linux-kernel - Re: [git pull] core, x86: make LIST

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Date:	Mon, 14 Jul 2008 08:59:44 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...ranet.com>
Subject: Re: [git pull] core, x86: make LIST_POISON less deadly



On Mon, 14 Jul 2008, Ingo Molnar wrote:
> > 
> > Why not use something sane like 0xdead000000000000, which has the high 
> > bit set but very fundamentally isn't a valid pointer, and never will 
> > be? And which is a *lot* more visually obvious too!
> 
> initially i suggested that too - but such addresses raise a #GP instead 
> of a page fault so their decoding is a bit harder.

But raising a GP is exactly what you want: a PF is an indication that the 
address was actually half-way valid, and will not fault at all on some 
(possibly future) machine.

> We dont do any instruction decoding in #GP handlers to figure out what 
> happened, while in the pagefault case we know which address faulted, 
> etc.

Why would we care? It would be very obvious from the instruction 
disassembly plus the register contents. No need to decode instructions.

> Perhaps we could try to make #GP handlers a bit more informative - 
> although decoding instructions will make things a bit more fragile 
> inevitably.
> 
> Perhaps make it 0xffffcdead0000000 ?

I'm really not seeing the reason for not just doing it right.

			Linus
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