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Date:   Mon, 5 Jul 2010 14:09:31 +0100
From:   Matt Fleming <matt@console-pimps.org>
To:     Ralf Baechle <ralf@linux-mips.org>
Cc:     linux-mips@linux-mips.org, Adam Jiang <jiang.adam@gmail.com>
Subject: Re: How to detect STACKOVEFLOW on mips
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On Mon, Jul 05, 2010 at 11:56:27AM +0100, Ralf Baechle wrote:
> On Wed, Jun 30, 2010 at 10:57:41PM +0100, Matt Fleming wrote:
> 
> > On Wed, 30 Jun 2010 15:50:06 +0100, Ralf Baechle <ralf@linux-mips.org> wrote:
> > > 
> > > There used to be some code for other architectures that zeros the stack
> > > page and counts how much of that has been overwritten by the stack.  That
> > > was never ported to MIPS.
> > > 
> > > Another helper to find functions that do excessive static allocations is
> > > "make checkstack".
> > 
> > Both SH and sparc use the mcount function (enabled with the -pg switch
> > to gcc) to check the stack has not overflowed. The relevant code is in
> > arch/{sh,sparc}/lib/mcount.S. This checks the stack pointer value on
> > every function call. Yeah, it's heavy-weight, but an implementation for
> > MIPS should be able to catch almost the exact point at which stack
> > overflow occurs.
> 
> Which often isn't so helpful.  The alarm gets triggered on the last stack
> pointer decrement but according to murphy the overflow has happened 10
> levels up in the callchain.

Last decrement? The alarm should be triggered the next time the
function in which the overflow occurs makes a function call. I don't
see how you could go down a level of the callchain and not trigger the
alarm if the overflow has happened?