linux-kernel - Re: [Xen-devel] [PATCH 9/9] xen/p2m/debugfs: Fix potential pointer exception.

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Date:	Fri, 30 Sep 2011 09:18:17 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Dan Carpenter <error27@...il.com>
Subject: Re: [Xen-devel] [PATCH 9/9] xen/p2m/debugfs: Fix potential pointer
 exception.

On Thu, 2011-09-29 at 20:52 +0100, Konrad Rzeszutek Wilk wrote:
> We could be referencing the last + 1 element of level_name[]
> array which would cause a pointer exception.

If we end up accessing it does that not mean something, i.e. should it
not be a real string here and not NULL? Otherwise isn't it a bug in the
lookup code that we end up looking there?

I think this lookup correspond to the initialisation of lvl=4 and
falling through the subsequent list of checks without matching one. In
which case I think level_name[4] should be "unknown" or even "error".

I don't think you can hit type_name[4] in the same way, type and
prev_type are always one of the TYPE_* defines, which have values 0..3
inclusive. You could make this more obvious and defend against future
changes breaking this with:
	... type_name[] = {
		[TYPE_IDENTITY] = "identity",
		[TYPE_MISSING] = "missing"
		...
	};

Ian.

> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  arch/x86/xen/p2m.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 58efeb9..bc4cf0a 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -786,9 +786,9 @@ EXPORT_SYMBOL_GPL(m2p_find_override_pfn);
>  int p2m_dump_show(struct seq_file *m, void *v)
>  {
>  	static const char * const level_name[] = { "top", "middle",
> -						"entry", "abnormal" };
> +						"entry", "abnormal", NULL};
>  	static const char * const type_name[] = { "identity", "missing",
> -						"pfn", "abnormal"};
> +						"pfn", "abnormal", NULL};
>  #define TYPE_IDENTITY 0
>  #define TYPE_MISSING 1
>  #define TYPE_PFN 2


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