Date:	Sun, 22 Aug 1999 13:27:47 +0300 (IDT)
From:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
X-Sender:	eliz AT is
To:	Martin Str|mberg <ams AT ludd DOT luth DOT se>
cc:	DJGPP-WORKERS <djgpp-workers AT delorie DOT com>
Subject:	Re: stack overruns (was: fixed stack size)
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On Sun, 22 Aug 1999, Martin Str|mberg wrote:

> Hans-Bernhard said:
> 
> > Same problem as with the guard page approach: if the stack is overflown,
> > if will often happen in one large step, without touching all addresses in
> > between. Think of someone using a double a[200000]; local variable.
> > Stack corruption happens, but your guard value will only be hit if
> > that array is actually modified.
> 
> I'm aware of that, but this one is very cheap to implement. And it
> will catch too deep function nesting, I think.

I don't think so.  Most stack overflows are from declarations like 
Hans-Bernhard mentioned: "double a[200000];".  These are compiled into a 
single instruction that decrements ESP.  Your sugestion will only work if 
the exact location where you put the special value is written to by the 
code *before* it writes into lower addresses.  If the code only reads from 
the array, or writes to other array elements, it won't be caught.

For example, the following snippet is a clear disaster, but it won't be 
caught by your suggestion:

	int foo (int bar)
	{
	  double a[200000];

	  a[0] = bar * M_PI;
	  printf ("%f\n", a[0]);
	}

> Does anybody know why cc1 or whatever programs must have a larger
> stack?

Probably because GCC uses alloca a lot.  But I didn't look close enough 
to say for sure.

> It surely isn't because it declares huge arrays locally without
> trying to use them, right?

The problem is that the damage (and the crash) can happen before the code 
accesses the allocated array.  See the snippet above: the damage happens 
in printf, whether or not the code accesses a[] after the call to printf.

> By the way, what data is first overwritten by a stack overrun?

The data in .bss section.

The memory of a DJGPP program is organized like this (in ascending order 
of the linear addresses): .code, .data, .bss, stack, heap.

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