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

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Message-Id: <20100118113026.4c6efd26.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Mon, 18 Jan 2010 11:30:26 +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 11:13:04 +0900
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp> wrote:

> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:
> 
> > On Mon, 18 Jan 2010 10:38:27 +0900
> > OGAWA Hirofumi <hirofumi@...l.parknet.co.jp> wrote:
> >
> >> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:
> >> 
> >> >> So, lseek() returns (uses) it?
> >> >
> >> > lseek can return negative value, as far as I know.
> >> 
> >> Umm..., how do you know the difference of -EOVERFLOW and fpos == -75? 
> >> 
> >
> > Ah, sorry. I read wrong.
> >
> > For /dev/mem, it uses its own lseek function which allows negative f_pos
> > value. Other usual file system doesn't allow negative f_pos.
> >
> > It's ok not to return -EOVEFLOW for /dev/mem because there is no file end.
> 
> No, no. I think it has the problem.
> 
> E.g. /dev/mem returns -75 as fpos, so, lseek(2) returns -75 to
> userland. Then the userland (e.g. glibc) convert it as
> error. I.e. finally, errno == -75, and lseek(3) returns -1, right?
> 
Maybe possible.

Hmm. Then, /dev/mem's llseek need some fix not to return pos < -PAGESIZE.
Wu-san, could you add additional bug fix to lseek()'s f_pos handling in
/dev/mem ?

Thanks,
-Kame


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