Re: NULL pointer dereference panic

On Tue, Jun 20, 2006 at 08:30:50AM -0700, Tim Kientzle wrote:
> 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

Thanks for this observation!

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

"Why do we call our software `beta'?  'Coz it's beta than nothing" :-)
The logic is older than FreeBSD itself, and alas, new compiler tricks
don't allow for an alternative one that is simple enough to be used in
the kernel.

-- 
Yar