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

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Date:	Mon, 18 Jan 2010 10:25:44 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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

On Mon, 18 Jan 2010 10:17:48 +0900
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp> wrote:

> 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?
yes. Some file (/dev/kmem) requires that. 

> So, lseek() returns (uses) it?

lseek can return negative value, as far as I know.

Thanks,
-Kame

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