fileXray Example: FreespaceFS

In a previous blog post we saw how the trawling mechanism in fileXray provides a way of looking for patterns on an HFS+ volume.

There are times when you really must be able to just manually “go through” the free (unallocated) space in a volume. Perhaps you are an end user who wants to look for lost data using some unusual technique. Perhaps you are a forensics or security professional who wants a convenient and easy mechanism to isolate the free extents of an HFS+ volume, and then be able to examine those extents using tools of your choice. The Free Space File System (FreespaceFS), one of fileXray’s built-in virtual file systems, provides just that mechanism.

Simply put, FreespaceFS contains virtual files that represent the free extents of a given HFS+ volume. The idea is to isolate free space in easy-to-read contiguous chunks, exposing each chunk as a virtual file that can be normally read, which makes searching through free space much more convenient and faster in most cases.

When you mount an HFS+ volume through FreespaceFS, a top-level virtual directory called freespace in the resultant volume contains one or more virtual subdirectories whose names are of the format X_Y. X is a monotonically increasing decimal number starting at 0. Y represents a block number in hexadecimal, which is the starting block number of the first extent within that directory. Consider the following example.

# Create a mount point.
$ mkdir /Volumes/freespace

# Use the Free Space File System to mount the root volume.
$ sudo fileXray --userfs_type freespace --userfs_mount /Volumes/freespace
$ ls -las /Volumes/freespace/freespace/
total 0
0 drwxr-xr-x    38 root  wheel  0 Nov  2 20:58 .
0 drwxr-xr-x     3 root  wheel  0 Nov  2 20:58 ..
0 dr-xr-xr-x  1026 root  wheel  0 Nov  2 20:58 00000000_00014d87
0 dr-xr-xr-x  1026 root  wheel  0 Nov  2 20:58 00000001_003e4772
0 dr-xr-xr-x  1026 root  wheel  0 Nov  2 20:58 00000002_004e8ae8
0 dr-xr-xr-x  1026 root  wheel  0 Nov  2 20:58 00000003_00550783
0 dr-xr-xr-x  1026 root  wheel  0 Nov  2 20:58 00000004_005b8023
0 dr-xr-xr-x  1026 root  wheel  0 Nov  2 20:58 00000005_00bda9bd
…
0 dr-xr-xr-x  1026 root  wheel  0 Nov  2 20:58 00000034_02d66310
0 dr-xr-xr-x   389 root  wheel  0 Nov  2 20:58 00000035_02ec061a

Inside each such directory named X_Y, there are at most 1024 virtual files—a new directory is created after the previous one is populated with 1024 files. Each file represents a free extent—that is, a range of contiguous free blocks. Each file’s name is of the form U_V. U is the extent’s starting block number and V is the number of blocks in the extent. Both U and V are represented in hexadecimal. As noted earlier, the value of U for the first extent contained within the X_Y directory is the same as the value of Y.

Reading from such a file will return data from the volume blocks the file represents. The following excerpt shows the last few contents of the last X_Y directory in the above example.

$ ls -asl /Volumes/freespace/freespace/00000035_02ec061a
…
     848 -rw-r--r--    1 root  wheel   424K Nov  2 21:18 02f0db4e-02f0dbb7
      88 -rw-r--r--    1 root  wheel    44K Nov  2 21:18 02f0dc69-02f0dc73
    2272 -rw-r--r--    1 root  wheel   1.1M Nov  2 21:18 02f0dc77-02f0dd92
  114176 -rw-r--r--    1 root  wheel    56M Nov  2 21:18 02f0dd94-02f11553
  530184 -rw-r--r--    1 root  wheel   259M Nov  2 21:18 02f11556-02f21836
   16608 -rw-r--r--    1 root  wheel   8.1M Nov  2 21:18 02f21840-02f2205b
92345904 -rw-r--r--    1 root  wheel    44G Nov  2 21:18 02f2205d-03a24322
$

Note that that last file, 02f2205d-03a24322, contains about 44GB of free space.

In case you are wondering if there is an analog for used extents, the answer is yes: fileXray also provides a “Used Space File System” that exposes the in-use (allocated) extents of an HFS+ volume as virtual files.

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