS_ONSTACK flag in the thread's sigaltstack state in the kernel.
Thereafter you are "on the signal stack" and don't switch SP before
pushing a handler frame no matter what the SP value is.  Only when you
sigreturn from the original handler context do you clear the SS_ONSTACK
flag so that a new handler frame will start over at the base of the
alternate signal stack.

The undesireable effect of the Linux behavior is that an overflow of the
alternate signal stack can not only go undetected, but lead to a ring
buffer effect of clobbering the original handler frame at the base of the
signal stack for each successive signal that comes just after the
overflow.  This is what Shi Weihua's test case demonstrates.  Normally
this does not come up because of the signal mask, but the test case uses
SA_NODEFER for its SIGSEGV handler.

The other subtle part of the existing Linux semantics is that a simple
longjmp out of a signal handler serves to take you off the signal stack
in a safe and reliable fashion without having used sigreturn (nor having
just returned from the handler normally, which means the same).  After
the longjmp (or even informal stack switching not via any proper libc or
kernel interface), the alternate signal stack stands ready to be used
again.

A paranoid program would allocate a PROT_NONE red zone around its
alternate signal stack.  Then a small overflow would trigger a SIGSEGV in
handler setup, and be fatal (core dump) whether or not SIGSEGV is
blocked.  As with thread stack red zones, that cannot catch all overflows
(or underflows).  e.g., a local array as large as page size allocated in
a function called from a handler, but not actually touched before more
calls push more stack, could cause an overflow that silently pushes into
some unrelated allocated pages.

The BSD behavior does not do anything in particular about overflow.  But
it does at least avoid the wraparound or "ring buffer effect", so you'll
just get a straightforward all-out overflow down your address space past
the low end of the alternate signal stack.  I don't know what the BSD
behavior is for longjmp out of an SA_ONSTACK handler.

The POSIX wording relating to sigaltstack is pretty minimal.  I don't
think it speaks to this issue one way or another.  (The program that
overflows its stack is clearly in undefined behavior territory of one
sort or another anyhow.)

Given the longjmp issue and the potential for highly subtle complications
in existing programs relying on this in arcane ways deep in their code, I
am very dubious about changing the behavior to the BSD style persistent
flag.  I think Shi Weihua's patches have a similar effect by tracking the
SP used in the last handler setup.

I think it would be sensible for the signal handler setup code to detect
when it would itself be causing a stack overflow.  Maybe something like
the following patch (untested).  This issue exists in the same way on all
machines, so ideally they would all do a similar check.

When it's the handler function itself or its callees that cause the
overflow, rather than the signal handler frame setup alone crossing the
boundary, this still won't help.  But I don't see any way to distinguish
that from the valid longjmp case.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 arch/x86/kernel/signal_32.c |    7 +++++++
 1 file changed, 7 insertions(+)

Index: linux-x86.q/arch/x86/kernel/signal_32.c
===================================================================
--- linux-x86.q.orig/arch/x86/kernel/signal_32.c
+++ linux-x86.q/arch/x86/kernel/signal_32.c
@@ -296,6 +296,13 @@ get_sigframe(struct k_sigaction *ka, str
 	/* Default to using normal stack */
 	esp = regs->esp;
 
+	/*
+	 * If we are on the alternate signal stack and would overflow it, don't.
+	 * Return an always-bogus address instead so we will die with SIGSEGV.
+	 */
+	if (on_sig_stack(esp) && !likely(on_sig_stack(esp - frame_size)))
+		return (void __user *) -1L;
+
 	/* This is the X/Open sanctioned signal stack switching.  */
 	if (ka->sa.sa_flags & SA_ONSTACK) {
 		if (sas_ss_flags(esp) == 0)

Re: Fw: Re: [PATCH 1/3] signal(i386): alternative signal stack wraparound occurs

[Roland McGrath]

> thanks Roland for the detailed analysis. I've queued up the patch below 
> in the x86 tree. I suspect we can wait with this for v2.6.25, due to 
> this being long-standing behavior of Linux? Thus we could observe the 
> effects of this patch for a longer time.

It's certainly nothing new.  The failure mode of concern is only for a
buggy program to fail to crash as quickly and gracefully as it might.


