linux-kernel - Re: [PATCH v3 1/1] vfs: iversion truncate bug fix

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Date:	Thu, 05 Jan 2012 14:49:54 -0500
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Dave Chinner <david@...morbit.com>, Greg KH <greg@...ah.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Kasatkin <dmitry.kasatkin@...el.com>,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, viro@...iv.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/1] vfs: iversion truncate bug fix

On Thu, 2012-01-05 at 13:14 -0500, Christoph Hellwig wrote:
> On Thu, Jan 05, 2012 at 03:53:54PM +1100, Dave Chinner wrote:
> > That's seems like a rather unreliable way of detecting that a file
> > has changed to me.  I mean, only ext4 uses inode_inc_version() when
> > it internally dirties an inode, and only ext4 sets the MS_I_VERSION
> > so that inode_inc_version is only called for ext4 inodes when
> > timestamps change.
> 
> And even ext4 only does it when using the non-default "i_version"
> mount option.
> 
> > Hence just adding an increment to the truncate case doesn't seem to
> > be sufficient to me. e.g. what about the equivalent case of having a
> > hole punched in the file via fallocate? The file has definitely
> > changed, but i_version won't change....
> > 
> > Perhaps bumping i_version in __mark_inode_dirty() might be the best
> > way to capture all changes (other than timestamp updates) to any
> > inode regardless of the filesystem type?
> 
> It has the same problem as the timestamp updates doing that right now -
> the fs can't do locking around it, and it can't return errors.  That's
> something affecting at least btrfs, xfs and IIRC ubifs, and probably
> the cluster filesystems as well.  The right answer is to replace the
> timestmap updates which are the only places doing that with a method
> as Josef had planned to do, and then we can include the i_version
> updates in there.
> 
> That assumes we figure out a coherent way to do it - except for the
> conditional abuse in file_updates_times it's currently entirely under
> fs control.  So the best way to fix it would be to:
> 
>  - move the fs-private use into those filesystems actually using it.
>    Note that a lot less actually check for it rather than just updating
>    it based on some cargo cult, and most only do so for directories.
>  - figure a why what exact change count semantics NFS (and IMA) want
>    and find a way to implement them so that the fs can tell the callers
>    that they don't exist.
> 
> Btw, does IMA care about these beeing persistent?

By 'persistent' I assume you mean across boots.  IMA (and IMA-appraisal)
measure and appraise files the first time they're accessed/executed.  So
no, it does not need to be persistent. IMA/IMA-appraisal just need some
way to detect file change in order to know whether the file needs to be
re-measured/appraised on subsequent access.

Mimi

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