linux-ext4 - Re: [PATCH 4/7] ext4: fsync should wait for DIO writers

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Message-ID: <874nn25gqi.fsf@openvz.org>
Date:	Thu, 13 Sep 2012 15:01:25 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Zheng Liu <gnehzuil.liu@...il.com>
Cc:	linux-ext4@...r.kernel.org, tytso@....edu, jack@...e.cz,
	wenqing.lz@...bao.com
Subject: Re: [PATCH 4/7] ext4: fsync should wait for DIO writers

On Thu, 13 Sep 2012 18:46:21 +0800, Zheng Liu <gnehzuil.liu@...il.com> wrote:
> On Sun, Sep 09, 2012 at 09:27:11PM +0400, Dmitry Monakhov wrote:
> > fsync and punch_hole are the places where we have to wait for all
> > existing writers (writeback, aio, dio), but currently we simply
> > flush pended end_io request which is not sufficient.
> > Even more i_mutex is not holded while punch_hole which obviously
> > result in dangerous data corruption due to write-after-free.
> 
> Hi Dmitry,
> 
> Lukas already has a patch to take i_mutex locking before punching a
> hole.  Just a reminding. :-)
Yes, i've found it after patch was submitted, but this bug make me
nervous a bit because we have broken punch_hole implementation
long time ago, it allow to destroy data easily, user are able to
call it if has WR permission for a file.
So if you ask be i'll vote for hide it under CAP_SYS_RESOURCE until
proper implementation appears.
Same it true for EXT4_IO_MOVE_EXT because it allow to kernel panic
since v2.6.30-6558-g748de67, so all primary distros (RH6,Deb6) are
affected :( 

> 
> https://patchwork.kernel.org/patch/1247271/
> 
> Regards,
> Zheng
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