Thanks,
Roland

Re: Fw: Re: [PATCH 1/3] signal(i386): alternative signal stack wraparound occurs

[Shi Weihua]

Roland McGrath wrote::
> cf http://lkml.org/lkml/2007/10/3/41
> 
> To summarize: on Linux, SA_ONSTACK decides whether you are already on the
> signal stack based on the value of the SP at the time of a signal.  If
> you are not already inside the range, you are not "on the signal stack"
> and so the new signal handler frame starts over at the base of the signal
> stack.
> 
> sigaltstack (and sigstack before it) was invented in BSD.  There, the
> SA_ONSTACK behavior has always been different.  It uses a kernel state
> flag to decide, rather than the SP value.  When you first take an
> SA_ONSTACK signal and switch to the alternate signal stack, it sets the
> SS_ONSTACK flag in the thread's sigaltstack state in the kernel.
> Thereafter you are "on the signal stack" and don't switch SP before
> pushing a handler frame no matter what the SP value is.  Only when you
> sigreturn from the original handler context do you clear the SS_ONSTACK
> flag so that a new handler frame will start over at the base of the
> alternate signal stack.
> 
> The undesireable effect of the Linux behavior is that an overflow of the
> alternate signal stack can not only go undetected, but lead to a ring
> buffer effect of clobbering the original handler frame at the base of the
> signal stack for each successive signal that comes just after the
> overflow.  This is what Shi Weihua's test case demonstrates.  Normally
> this does not come up because of the signal mask, but the test case uses
> SA_NODEFER for its SIGSEGV handler.
> 
> The other subtle part of the existing Linux semantics is that a simple
> longjmp out of a signal handler serves to take you off the signal stack
> in a safe and reliable fashion without having used sigreturn (nor having
> just returned from the handler normally, which means the same).  After
> the longjmp (or even informal stack switching not via any proper libc or
> kernel interface), the alternate signal stack stands ready to be used
> again.
> 
> A paranoid program would allocate a PROT_NONE red zone around its
> alternate signal stack.  Then a small overflow would trigger a SIGSEGV in
> handler setup, and be fatal (core dump) whether or not SIGSEGV is
> blocked.  As with thread stack red zones, that cannot catch all overflows
> (or underflows).  e.g., a local array as large as page size allocated in
> a function called from a handler, but not actually touched before more
> calls push more stack, could cause an overflow that silently pushes into
> some unrelated allocated pages.
> 
> The BSD behavior does not do anything in particular about overflow.  But
> it does at least avoid the wraparound or "ring buffer effect", so you'll
> just get a straightforward all-out overflow down your address space past
> the low end of the alternate signal stack.  I don't know what the BSD
> behavior is for longjmp out of an SA_ONSTACK handler.
> 
> The POSIX wording relating to sigaltstack is pretty minimal.  I don't
> think it speaks to this issue one way or another.  (The program that
> overflows its stack is clearly in undefined behavior territory of one
> sort or another anyhow.)
> 
> Given the longjmp issue and the potential for highly subtle complications
> in existing programs relying on this in arcane ways deep in their code, I
> am very dubious about changing the behavior to the BSD style persistent
> flag.  I think Shi Weihua's patches have a similar effect by tracking the
> SP used in the last handler setup.
> 
> I think it would be sensible for the signal handler setup code to detect
> when it would itself be causing a stack overflow.  Maybe something like
> the following patch (untested).  This issue exists in the same way on all
> machines, so ideally they would all do a similar check.  
> 
> When it's the handler function itself or its callees that cause the
> overflow, rather than the signal handler frame setup alone crossing the
> boundary, this still won't help.  But I don't see any way to distinguish
> that from the valid longjmp case.

Thank you for your detailed explanation and patch.
I tested your patch, unfortunately it can not stop all kinds of overflow.

The esp is a user space pointer, so the user can move it. 
For example, the user defines "int i[1000];" in  the handler function.
Please run the following test program, and pay attention to "int i[1000];".
-------------------------------------------------------------------------
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>

volatile int counter = 0;

#ifdef __i386__
void print_esp()
{
	unsigned long esp;
	__asm__ __volatile__("movl %%esp, %0":"=g"(esp));

	printf("esp = 0x%08lx\n", esp);
}
#endif

void segv_handler()
{
#ifdef __i386__
	print_esp();
#endif

	int *c = NULL;
	counter++;
	printf("%d\n", counter);

	int i[1000];	// <- Pay attention here.

	*c = 1;			// SEGV
}

int main()
{
	int *c = NULL;
	char *s = malloc(SIGSTKSZ);
	stack_t stack;
	struct sigaction action;

	memset(s, 0, SIGSTKSZ);
	stack.ss_sp = s;
	stack.ss_flags = 0;
	stack.ss_size = SIGSTKSZ;
	int error = sigaltstack(&stack, NULL);
	if (error) {
		printf("Failed to use sigaltstack!\n");
		return -1;
	}

	memset(&action, 0, sizeof(action));
	action.sa_handler = segv_handler;
	action.sa_flags = SA_ONSTACK | SA_NODEFER;

	sigemptyset(&action.sa_mask);

	sigaction(SIGSEGV, &action, NULL);

	*c = 0;			//SEGV

	if (!s)
		free(s);

	return 0;
}
-------------------------------------------------------------------------

So, the patch I posted is still needed 
Surely, adding a variable to sched.h is not a good idea. 
Could you tell me a better place to store the previous esp?

