Gradual Evidence Acquisition From an Erroneous Drive

<2021-10-23>

tl;dr

A hard drive is a relatively fragile data store. After discovering the first indicators of a drive failure, the hard drive might suddenly die. The present blog post, therefore, discusses the gradual acquisition of evidence from an erroneous drive by utilizing the synergy of the open source tools ddrescue and partclone. To spare the mechanics of the drive and acquire the most critical data first, partclone is used to create a so-called domain file, where the used blocks of the file system are noted.

# Record actually used blocks in domainfile
partclone.[fstype] -s /dev/sdXY -D --offset_domain=$OFF_IN_BYTES \
    -o domainfile

This is the basis for ddrescue's recovery process, where a blocksize-changing data recovery algorithm is utilized that will only cover these areas for the moment.

# Restrict rescue area via domainfile
ddrescue --domain-log domainfile /dev/sdX out.img mapfile

Afterward, additional runs can be conducted to acquire the remaining sectors.

Background

Since HDDs are rather sensitive mechanical components, it is not uncommon for them to exhibit read errors after a certain amount of usage and the wear and tear of the magnetic platters or alternatively as a consequence of shock events, which lead to a mechanical damage inside the drive. So-called head crashes, which most commonly occur when the HDD drops during regular operation might be lethal for an HDD and would require a complete dismantling of the drive in specialized laboratory. Grinding sounds are typical for such a scenario and require an immediate stop of operation. However, minor shock events of the HDD and/or when the actuator arm is in its "parking position" might not lead to great physical damage, but result in mechanical dysfunctioning and read/write errors. This regularly leads to clicking, knocking, or ticking noises, which stem from abnormal behavior of the disk's read and write head, when it is repeatedly trying to read a sector. If a hard disk makes noise, data loss is likely to occur soon or has already happened. Grinding or screeching noise should be an indicator to power down the device immediately and hand it over to a specialized data recovery laboratory, to secure the remaining evidence. Given minor clicking or knocking noise, one might try to recover the data with the help of specialized software as soon as possible, as it is discussed in this blog post.

Acquisition of data from erroneous drives

Standard approach with GNU `ddrescue`

GNU ddrescue is the go-to tool to perform data recovery task with open source tooling ¹. It maximizes the amount of recovered data by reading the unproblematic sectors first and scheduling areas with read errors for later stages by keeping track of all visited sectors in a so-called mapfile. ddrescue has an excellent and exhaustive manual to consult ². To get a first glimpse ddrescue's procedure, which employs a block-size-changing algorithm, can be summarized as follows: Per default, its operation is divided into four phases, where the first and last one can be divided into passes, while each phase consults the mapfile to keep track of the status of each sector (or area) in its mapfile.

Copying: Read non-tried parts, forward and backward with increasing granularity in each pass. Record the blocks, which could not be read, as non-trimmed in the mapfile.
Trimming: Blocks that were marked as non-trimmed are trimmed in this phase, meaning to read from the edge forward sector by sector until a read error is encountered. Then read the sectors backward from the edge at the block's end until the sector read fails and keep track of the sectors in between as non-scraped in the mapfile.
Scraping: In this phase, non-scraped blocks are scraped forward sector by sector while marking unreadable sectors as bad.
Retrying: Lastly, the bad sectors can be read $n$-times with reversed directions for each try, which is disabled by default and can be set via the parameter --retry-passes=n. Unreadable sectors are filled with zeros in the resulting image (or device).

Using ddrescue with its sane default settings is as simple as running

ddrescue /dev/sdX out.img mapfile

To activate direct disk access and omit kernel caching, one must use -d/--idirect and set the sector sizes via -b/--sector-size. An indicator for kernel caching is when the positions and sizes in the mapfile are always a multiple of the sector size ³.

