linux-kernel - Re: [PATCH][RESEND] mm: Fix off-by-one bug in print_nodes

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Message-ID: <alpine.DEB.2.00.1112181439500.1364@chino.kir.corp.google.com>
Date:	Sun, 18 Dec 2011 14:44:24 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Ryota Ozaki <ozaki.ryota@...il.com>
cc:	linux-kernel@...r.kernel.org, Greg Kroah-Hartman <gregkh@...e.de>,
	linux-mm@...ck.org, stable@...nel.org
Subject: Re: [PATCH][RESEND] mm: Fix off-by-one bug in print_nodes_state

On Sun, 18 Dec 2011, Ryota Ozaki wrote:

> /sys/devices/system/node/{online,possible} involve a garbage byte
> because print_nodes_state returns content size + 1. To fix the bug,
> the patch changes the use of cpuset_sprintf_cpulist to follow the
> use at other places, which is clearer and safer.
> 

It's not a garbage byte, sysdev files use a buffer created with 
get_zeroed_page(), so extra byte is guaranteed to be zero since 
nodelist_scnprintf() won't write to it.  So the issue here is that 
print_nodes_state() returns a size that is off by one according to 
ISO C99 although it won't cause a problem in practice.

> This bug was introduced since v2.6.24.
> 

It's not a bug, the result of a 4-node system would be "0-3\n\0" and 
returns 5 correctly.  You can verify this very simply with strace.
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