linux-kernel - Re: [PATCH 6/6] vfs: introduce FMODE_NEG_OFFSET for allowing negative f

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Date:	Mon, 18 Jan 2010 10:17:48 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Eric Paris <eparis@...hat.com>, Nick Piggin <npiggin@...e.de>,
	Andi Kleen <andi@...stfloor.org>,
	David Howells <dhowells@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	<linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 6/6] vfs: introduce FMODE_NEG_OFFSET for allowing negative f_pos

KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:

>> > +static int
>> > +__negative_fpos_check(struct file *file, loff_t pos, size_t count)
>> > +{
>> > +	/*
>> > +	 * pos or pos+count is negative here, check overflow.
>> > +	 * too big "count" will be caught in rw_verify_area().
>> > +	 */
>> > +	if ((pos < 0) && (pos + count < pos))
>> > +		return -EOVERFLOW;
>> > +	if (file->f_mode & FMODE_NEG_OFFSET)
>> > +		return 0;
>> > +	return -EINVAL;
>> > +}
>> > +
>> >  /*
>> >   * rw_verify_area doesn't like huge counts. We limit
>> >   * them to something that fits in "int" so that others
>> > @@ -222,8 +236,11 @@ int rw_verify_area(int read_write, struc
>> >  	if (unlikely((ssize_t) count < 0))
>> >  		return retval;
>> >  	pos = *ppos;
>> > -	if (unlikely((pos < 0) || (loff_t) (pos + count) < 0))
>> > -		return retval;
>> > +	if (unlikely((pos < 0) || (loff_t) (pos + count) < 0)) {
>> > +		retval = __negative_fpos_check(file, pos, count);
>> > +		if (retval)
>> > +			return retval;
>> > +	}
>> >  
>> >  	if (unlikely(inode->i_flock && mandatory_lock(inode))) {
>> >  		retval = locks_mandatory_area(
>> 
>> Um... How do lseek() work? It sounds like to violate error code range.
>
> This is for read-write. As far as I know, 
>   - generic_file_llseek,
>   - default_llseek
>   - no_llseek
>
> doesn't call this function. 

It seems to allow to set negative value to ->f_pos, right? So, lseek()
returns (uses) it?

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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