Mount FreeBSD UFS from logical partition under Linux

Question

How can I mount the FreeBSD UFS boot partition under Ubuntu in this setup:

• a single HDD which contains
• an MBR partition table which contains
• some primary Linux partitions and an extended partition which contains
• a Linux logical partition and a FreeBSD logical partition which contains
• the FreeBSD disklabel (so the logical partition is the "slice") which contains
• the FreeBSD boot (UFS) and swap partitions

Here is the MBR partitioning:

ubuntu$ sudo fdisk -l /dev/sda

Disk /dev/sda: 42.9 GB, 42949672960 bytes

255 heads, 63 sectors/track, 5221 cylinders, total 83886080 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0005d5af

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048     1953791      975872   83  Linux
/dev/sda2         1953792    11718655     4882432   83  Linux
/dev/sda3        11718656    13672447      976896   82  Linux swap /     Solaris
/dev/sda4        13674494    83884031    35104769    5  Extended
/dev/sda5        13674496    33204223     9764864   83  Linux
/dev/sda6        33206272    83884031    25338880   a5  FreeBSD

And here is the disklabel:

freebsd$ disklabel /dev/ada0s6
# /dev/ada0s6:
8 partitions:
#          size     offset    fstype   [fsize bsize bps/cpg]
  a:   48580592         16    4.2BSD        0     0     0
  b:    2097152   48580608      swap                    
  c:   50677760          0    unused        0     0     # "raw" part, don't edit

I can boot FreeBSD using the following /etc/grub.d/40_custom:

#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.

menuentry "FreeBSD" {
    insmod part_bsd
    insmod ufs2
    set root="(hd0,msdos6,bsd1)"
    kfreebsd /boot/kernel/kernel
    set kFreeBSD.acpi_load=YES
    set kFreeBSD.hint.acpi.0.disabled=0
    set kFreeBSD.vfs.root.mountfrom=ufs:/dev/ada0s6a
    kfreebsd_loadenv /boot/device.hints
}

That way I can access the FreeBSD partition from grub2 with no problems. But Linux does not detect any BSD partitions:

ubuntu$ ls /dev/sda*
/dev/sda  /dev/sda1  /dev/sda2  /dev/sda3  /dev/sda4  /dev/sda5  /dev/sda6

Versions: Ubuntu 14.04 with kernel 4.2.0-27-generic on x86_64, FreeBSD 10.3 RELEASE amd64, both fresh installs.

Answer 1

A workaround is to calculate the offset of the BSD partition within the logical partition and use a loop device with offset:

mount -t ufs -o loop,offset=8192,ro,ufstype=ufs2 /dev/sda6 /mnt

Answer 2

Probably you have been confused by different harddisk naming convention in Linux and FreeBSD. From your output one can clearly see that Linux has detected your ufs partition and it is /dev/sda6. So, you just need to do the following

sudo modprobe ufs
sudo mount -t ufs -o ufstype=ufs2 /dev/sda6 /mnt

Answer 3

Use fdisk /dev/sdX using command b (for BSD disklabels) followed by p (for print) to get the list of BSD disklabels/slices. This will look like this:

Slice   Start     End Sectors   Size Type     Fsize Bsize   Cpg
a     4082400 4606687  524288   256M 4.2BSD    2048 16384 32776
b     4606688 5079391  472704 230.8M swap         0     0     0
c     4082400 8164799 4082400     2G unused       0     0     0
d     5079392 5603679  524288   256M 4.2BSD    2048 16384 32776
e     5603680 6127967  524288   256M 4.2BSD    2048 16384 32776
f     6127968 8164799 2036832 994.6M 4.2BSD    2048 16384 28552

This gives you the start sector for each partition. The sector multiplied by the sector size (512 bytes; see fdisk output) gives you an offset that you can use with with mount.

For example for slice f:

mount -t ufs -o loop,offset=$((6127968 * 512)),ro,ufstype=ufs2 /dev/sdX /mnt/freebsd

Answer 4

Mounting the specific partition /dev/sdb2 without the need for offset calculation

1. Load the UFS file system module

 sudo modprobe ufs

2. Create a mount point for UFS

 sudo mkdir -p /mnt/ufs

3. Mount the partition without offset (offset is not required when mounting a specific partition)

 sudo mount -t ufs -o rw,ufstype=ufs2 /dev/sdb2 /mnt/ufs

4. Check the contents of the mounted partition

  ls -la /mnt/ufs

Mounting a Disk Image or Device with UFS File System Using Offset Calculation

1. Determine the starting sector of the desired partition, this command lists the partition table of the disk image or device

 sudo fdisk -l /path/to/disk/image

2. Calculate the offset - your starting sector * 512 bytes Replace 'your_starting_sector' with the actual sector number obtained from the output of fdisk

  your_start_sector=<obtained_from_fdisk_output>


  offset=$((your_ starting_sector * 512))

3. Mount the image or device with the calculated offset. The offset is used to mount the desired partition

  sudo mount -t ufs -o loop,offset=$offset,rw,ufstype=ufs2 /path/to/disk/image /mnt/ufs

Example:

This is an example of mounting a partition inside a disk image so that no one gets confused.

Retrieving Partition Information from the Disk Image

To view the partition table inside the disk image, we use fdisk. This will help identify the starting sector of the desired partition.

sudo fdisk -l /path/to/disk/image.img

Our Output:

Disk /path/to/disk/image.img: 42.9 GB, 42949672960 bytes
255 heads, 63 sectors/track, 5221 cylinders, total 83886080 sectors
Units = sectors of 1 * 512 = 512 bytes

  Device      Boot     Start       End       Blocks    Id  System
/dev/loop0p1   *        2048     1953791      975872   83  Linux
/dev/loop0p2         1953792    11718655     4882432   a5  FreeBSD

the partition /dev/loop0p2 (FreeBSD) starts at sector 1953792.

Calculating the Offset:

To calculate the offset in bytes, multiply the starting sector of the desired partition by the sector size (512 bytes in most cases).

If the starting sector is 1953792, the offset is calculated as follows:

offset=$((1953792 * 512))

The offset will be 1001885184 bytes.

Mounting the Partition Using the Offset:

Now that the offset has been calculated, you can mount the partition by specifying the path to the disk image and the offset:

sudo mount -t ufs -o loop,offset=1001885184,rw,ufstype=ufs2 /path/to/disk/image.img /mnt/ufs

This command mounts the partition starting from the specified offset inside the disk image.

Checking the Contents of the Mounted Partition:

After successfully mounting, you can check the contents of the partition:

ls -la /mnt/ufs

---

An offset is not needed when mounting a specific partition on a physical device, such as /dev/sdb2. In this case, the system already knows where the partition starts because the operating system reads the partition table from the device and manages access to each partition accordingly.

However, when working with a disk image, the partition table needs to be "emulated" manually — meaning you must locate the partition and specify the offset. This is because you're not mounting a physical device that automatically understands the partition structure, but rather a file that is a copy of the disk.