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

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Date:	Thu, 13 Sep 2012 20:36:32 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
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, Sep 13, 2012 at 03:01:25PM +0400, Dmitry Monakhov wrote:
> 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 :( 

IMHO, it will be better when this patch is applied, and I prefer to fix
the problem rather than hidding it using CAP_SYS_RESOURCE. ;-)

Regards,
Zheng
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