Here is a new patch rebased on 2.6.24-rc3-git2.

Signed-off-by: Shi Weihua <shiwh@cn.fujitsu.com> 

--- /shiwh-tmp/linux-2.6.24-rc3-git2.orig/arch/x86/kernel/signal_32.c	2007-11-28 09:15:34.000000000 +0800
+++ /shiwh-tmp/linux-2.6.24-rc3-git2/arch/x86/kernel/signal_32.c	2007-11-28 13:19:13.000000000 +0800
@@ -297,7 +297,8 @@ get_sigframe(struct k_sigaction *ka, str
 
 	/* This is the X/Open sanctioned signal stack switching.  */
 	if (ka->sa.sa_flags & SA_ONSTACK) {
-		if (sas_ss_flags(esp) == 0)
+		if (sas_ss_flags(esp) == 0 &&
+		    !on_sig_stack(current->pre_ss_sp))
 			esp = current->sas_ss_sp + current->sas_ss_size;
 	}
 
@@ -330,9 +331,15 @@ static int setup_frame(int sig, struct k
 
 	frame = get_sigframe(ka, regs, sizeof(*frame));
 
+	if ((ka->sa.sa_flags & SA_ONSTACK) &&
+	    !sas_ss_flags((unsigned long)frame))
+		goto give_sigsegv;
+
 	if (!access_ok(VERIFY_WRITE, frame, sizeof(*frame)))
 		goto give_sigsegv;
 
+	current->pre_ss_sp = (unsigned long)frame;
+
 	usig = current_thread_info()->exec_domain
 		&& current_thread_info()->exec_domain->signal_invmap
 		&& sig < 32
@@ -422,9 +429,15 @@ static int setup_rt_frame(int sig, struc
 
 	frame = get_sigframe(ka, regs, sizeof(*frame));
 
+	if ((ka->sa.sa_flags & SA_ONSTACK) &&
+	    !sas_ss_flags((unsigned long)frame))
+		goto give_sigsegv;
+
 	if (!access_ok(VERIFY_WRITE, frame, sizeof(*frame)))
 		goto give_sigsegv;
 
+	current->pre_ss_sp = (unsigned long)frame;
+
 	usig = current_thread_info()->exec_domain
 		&& current_thread_info()->exec_domain->signal_invmap
 		&& sig < 32
--- /shiwh-tmp/linux-2.6.24-rc3-git2.orig/include/linux/sched.h	2007-11-28 09:15:52.000000000 +0800
+++ /shiwh-tmp/linux-2.6.24-rc3-git2/include/linux/sched.h	2007-11-28 13:20:04.000000000 +0800
@@ -1059,6 +1059,7 @@ struct task_struct {
 	struct sigpending pending;
 
 	unsigned long sas_ss_sp;
+	unsigned long pre_ss_sp;
 	size_t sas_ss_size;
 	int (*notifier)(void *priv);
 	void *notifier_data;
--- /shiwh-tmp/linux-2.6.24-rc3-git2.orig/kernel/signal.c	2007-11-28 09:15:52.000000000 +0800
+++ /shiwh-tmp/linux-2.6.24-rc3-git2/kernel/signal.c	2007-11-28 13:20:54.000000000 +0800
@@ -2403,6 +2403,9 @@ do_sigaltstack (const stack_t __user *us
 
 		current->sas_ss_sp = (unsigned long) ss_sp;
 		current->sas_ss_size = ss_size;
+
+		/* reset previous sp */
+		current->pre_ss_sp = 0;
 	}
 
 	if (uoss) {

Thanks,
Shi Weihua

> 
> 
> Thanks,
> Roland
> 
> ---
> 
> diff --git a/arch/x86/kernel/signal_32.c b/arch/x86/kernel/signal_32.c
> index d58d455..0000000 100644  
> --- a/arch/x86/kernel/signal_32.c
> +++ b/arch/x86/kernel/signal_32.c
> @@ -295,6 +295,13 @@ get_sigframe(struct k_sigaction *ka, str
>  	/* Default to using normal stack */
>  	esp = regs->esp;
>  
> +	/*
> +	 * If we are on the alternate signal stack and would overflow it, don't.
> +	 * Return an always-bogus address instead so we will die with SIGSEGV.
> +	 */
> +	if (on_sig_stack(esp) && !likely(on_sig_stack(esp - frame_size)))
> +		return (void __user *) -1L;
> +
>  	/* This is the X/Open sanctioned signal stack switching.  */
>  	if (ka->sa.sa_flags & SA_ONSTACK) {
>  		if (sas_ss_flags(esp) == 0)

Re: [PATCH 1/3] signal(i386): alternative signal stack wraparound occurs

[Ingo Molnar]

* Shi Weihua <shiwh@cn.fujitsu.com> wrote:

> > When it's the handler function itself or its callees that cause the 
> > overflow, rather than the signal handler frame setup alone crossing 
> > the boundary, this still won't help.  But I don't see any way to 
> > distinguish that from the valid longjmp case.
> 
> Thank you for your detailed explanation and patch. I tested your 
> patch, unfortunately it can not stop all kinds of overflow.
[...]
> So, the patch I posted is still needed 

thanks, i've picked up your fix for x86.git, for 2.6.25 merging.

