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

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Date:	Mon, 14 Jul 2008 17:12:47 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Mon, 14 Jul 2008, Ingo Molnar wrote:
> >  
> > +config ILLEGAL_POINTER_VALUE
> > +       hex
> > +       default 0 if X86_32
> > +       default 0xffffc10000000000 if X86_64
> 
> This looks like a singularly bad pointer value on x86-64.
> 
> Why not pick something that is *guaranteed* to fault? The above looks 
> like any future setup that supports 41 bits of addressing and has 
> extended the page tables (yes, it will happen eventually) will find 
> that to be a perfectly valid address?
> 
> It's also visually confusing, since it's visually very close to a real 
> kernel pointer too.
> 
> Grr.
> 
> 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.

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.

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 ?

in any case, please ignore this topic until it's all worked out - no 
other topics depend on this one so it can be skipped safely.

	Ingo
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