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Re: [ 68/89] xfs: fix _xfs_buf_find oops on blocks beyond the filesystem

To: Ben Myers <bpm@xxxxxxx>
Subject: Re: [ 68/89] xfs: fix _xfs_buf_find oops on blocks beyond the filesystem end
From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Date: Thu, 14 Feb 2013 12:05:01 -0800
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, stable@xxxxxxxxxxxxxxx, Dave Chinner <dchinner@xxxxxxxxxx>, Brian Foster <bfoster@xxxxxxxxxx>, CAI Qian <caiqian@xxxxxxxxxx>, xfs@xxxxxxxxxxx
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On Thu, Feb 14, 2013 at 01:55:12PM -0600, Ben Myers wrote:
> Greg,
> 
> On Thu, Feb 14, 2013 at 11:26:14AM -0800, Greg Kroah-Hartman wrote:
> > On Thu, Feb 14, 2013 at 11:07:30AM +1100, Dave Chinner wrote:
> > > [cc xfs@xxxxxxxxxxx]
> > > 
> > > On Wed, Feb 13, 2013 at 08:18:45AM -0800, Greg Kroah-Hartman wrote:
> > > > On Wed, Feb 13, 2013 at 04:30:32PM +0100, Paolo Bonzini wrote:
> > > > > Il 01/02/2013 14:08, Greg Kroah-Hartman ha scritto:
> > > > > > 3.7-stable review patch.  If anyone has any objections, please let 
> > > > > > me know.
> > > > > > 
> > > > > > ------------------
> > > > > > 
> > > > > > From: Dave Chinner <dchinner@xxxxxxxxxx>
> > > > > > 
> > > > > > commit eb178619f930fa2ba2348de332a1ff1c66a31424 upstream.
> > > > > > 
> > > > > > When _xfs_buf_find is passed an out of range address, it will fail
> > > > > > to find a relevant struct xfs_perag and oops with a null
> > > > > > dereference. This can happen when trying to walk a filesystem with a
> > > > > > metadata inode that has a partially corrupted extent map (i.e. the
> > > > > > block number returned is corrupt, but is otherwise intact) and we
> > > > > > try to read from the corrupted block address.
> > > > > > 
> > > > > > In this case, just fail the lookup. If it is readahead being issued,
> > > > > > it will simply not be done, but if it is real read that fails we
> > > > > > will get an error being reported.  Ideally this case should result
> > > > > > in an EFSCORRUPTED error being reported, but we cannot return an
> > > > > > error through xfs_buf_read() or xfs_buf_get() so this lookup failure
> > > > > > may result in ENOMEM or EIO errors being reported instead.
> > > > > 
> > > > > It looks like this breaks xfs_growfs.  See
> > > > > http://bugzilla.redhat.com/show_bug.cgi?id=909602.
> > > 
> > > Entirely possible, as the filesystem size is not updated until after
> > > all the new metadata is written to disk. in 3.8, there's this commit:
> > > 
> > > commit fd23683c3b1ab905cba61ea2981c156f4bf52845
> > > Author: Dave Chinner <dchinner@xxxxxxxxxx>
> > > Date:   Mon Nov 12 22:53:59 2012 +1100
> > > 
> > >     xfs: growfs: use uncached buffers for new headers
> > >     
> > >     When writing the new AG headers to disk, we can't attach write
> > >     verifiers because they have a dependency on the struct xfs-perag
> > >     being attached to the buffer to be fully initialised and growfs
> > >     can't fully initialise them until later in the process.
> > >     
> > >     The simplest way to avoid this problem is to use uncached buffers
> > >     for writing the new headers. These buffers don't have the xfs-perag
> > >     attached to them, so it's simple to detect in the write verifier and
> > >     be able to skip the checks that need the xfs-perag.
> > >     
> > >     This enables us to attach the appropriate buffer ops to the buffer
> > >     and henc calculate CRCs on the way to disk. IT also means that the
> > >     buffer is torn down immediately, and so the first access to the AG
> > >     headers will re-read the header from disk and perform full
> > >     verification of the buffer. This way we also can catch corruptions
> > >     due to problems that went undetected in growfs.
> > >     
> > >     Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> > >     Reviewed-by Rich Johnston <rjohnston@xxxxxxx>
> > >     Signed-off-by: Ben Myers <bpm@xxxxxxx>
> > > 
> > > As part of the metadata verifier feature. It means that growfs no
> > > longer uses cached buffers, and hence does not pass through
> > > _xfs_buf_find() and hence will not trigger the beyond-EOFS that the
> > > above commit adds.
> > > 
> > > > Ick, not good.
> > > > 
> > > > Dave, any thoughts here?  Should I drop this from the 3.7-stable queue?
> > > 
> > > Yeah, drop it.
> > > 
> > > But what I'm now wondering is how this patch got proposed for
> > > 3.7-stable. I don't recall seeing anything about this being
> > > proposed.
> > > 
> > > <trolls email archives>
> > > 
> > > Oh, it happened while I was at LCA and didn't have any access to Red
> > > Hat email and there was a private thread about it. By the time I
> > > read it the stable kernel was already released and so it immediately
> > > dropped from my attention.
> > > 
> > > XFS Maintainers: Major process fail. Patches that are being proposed
> > > for backports need to be posted to the XFS list, reviewed and tested
> > > before saying they are OK to go.  We have several growfs tests in
> > > xfstests would have failed if this was actually tested.
> > > 
> > > Stable folk: This is the reason why I, quite frankly, don't want to
> > > support stable kernels *at all*. The overhead of backporting and
> > > testing a patch to a single kernel target to ensure there are no
> > > unintended regressions is significant, and there are so many stable
> > > kernels no it's just a waste of developer time to try to support
> > > them. And in this case, the process simply wasn't executed and an
> > > unintended regression that is >this close< to causing filesystem
> > > corruption slipped through to the stable series.....
> > 
> > Ok, how about I never apply any xfs stable kernel patch, unless you send
> > it to stable@xxxxxxxxxxxxxxx?
> 
> Dave has made it clear that he doesn't want to be involved in maintaining
> -stable kernels.  However, my team at SGI is interested in maintaining -stable
> kernels.  We're not going to use the fact that there is a risk of regression 
> as
> an excuse to starve -stable of relevant fixes, just as we do not use it as an
> excuse to starve the upstream branch of feature content.
> 
> > I have that rule in place for some other subsystems that don't want me
> > applying stuff that they aren't aware of, and have no problem doing the same
> > thing here.
> > 
> > Just let me know.
> 
> Here are the usual suspects:
> 
> Ben Myers <bpm@xxxxxxx>
> Mark Tinguely <tinguely@xxxxxxx>
> Dave Chinner <dchinner@xxxxxxxxxx>
> Eric Sandeen <sandeen@xxxxxxxxxx>

Ok, but for this specific patch, did I do something wrong in taking it?

I guess I'll just let you send me xfs patches, is that ok with everyone
else?  Dave can just ignore them, especially given redhat's horrible
email system :)

thanks,

greg k-h

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