Re: [PATCH 2/2] ext4: Spot-check block group sub-table locations

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Sun, Sep 29, 2013 at 02:44:01PM +0800, Zheng Liu wrote:
> On Fri, Sep 27, 2013 at 05:14:04PM -0700, Darrick J. Wong wrote:
> > Perform a quick sanity check of bitmap and inode table block numbers when
> > loading them, and if there's something suspicious, mark the block group
> > corrupt.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> 
> One comment below.  Otherwise the patch looks good to me.
> Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>
> 
> > ---
> >  fs/ext4/balloc.c |    7 +++++++
> >  fs/ext4/ext4.h   |    2 ++
> >  fs/ext4/ialloc.c |    6 ++++++
> >  fs/ext4/inode.c  |    9 +++++++++
> >  fs/ext4/super.c  |   21 +++++++++++++++++++++
> >  5 files changed, 45 insertions(+)
> [...]
> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> > index 2c2e6cb..d22248e 100644
> > --- a/fs/ext4/super.c
> > +++ b/fs/ext4/super.c
> > @@ -2049,6 +2049,27 @@ void ext4_group_desc_csum_set(struct super_block *sb, __u32 block_group,
> >  	gdp->bg_checksum = ext4_group_desc_csum(EXT4_SB(sb), block_group, gdp);
> >  }
> >  
> > +/* returns 1 if the location of a blockgroup data item seems sane */
> > +int ext4_is_sane_bgdata_location(struct super_block *sb, ext4_group_t grp,
> > +				 ext4_fsblk_t blk)
> > +{
> > +	struct ext4_sb_info *sbi = EXT4_SB(sb);
> > +	ext4_fsblk_t first_block = le32_to_cpu(sbi->s_es->s_first_data_block);
> > +	ext4_fsblk_t last_block;
> > +
> > +	if (EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_FLEX_BG))
> > +		last_block = ext4_blocks_count(sbi->s_es) - 1;
> > +	else {
> > +		first_block += grp * sbi->s_blocks_per_group;
> > +		last_block = first_block + (EXT4_BLOCKS_PER_GROUP(sb) - 1);
> > +	}
> > +
> > +	if (last_block >= ext4_blocks_count(sbi->s_es))
> > +		last_block = ext4_blocks_count(sbi->s_es) - 1;
> > +
> > +	return blk >= first_block && blk <= last_block;
> > +}
> > +
> >  /* Called at mount-time, super-block is locked */
> >  static int ext4_check_descriptors(struct super_block *sb,
> >  				  ext4_group_t *first_not_zeroed)
> 
> FWIW, I notice that in ext4_check_descriptors() we also check the
> location of bitmap.  So maybe we could use ext4_is_sane_bgdata_location
> in that function.

That could be done, but all we'd be doing is trading this:

	block_bitmap = ext4_block_bitmap(sb, gdp);
	if (block_bitmap < first_block || block_bitmap > last_block) {

for this slightly slower code:

	if (ext4_is_sane_bgdata_location(sb, gdp,
					 ext4_block_bitmap(sb, gdb))) {

I don't know if that's worth the code churn or not.  It's an easy change, but I
don't think it adds much.  Ted -- if you want me to make the change I'll do it,
but if you're content to leave the patch as it is, then so am I.

--D
> 
>                                                 - Zheng