linux-ext4 - Re: [PATCH 36/74] libext2fs: detect

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Date:	Wed, 15 Jan 2014 13:00:09 -0800
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 36/74] libext2fs: detect

On Thu, Dec 12, 2013 at 11:39:38PM -0500, Theodore Ts'o wrote:
> It looks like the subject line for this commit was truncated --- I
> could try to make something up, but could you suggest something?

I have no recollection if I ever sent a reply to this, but the subject line
should be:

"libext2fs: detect superblock when jumping ahead trying to read gdt"

--D
> 
> Thanks!!
> 
> 				- Ted
> 
> 
> On Tue, Dec 10, 2013 at 05:22:20PM -0800, Darrick J. Wong wrote:
> > If ext2fs_descriptor_block_loc2() is called with a meta_bg filesystem
> > and group_block is not the normal value, the function will return the
> > location of the backup group descriptor block in the next block group.
> > Unfortunately, it fails to account for the possibility that the backup
> > group contains a backup superblock but the regular superblock does
> > not.  This is the case with block groups 48-49 on a meta_bg fs with 1k
> > blocks; in this case, libext2fs will fail to open the filesystem.
> > 
> > Therefore, teach the function to adjust for superblocks in the backup
> > group, if necessary.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> > ---
> >  lib/ext2fs/openfs.c |   19 +++++++++++++++----
> >  1 file changed, 15 insertions(+), 4 deletions(-)
> > 
> > 
> > diff --git a/lib/ext2fs/openfs.c b/lib/ext2fs/openfs.c
> > index b2a8abb..92d9e40 100644
> > --- a/lib/ext2fs/openfs.c
> > +++ b/lib/ext2fs/openfs.c
> > @@ -47,7 +47,7 @@ blk64_t ext2fs_descriptor_block_loc2(ext2_filsys fs, blk64_t group_block,
> >  	bg = EXT2_DESC_PER_BLOCK(fs->super) * i;
> >  	if (ext2fs_bg_has_super(fs, bg))
> >  		has_super = 1;
> > -	ret_blk = ext2fs_group_first_block2(fs, bg) + has_super;
> > +	ret_blk = ext2fs_group_first_block2(fs, bg);
> >  	/*
> >  	 * If group_block is not the normal value, we're trying to use
> >  	 * the backup group descriptors and superblock --- so use the
> > @@ -57,10 +57,21 @@ blk64_t ext2fs_descriptor_block_loc2(ext2_filsys fs, blk64_t group_block,
> >  	 * have the infrastructure in place to do that.
> >  	 */
> >  	if (group_block != fs->super->s_first_data_block &&
> > -	    ((ret_blk + fs->super->s_blocks_per_group) <
> > -	     ext2fs_blocks_count(fs->super)))
> > +	    ((ret_blk + has_super + fs->super->s_blocks_per_group) <
> > +	     ext2fs_blocks_count(fs->super))) {
> >  		ret_blk += fs->super->s_blocks_per_group;
> > -	return ret_blk;
> > +
> > +		/*
> > +		 * If we're going to jump forward a block group, make sure
> > +		 * that we adjust has_super to account for the next group's
> > +		 * backup superblock (or lack thereof).
> > +		 */
> > +		if (ext2fs_bg_has_super(fs, bg + 1))
> > +			has_super = 1;
> > +		else
> > +			has_super = 0;
> > +	}
> > +	return ret_blk + has_super;
> >  }
> >  
> >  blk_t ext2fs_descriptor_block_loc(ext2_filsys fs, blk_t group_block, dgrp_t i)
> > 
> --
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