linux-kernel - Re: [PATCH] [5/12] x86_64: Make patching more robust, fix paravirt issue

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Date:	Mon, 20 Aug 2007 11:50:28 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andi Kleen <ak@...e.de>, patches@...-64.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH] [5/12] x86_64: Make patching more robust, fix paravirt
	issue

On Fri, 2007-08-17 at 17:05 -0700, Jeremy Fitzhardinge wrote:
> Andi Kleen wrote:
> > Commit 19d36ccdc34f5ed444f8a6af0cbfdb6790eb1177 "x86: Fix alternatives
> > and kprobes to remap write-protected kernel text" uses code which is
> > being patched for patching.
> >
> > In particular, paravirt_ops does patching in two stages: first it
> > calls paravirt_ops.patch, then it fills any remaining instructions
> > with nop_out().  nop_out calls text_poke() which calls
> > lookup_address() which calls pgd_val() (aka paravirt_ops.pgd_val):
> > that call site is one of the places we patch.
> >
> > If we always do patching as one single call to text_poke(), we only
> > need make sure we're not patching the memcpy in text_poke itself.
> > This means the prototype to paravirt_ops.patch needs to change, to
> > marshal the new code into a buffer rather than patching in place as it
> > does now.  It also means all patching goes through text_poke(), which
> > is known to be safe (apply_alternatives is also changed to make a
> > single patch).
> >   
> 
> Hi Andi,
> 
> This patch breaks Xen booting.  I get infinite recursive faults during
> patching when this patch is present.  If I boot with
> "noreplace-paravirt" it works OK, and it works as expected if I back
> this patch out.  I haven't tracked down the exact failure mode; its a
> little hard to debug because it overwrites all kernel memory with
> recursive fault stackframes and then finally traps out to Xen when it
> hits the bottom of memory.
> 
> I think we should back this one out before .23.

Then back out 19d36ccdc34f5ed444f8a6af0cbfdb6790eb1177 too, which broke
lguest booting, and this tried to fix.

Rusty.

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