linux-kernel - Re: 2.6.33-rc2: Xen/Guest switching to user mode with no user page tables

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Date:	Sat, 9 Jan 2010 00:50:39 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Christian Kujau <lists@...dbynature.de>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>, brgerst@...il.com
Subject: Re: 2.6.33-rc2: Xen/Guest switching to user mode with no user page
	tables

On Thu, Jan 07, 2010 at 11:30:46AM -0800, Christian Kujau wrote:
> On Thu, 7 Jan 2010 at 11:19, H. Peter Anvin wrote:
> > The big difference between the code before and after this commit is that
> > before, kernel_thread() would initialize the pt_regs structure with
> > whatever state happened to be passed into it by the caller, whereas
> > afterwards it is initialized to zero.
> 
> To be honest, bisection was kinda hazy in the last step (see my previous 
> mails), but from looking at the bisection log, it's definitely one of 
> your/Brians commit (sorry!), so it may be 3bd95dfb in combination with the 
> other 4 changes. However, only with 3bd95dfb applied, the DomU wouldn't 
> start at all. With the only other patches applied, the DomU would start, 
> and then die with a GPF.
> 
> Christian.
> -- 
> BOFH excuse #191:
> 
> Just type 'mv * /dev/null'.
> 

OK, perhaps the patch below is not _that_ stupid so I
would like to get it reviewed and tested if possible.
Just a thought. Wonder if it help but definitely it will
not harm anyway :)

	-- Cyrill
---
x86: kernel_thread -- initialize SS to a known state

Before the kernel_thread was converted into "C" we had
pt_regs::ss set to __KERNEL_DS (by SAVE_ALL asm macro).

Though I must admit I didn't find any *explicit* load of
%ss from this structure the better to be on a safe side
and set it to a known value.

Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
---
 arch/x86/kernel/process.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-2.6.git/arch/x86/kernel/process.c
=====================================================================
--- linux-2.6.git.orig/arch/x86/kernel/process.c
+++ linux-2.6.git/arch/x86/kernel/process.c
@@ -288,6 +288,8 @@ int kernel_thread(int (*fn)(void *), voi
 	regs.es = __USER_DS;
 	regs.fs = __KERNEL_PERCPU;
 	regs.gs = __KERNEL_STACK_CANARY;
+#else
+	regs.ss = __KERNEL_DS;
 #endif
 
 	regs.orig_ax = -1;
--
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