linux-kernel - Re: [RFC][PATCH] vfs: always protect diretory file->fpos with inode mutex

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Date:	Wed, 20 Feb 2013 09:49:36 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Jan Kara <jack@...e.cz>
CC:	<linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<andi@...stfloor.org>, Wuqixuan <wuqixuan@...wei.com>,
	Al Viro <viro@...IV.linux.org.uk>, <gregkh@...uxfoundation.org>
Subject: Re: [RFC][PATCH] vfs: always protect diretory file->fpos with inode
 mutex

On 2013/2/19 20:59, Jan Kara wrote:
> On Tue 19-02-13 19:47:30, Li Zefan wrote:
>> On 2013/2/19 17:19, Jan Kara wrote:
>>> On Tue 19-02-13 09:22:40, Li Zefan wrote:
>>>> There's a long long-standing bug...As long as I don't know when it dates
>>>> from.
>>>>
>>>> I've written and attached a simple program to reproduce this bug, and it can
>>>> immediately trigger the bug in my box. It uses two threads, one keeps calling
>>>> read(), and the other calling readdir(), both on the same directory fd.
>>>   So the fact that read() or even write() to fd opened O_RDONLY has *any*
>>> effect on f_pos looks really unexpected to me. I think we really should
>>> have there:
>>> 	if (ret >= 0)
>>> 		file_pos_write(...);
>>
>> I thought about this. The problem is then we have to check every fop->write()
>> to see if any of them can return -errno with file->f_pos changed and fix them,
>> though it's do-able.
>   But returning error and advancing f_pos would be a bug - specification
> says write() returns the number of bytes written or -1 and f_pos should be
> advanced by the number of bytes written.
> 

Oh, I had an illusion that vfs saves f_pos and calls write() and restore f_pos
if write() fails.

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