# Check the disk's sector size
SECTOR_IN_BYTES=$(cat /sys/block/sdX/queue/physical_block_size)
# Run ddrescue with direct disk access
ddrescue -d -b $SECTOR_IN_BYTES /dev/sdX out.img mapfile

Gradual approach by combining `partclone` and `ddrescue`

While the straightforward sector-by-sector copying of a failing HDD with ddrescue often yields good results, it might be very slow. Given the fact, that acquiring evidence after the damage is a race against the clock because, with every rotation of the platter the probability of an ultimate drive failing increases, one might want to ensure, that critical data gets acquired first by determining the actually used blocks of the filesystem and prioritizing those ⁴. To accomplish this, the open source tool for cloning partitions partclone comes into the (inter)play with ddrescue.

partclone "provide[s] utilities to backup used blocks" and supports most of the widespread filesystems, like ext{2,3,4}, btrfs, xfs, NTFS, FAT, ExFAT and even Apple's HFS+ ⁵. One of its features is the ability to list "all used blocks as domain files", so that "it could make ddrescue smarter and faster when dumping a partition" ⁴. partclone operates similarily to ddrutility's tool ddru_ntfsbitmap, which extracts the bitmap file from an NTFS partition and creates a domain file ⁶ but works with other filesystems as well by looking at their block allocation structures to determine used blocks and store those in the a/m domain mapfile ⁷. The term rescue domain describes the "[b]lock or set of blocks to be acted upon" ⁸. By specifying --domain-mapfile=file the tool is restricted to look only at areas, which are marked with a + ⁹.

Generating a domain mapfile

To generate a domain, file simply use partclone with the -D flag and specify the resulting domain file via -o

partclone.[fstype] -s /dev/sdXY -D -o sdXY.mapfile

If you want to run ddrescue on the whole disk and not just the partition, to image the whole thing iteratively, it is necessary to use --offset_domain=N, which specifies the offset in bytes to the start of the partition. This will be added to all position values in the resulting domain mapfile. To create such a file use the following commands:

# Retrieve the offset in sectors
OFF_IN_SECTORS=$(mmls /dev/sdXY | awk '{ if ($2 == "001") print $3}')

# Retrieve sector size
SECTOR_IN_BYTES=$(mmls /dev/sdX | grep -P 'in\s\d*\-byte sectors' | \
    grep -oP '\d*')

# Calculate offset
OFF_IN_BYTES=$((OFF_IN_SECTORS * SECTOR_IN_BYTES))

# Create domain file
partclone.[fstype] -s /dev/sdXY -D --offset_domain=$OFF_IN_BYTES \
    -o domainfile

The resulting domain file looks like illustrated in the following listing:

cat domainfile

# Domain logfile created by unset_name v0.3.13
# Source: /dev/sdXY
# Offset: 0x3E900000
# current_pos  current_status
0xF4240000     ?
#      pos        size  status
0x3E900000  0x02135000  +
0x40A35000  0x05ECB000  ?
0x46900000  0x02204000  +
0x48B04000  0x005FC000  ?
<snip>

The offset at the top denotes the beginning of the file system. The current_pos-value corresponds to the last sector used by the file system. Used areas are marked with a + and unused areas with a ? ⁷.

Acquiring the used blocks with `ddrescue`

To acquire only those areas with ddrescue that are actually used by the file system and, therefore, have been denoted with a + in the domain file, use the following command.

# Clone only blocks, which are actually used (of part Y)
ddrescue --domain-log domainfile /dev/sdX out.img mapfile
# Check if acquisition was successful
fsstat -o $OFF_IN_SECTORS out.img

Since you already know the offset, you might omit to clone the partition table on the first run. After completion of the a/m command, you can be sure that the mission-critical file system blocks have been acquired which can be double-checked by diffing the domain file and the mapfile, like this diff -y domainfile mapfile.

Acquiring the remaining blocks with `ddrescue`

So the additional sectors, which might contain previously deleted data, of the disk can be imaged in a subsequent and lengthy run without having to fear a definitive drive failure too much. To do this simply supply ddrescue the mapfile, which recorded all previously generated in the previous run without restricting the rescue domain this time, so that it will add the remaining blocks, which were either zero filled or omitted entirely:

# Clone remaining blocks
ddrescue /dev/sdX out.img mapfile
# Check result by inspecting the partition table
mmls out.img

After the completion of this procedure, which is a fragile process on its own, some kind of integrity protection should be employed, even though the source media could not be hashed itself. For example, this could be done by hashing the artifacts and signing the resulting file, which contains the hashes.

Summary

The present blog post discussed the usage of ddrescue as well as gradual imaging of damaged drives. To acquire mission-critical data first and rather timely, partclone was used to determine the blocks that are actually used by the file system residing on the partition in question. This information was recorded in a so-called domain file and fed to ddrescue via the command line parameter --domain-log, so that the tool limits its operation on the blocks specified there. Afterward, another lengthy run could be initiated to image the remaining sectors.