Re: [PATCH 2/3] metadump: Only verify obfuscated metadata being dumped

[Eric Sandeen <sandeen@sandeen.net>]

On 2/28/14, 12:25 AM, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> The discontiguous buffer support series added a verifier check on
> the metadata buffers before they go written to the metadump image.
> If this failed, it returned an error, and the result would be that
> we stopped processing the metadata and exited, truncating the dump.
> 
> xfs_metadump is supposed to dump the metadata in the filesystem for
> forensic analysis purposes, which means we actually want it to
> retain any corruptions it finds in the filesystem. Hence running the
> verifier - even to recalculate CRCs - when the metadata is
> unmodified is the wrong thing to be doing. And stopping the dump
> when we come across an error is even worse.
> 
> We still need to do CRC recalculation when obfuscating names and
> attributes. Hence we need to make running the verifier conditional
> on the buffer or inode:
> 	a) being uncorrupted when read, and
> 	b) modified by the obfuscation code.
> 
> If either of these conditions is not true, then we don't run the
> verifier or recalculate the CRCs.

Lord how I hate "if (!dont_obfuscate)" but that's not your fault.  ;)

Reviewed-by: Eric Sandeen <sandeen@redhat.com>

> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  db/io.h       |  1 +
>  db/metadump.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++-------------
>  2 files changed, 49 insertions(+), 13 deletions(-)
> 
> diff --git a/db/io.h b/db/io.h
> index 4f24c83..d8cf383 100644
> --- a/db/io.h
> +++ b/db/io.h
> @@ -41,6 +41,7 @@ typedef struct iocur {
>  	int			ino_crc_ok:1;
>  	int			ino_buf:1;
>  	int			dquot_buf:1;
> +	int			need_crc:1;
>  } iocur_t;
>  
>  #define DB_RING_ADD 1                   /* add to ring on set_cur */
> diff --git a/db/metadump.c b/db/metadump.c
> index 14902a7..3248009 100644
> --- a/db/metadump.c
> +++ b/db/metadump.c
> @@ -190,26 +190,36 @@ write_buf_segment(
>  	return 0;
>  }
>  
> +/*
> + * we want to preserve the state of the metadata in the dump - whether it is
> + * intact or corrupt, so even if the buffer has a verifier attached to it we
> + * don't want to run it prior to writing the buffer to the metadump image.
> + *
> + * The only reason for running the verifier is to recalculate the CRCs on a
> + * buffer that has been obfuscated. i.e. a buffer than metadump modified itself.
> + * In this case, we only run the verifier if the buffer was not corrupt to begin
> + * with so that we don't accidentally correct buffers with CRC or errors in them
> + * when we are obfuscating them.
> + */
>  static int
>  write_buf(
>  	iocur_t		*buf)
>  {
> +	struct xfs_buf	*bp = buf->bp;
>  	int		i;
>  	int		ret;
>  
>  	/*
>  	 * Run the write verifier to recalculate the buffer CRCs and check
> -	 * we are writing something valid to disk
> +	 * metadump didn't introduce a new corruption. Warn if the verifier
> +	 * failed, but still continue to dump it into the output file.
>  	 */
> -	if (buf->bp && buf->bp->b_ops) {
> -		buf->bp->b_error = 0;
> -		buf->bp->b_ops->verify_write(buf->bp);
> -		if (buf->bp->b_error) {
> -			fprintf(stderr,
