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

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Message-ID: <4EEE6DC0.2030007@gmail.com>
Date:	Sun, 18 Dec 2011 17:48:32 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Ryota Ozaki <ozaki.ryota@...il.com>, 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

(12/18/11 5:44 PM), David Rientjes wrote:
> 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.

Usually, /sys files don't output trailing '¥0'. And, '¥0' is not regular
io friendly. So I can imagine some careless programmer think it is 
garbage. Is there any benefit to show trailing '¥0'?

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