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

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Date:	Mon, 18 Jan 2010 11:13:04 +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:

> 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?

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