linux-ext4 - Re: [PATCH] ext4: Always verify extent tree blocks

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Date:	Thu, 11 Aug 2011 15:14:28 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	linux-ext4 List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: Always verify extent tree blocks

On Thu, Aug 11, 2011 at 03:33:38PM -0600, Andreas Dilger wrote:
> On 2011-08-11, at 3:13 PM, Darrick J. Wong wrote:
> > It turns out that ext4_ext_check only verifies the validity of the extent block
> > it's processing if the block has to be read in from the disk.  Unfortunately,
> > this means that the check is NOT done if the block is already in memory, which
> > means that if a file has a corrupted extent block, then the first IO peformed
> > on the file will find the corrupt block and fail, but a second IO will see that
> > the extent block is in memory, bypass the corruption check, and use garbage
> > data as if they were extent data.
> 
> It looks like ext4_ext_check() is fairly heavyweight, so calling it on every
> extent access may affect performance.  What about marking the extent or buffer

<shrug> I didn't think the simple header check was too terribly heavy...

... but it'll get more heavyweight when you add in metadata checksumming. :)

> bad in some way so that it always gets checked?  In the ext2 directory code
> it marks a directory page with PG_checked to indicate that it was validated
> on read, but there could be a number of different mechanisms to do this
> (including setting a bit in the magic so that ext4_ext_check() is aborted
> very quickly, possibly without any additional error on the console since
> one would already have been printed).

Ok, I guess I could add a BH_Checked = BH_JBDPrivateStart flag to ext4 and use
that to bypass the header check (and especially the checksum check) if it's
set.  Yes, I like that idea more... :)

Come to think of it I could probably reuse this in other places like the
directory handling code.  Okay, I'll roll that in.

--D
> 
> > A simple testcase is to allocate a file with enough extents to overflow the
> > inode i_block, umount, overwrite the extent block magic with garbage, then
> > mount the filesystem and try to access the file.  The first access causes the
> > kernel to spit out an error, but subsequent accesses seem to succeed.
> > 
> > Signed-off-by: Darrick J. Wong <djwong@...ibm.com>
> > ---
> > 
> > fs/ext4/extents.c |    6 +-----
> > 1 files changed, 1 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index ee4b391..bb07b79 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -744,8 +744,6 @@ ext4_ext_find_extent(struct inode *inode, ext4_lblk_t block,
> > 	i = depth;
> > 	/* walk through the tree */
> > 	while (i) {
> > -		int need_to_validate = 0;
> > -
> > 		ext_debug("depth %d: num %d, max %d\n",
> > 			  ppos, le16_to_cpu(eh->eh_entries), le16_to_cpu(eh->eh_max));
> > 
> > @@ -764,8 +762,6 @@ ext4_ext_find_extent(struct inode *inode, ext4_lblk_t block,
> > 				put_bh(bh);
> > 				goto err;
> > 			}
> > -			/* validate the extent entries */
> > -			need_to_validate = 1;
> > 		}
> > 		eh = ext_block_hdr(bh);
> > 		ppos++;
> > @@ -779,7 +775,7 @@ ext4_ext_find_extent(struct inode *inode, ext4_lblk_t block,
> > 		path[ppos].p_hdr = eh;
> > 		i--;
> > 
> > -		if (need_to_validate && ext4_ext_check(inode, eh, i))
> > +		if (ext4_ext_check(inode, eh, i))
> > 			goto err;
> > 	}
> > 
> > --
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> 
> 
> Cheers, Andreas
> 
> 
> 
> 
> 
> --
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