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Date:	Wed, 4 Jan 2012 16:56:59 -0700
From:	Andreas Dilger <adilger.kernel@...ger.ca>
To:	Jan Kara <jack@...e.cz>
Cc:	Djalal Harouni <tixxdz@...ndz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Darrick J. Wong" <djwong@...ibm.com>,
	Theodore Ts'o <tytso@....edu>,
	Yongqiang Yang <xiaoqiangnk@...il.com>,
	ext4 development <linux-ext4@...r.kernel.org>,
	linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] fs/ext{3,4}: fix potential race when setversion ioctl updates inode

On 2012-01-04, at 4:32 PM, Jan Kara wrote:
> On Wed 04-01-12 16:15:04, Andreas Dilger wrote:
>> On 2012-01-04, at 10:46 AM, Jan Kara wrote:
>>> On Tue 03-01-12 02:31:52, Djalal Harouni wrote:
>>>> 
>>>> The EXT{3,4}_IOC_SETVERSION ioctl() updates the inode without i_mutex,
>>>> this can lead to a race with the other operations that update the same
>>>> inode.
>>>> 
>>>> Patch tested.
>>> 
>>> OK, so I've taken the patch into my tree, just updated the changelog
>>> which result of our discussion in this thread. I also took the ext4 part
>>> since there is no risk of conflict and the patch looks obvious.
>> 
>> Actually, I'd like to hear more about whether this is a real problem, or
>> if it is just a theoretical problem found during code inspection or from
>> some static code analysis tool?
> 
> As far as I understood that was just a theoretical issue and I applied
> the patch just on the grounds that it is more consistent to use i_mutex for
> i_generation changes.
> 
>> With the metadata checksum feature we were discussing using the inode
>> generation as part of the seed for the directory leaf block checksum, so
>> that it wasn't possible to incorrectly access stale directory blocks from
>> a previous incarnation of the same inode number.
>> 
>> We were discussing just disabling this ioctl on filesystems with metadata
>> checksums, and printing a deprecation warning for filesystems without that
>> feature enabled.  I'm not aware of any real-world use for this ioctl, since
>> NFS cannot use it to reconstruct handles because there's no API to allocate
>> an inode with a specific number, so setting the generation is pointless.
> 
>  OK, I didn't know this. I'm fine with deprecating the ioctl if it's
> useless but since that's going to take a while I think the cleanup still
> makes some sense.

I'm not against landing the patch, and I agree that there is no question
about the performance impact of making this ioctl safe.  My real question
was whether there was a real-world use for this ioctl which might prevent
it from being deprecated.


Cheers, Andreas





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