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Date: Sat, 18 Apr 2009 09:54:58 -0400 (EDT)
From: Steven Rostedt <rostedt@goodmis.org>
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To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
cc: Peter Zijlstra <peterz@infradead.org>, LKML <linux-kernel@vger.kernel.org>,
       Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Masami Hiramatsu <mhiramat@redhat.com>
Subject: Re: [PATCH] x86 entry_64.S lockdep fix
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On Fri, 17 Apr 2009, Mathieu Desnoyers wrote:
> 
> I happened to have the following patch hanging around in my LTTng tree
> for a while. Would it solve your problem by any chance ? I had to move
> it a bit around in my patchset to put it before the nmi-safe int3
> handler patch I have, but it should apply correctly.
> 
> 
> x86 entry_64.S lockdep fix
> 
> Add missing lockdep irq on instrumentation to entry_64.S.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> ---
>  arch/x86/kernel/entry_64.S |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux-2.6-lttng/arch/x86/kernel/entry_64.S
> ===================================================================
> --- linux-2.6-lttng.orig/arch/x86/kernel/entry_64.S	2009-04-17 17:44:18.000000000 -0400
> +++ linux-2.6-lttng/arch/x86/kernel/entry_64.S	2009-04-17 17:53:42.000000000 -0400
> @@ -1420,9 +1420,9 @@ ENTRY(paranoid_exit)
>  	testl $3,CS(%rsp)
>  	jnz   paranoid_userspace
>  paranoid_swapgs:
> -	TRACE_IRQS_IRETQ 0
>  	SWAPGS_UNSAFE_STACK
>  paranoid_restore:
> +	TRACE_IRQS_IRETQ 0

This is buggy. If you go here via userspace, you just did a swapgs, and 
the %gs register (process context) is now zero. If you call kernel code 
that does anything with "current" you will crash the system.

-- Steve


>  	RESTORE_ALL 8
>  	jmp irq_return
>  paranoid_userspace:
> 
> -- 
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
>