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Date: Tue, 20 Jun 2006 08:30:50 -0700
From: Tim Kientzle <kientzle@freebsd.org>
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To: "Andrew R. Reiter" <arr@watson.org>
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Cc: Yar Tikhiy <yar@comp.chem.msu.su>, current@freebsd.org
Subject: Re: NULL pointer dereference panic
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Andrew R. Reiter wrote:
> On Tue, 20 Jun 2006, Yar Tikhiy wrote:
> 
> :On Tue, Jun 20, 2006 at 05:08:22AM +1000, Peter Jeremy wrote:
> :> On Mon, 2006-Jun-19 22:45:41 +0400, Yar Tikhiy wrote:
> :> >Peter, what gcc options did you build the kernel with?  My question
> :> >is unrelated to the panic, I'd just like to make stack traces look
> :> >sane in common cases :-)
> :> 
> :
> :When in basic i386 mode, gcc calls functions in the traditional
> :way.  E.g., the "foo(1, 2)" call will look as follows in asm:
> :
> :	pushl	$2
> :	pushl	$1
> :	call	foo
> :	addl	$8, %esp
> :
> :By merely decoding the addl instruction at the return pointer we
> :can find how many words of arguments the called function takes.

I'm not sure which -O flags enable this, but I've
certainly seen lazy stack cleanups in gcc-generated
code on i386:

    pushl $2
    pushl $1
    call foo
    pushl $3
    pushl $4
    call foo
    addl $16, %esp

This optimization would also confuse the stack-tracing
logic you describe.

Tim