linux-kernel - Re: [PATCH] block:Remove extra condition in end of disk check

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Date:	Tue, 25 Jun 2013 10:40:14 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
CC:	LKML <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...l.org>,
	Jens Axboe <axboe@...nel.dk>,
	Kiyoshi Ueda <k-ueda@...jp.nec.com>,
	Lin Ming <ming.m.lin@...el.com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] block:Remove extra condition in end of disk check

On 06/25/2013 04:21 AM, Tejun Heo wrote:
> On Mon, Jun 24, 2013 at 07:20:12PM +0530, Raghavendra K T wrote:
>>> @@ -1656,7 +1656,7 @@ static inline int bio_check_eod(struct bio *bio, unsigned int nr_sectors)
>>>   	if (maxsector) {
>>>   		sector_t sector = bio->bi_sector;
>>>
>>> -		if (maxsector < nr_sectors || maxsector - nr_sectors < sector) {
>>> +		if (maxsector - nr_sectors < sector) {
>
> If maxsector < nr_sectors, the subtraction will underflow making it a
> very large number and fail to detect the invalid condition, no?
>

Hi Tejun,
Thanks for the reply and explanation. You are right. underflow results
in invalid condition.

Considering maxsector and sectors are unsigned long, and nr_sector is
unsigned int, probably safer bet is
(max_sector < sector + nr_sector), but still it would leave scope for 
overflow.

Thanks again,
Raghu.

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