> Surely, adding a variable to sched.h is not a good idea. 
> Could you tell me a better place to store the previous esp?

i think sched.h is ok - it has a sas_ss_sp field already. Alternatively, 
if we only want this in x86, we could put it into the thread_struct - 
but i think eventually other architectures would want to use this too, 
right?

	Ingo

Re: [PATCH 1/3] signal(i386): alternative signal stack wraparound occurs

[Roland McGrath]

> > Thank you for your detailed explanation and patch. I tested your 
> > patch, unfortunately it can not stop all kinds of overflow.
> [...]
> > So, the patch I posted is still needed 
> 
> thanks, i've picked up your fix for x86.git, for 2.6.25 merging.

I just explained that not all overflows would be caught and that doing so
would violate the semantics of e.g. longjmp.  I don't see how the patch
you've included now doesn't still have all those problems.  I think it's wrong.


Thanks,
Roland

Re: [PATCH 1/3] signal(i386): alternative signal stack wraparound occurs

[Ingo Molnar]

* Roland McGrath <roland@redhat.com> wrote:

> > > Thank you for your detailed explanation and patch. I tested your 
> > > patch, unfortunately it can not stop all kinds of overflow.
> > [...]
> > > So, the patch I posted is still needed 
> > 
> > thanks, i've picked up your fix for x86.git, for 2.6.25 merging.
> 
> I just explained that not all overflows would be caught and that doing 
> so would violate the semantics of e.g. longjmp.  I don't see how the 
> patch you've included now doesn't still have all those problems.  I 
> think it's wrong.

ok, i've dropped it and reinstated your original fix - i missed that 
detail.

	Ingo

Re: [PATCH 1/3] signal(i386): alternative signal stack wraparound occurs

[Shi Weihua]

Roland McGrath wrote::
>>> Thank you for your detailed explanation and patch. I tested your 
>>> patch, unfortunately it can not stop all kinds of overflow.
>> [...]
>>> So, the patch I posted is still needed 
>> thanks, i've picked up your fix for x86.git, for 2.6.25 merging.
> 
> I just explained that not all overflows would be caught and that doing so
> would violate the semantics of e.g. longjmp.  I don't see how the patch
> you've included now doesn't still have all those problems.  I think it's wrong.
> 

I am sorry, i don't understand how this is related to the semantics of e.g. longjmp.
But, i am sure my patch solves all overflows. Ingo's patch can't catch the overflow 
which is caught by "int i[1000];" in the handler function.

Do you have more idea for me? Thanks.

Regards
Shi Weihua

By the way, I think Ingo's patch can be improved. 

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Shi Weihua <shiwh@cn.fujitsu.com>

---
--- linux-2.6.24-rc4-git1.orig/arch/x86/kernel/signal_32.c	2007-12-04 12:26:10.000000000 +0800
+++ linux-2.6.24-rc4-git1/arch/x86/kernel/signal_32.c	2007-12-05 11:13:56.000000000 +0800
@@ -297,8 +297,17 @@ get_sigframe(struct k_sigaction *ka, str
 
 	/* This is the X/Open sanctioned signal stack switching.  */
 	if (ka->sa.sa_flags & SA_ONSTACK) {
-		if (sas_ss_flags(esp) == 0)
+		int onstack = sas_ss_flags(esp);
+		if (onstack == 0)
 			esp = current->sas_ss_sp + current->sas_ss_size;
+		else if(onstack == SS_ONSTACK){
+			/*
+			 * If we are on the alternate signal stack and would overflow it, don't.
+			 * Return an always-bogus address instead so we will die with SIGSEGV.
+			 */
+			if (!likely(on_sig_stack(esp - frame_size)))
+				return (void __user *) -1L;
+		}
 	}
 
 	/* This is the legacy signal stack switching. */

Re: [PATCH 1/3] signal(i386): alternative signal stack wraparound occurs

[Roland McGrath]

> I am sorry, i don't understand how this is related to the semantics of
> e.g. longjmp.  But, i am sure my patch solves all overflows. Ingo's patch
> can't catch the overflow which is caught by "int i[1000];" in the handler
> function.

Please read the long explanation I wrote, which Ingo forwarded.

Thanks,
Roland