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

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Date:	Sun, 18 Dec 2011 16:28:40 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.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

(12/18/11 6:58 AM), 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.
> 
> This bug was introduced since v2.6.24.
> 
> Signed-off-by: Ryota Ozaki<ozaki.ryota@...il.com>
> ---
>   drivers/base/node.c |    8 +++-----
>   1 files changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 5693ece..ef7c1f9 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -587,11 +587,9 @@ static ssize_t print_nodes_state(enum node_states state, char *buf)
>   {
>   	int n;
> 
> -	n = nodelist_scnprintf(buf, PAGE_SIZE, node_states[state]);
> -	if (n>  0&&  PAGE_SIZE>  n + 1) {
> -		*(buf + n++) = '\n';
> -		*(buf + n++) = '\0';
> -	}
> +	n = nodelist_scnprintf(buf, PAGE_SIZE-2, node_states[state]);

PAGE_SIZE-1. This seems another off by one. buf[n++] = '¥n' mean
override old trailing '¥0' and buf[n] = '¥0' mean to append one byte.
Then totally, we append one byte.

> +	buf[n++] = '\n';
> +	buf[n] = '\0';
>   	return n;
>   }
> 

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