> -	_("%s: write verifer failed on bno 0x%llx/0x%x\n"),
> -				__func__, (long long)buf->bp->b_bn,
> -				buf->bp->b_bcount);
> -			return -buf->bp->b_error;
> +	if (buf->need_crc && bp && bp->b_ops && !bp->b_error) {
> +		bp->b_ops->verify_write(bp);
> +		if (bp->b_error) {
> +			print_warning(
> +				"obfuscation corrupted block at bno 0x%llx/0x%x",
> +				(long long)bp->b_bn, bp->b_bcount);
>  		}
>  	}
>  
> @@ -1359,12 +1369,15 @@ process_single_fsb_objects(
>  
>  			obfuscate_dir_data_block(dp, o,
>  						 last == mp->m_dirblkfsbs);
> +			iocur_top->need_crc = 1;
>  			break;
>  		case TYP_SYMLINK:
>  			obfuscate_symlink_block(dp);
> +			iocur_top->need_crc = 1;
>  			break;
>  		case TYP_ATTR:
>  			obfuscate_attr_block(dp, o);
> +			iocur_top->need_crc = 1;
>  			break;
>  		default:
>  			break;
> @@ -1444,6 +1457,7 @@ process_multi_fsb_objects(
>  
>  			obfuscate_dir_data_block(iocur_top->data, o,
>  						  last == mp->m_dirblkfsbs);
> +			iocur_top->need_crc = 1;
>  			ret = write_buf(iocur_top);
>  out_pop:
>  			pop_cur();
> @@ -1724,6 +1738,13 @@ process_inode_data(
>  	return 1;
>  }
>  
> +/*
> + * when we process the inode, we may change the data in the data and/or
> + * attribute fork if they are in short form and we are obfuscating names.
> + * In this case we need to recalculate the CRC of the inode, but we should
> + * only do that if the CRC in the inode is good to begin with. If the crc
> + * is not ok, we just leave it alone.
> + */
>  static int
>  process_inode(
>  	xfs_agnumber_t		agno,
> @@ -1731,17 +1752,30 @@ process_inode(
>  	xfs_dinode_t 		*dip)
>  {
>  	int			success;
> +	bool			crc_was_ok = false; /* no recalc by default */
> +	bool			need_new_crc = false;
>  
>  	success = 1;
>  	cur_ino = XFS_AGINO_TO_INO(mp, agno, agino);
>  
> +	/* we only care about crc recalculation if we are obfuscating names. */
> +	if (!dont_obfuscate) {
> +		crc_was_ok = xfs_verify_cksum((char *)dip,
> +					mp->m_sb.sb_inodesize,
> +					offsetof(struct xfs_dinode, di_crc));
> +	}
> +
>  	/* copy appropriate data fork metadata */
>  	switch (be16_to_cpu(dip->di_mode) & S_IFMT) {
>  		case S_IFDIR:
>  			success = process_inode_data(dip, TYP_DIR2);
> +			if (dip->di_format == XFS_DINODE_FMT_LOCAL)
> +				need_new_crc = 1;
>  			break;
>  		case S_IFLNK:
>  			success = process_inode_data(dip, TYP_SYMLINK);
> +			if (dip->di_format == XFS_DINODE_FMT_LOCAL)
> +				need_new_crc = 1;
>  			break;
>  		case S_IFREG:
>  			success = process_inode_data(dip, TYP_DATA);
> @@ -1756,6 +1790,7 @@ process_inode(
>  		attr_data.remote_val_count = 0;
>  		switch (dip->di_aformat) {
>  			case XFS_DINODE_FMT_LOCAL:
> +				need_new_crc = 1;
>  				if (!dont_obfuscate)
>  					obfuscate_sf_attr(dip);
>  				break;
> @@ -1770,6 +1805,9 @@ process_inode(
>  		}
>  		nametable_clear();
>  	}
> +
> +	if (crc_was_ok && need_new_crc)
> +		xfs_dinode_calc_crc(mp, dip);
>  	return success;
>  }
>  
> @@ -1840,9 +1878,6 @@ copy_inode_chunk(
>  
>  		if (!process_inode(agno, agino + i, dip))
>  			goto pop_out;
> -
> -		/* calculate the new CRC for the inode */
> -		xfs_dinode_calc_crc(mp, dip);
>  	}
>  skip_processing:
>  	if (write_buf(iocur_top))
> 

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