#2663 Support raw image files with MBR records that only set LBA values

[Jakub Kuczys]

I've been trying to extract files from the first partition of the following image (talking about the img file, not the xz archive that it is inside):
https://cdimage.ubuntu.com/releases/22.04/release/ubuntu-22.04.5-preinstalled-server-armhf+raspi.img.xz

However, this has been unsuccessful:

PS D:\> 7z l .\ubuntu-22.04.5-preinstalled-server-armhf+raspi.img

7-Zip 26.01 (x64) : Copyright (c) 1999-2026 Igor Pavlov : 2026-04-27

Scanning the drive for archives:
1 file, 4363124736 bytes (4161 MiB)

Listing archive: .\ubuntu-22.04.5-preinstalled-server-armhf+raspi.img


ERROR: .\ubuntu-22.04.5-preinstalled-server-armhf+raspi.img : Cannot open the file as archive



Errors: 1

Further inspection of the file reveals that the CHS Addresses (highlighted in red) for the partitions are set to 0:

However, the LBA of partition start (highlighted in green) and number of sectors in partition (highlighted in purple) are set which is enough to know where the partitions are. I believe that tools that work on partitions generally ignore the CHS values and use the LBA ones since the max values allowed by CHS are pretty limiting.

For comparision, if I manually edit the CHS addresses in the file to use correct values as shown on the image below, 7z manages to see the partitions and extract what I need:

PS D:\> 7z l .\ubuntu-22.04.5-preinstalled-server-armhf+raspi-modified.img

7-Zip 26.01 (x64) : Copyright (c) 1999-2026 Igor Pavlov : 2026-04-27

Scanning the drive for archives:
1 file, 4363124736 bytes (4161 MiB)

Listing archive: .\ubuntu-22.04.5-preinstalled-server-armhf+raspi-modified.img

--
Path = .\ubuntu-22.04.5-preinstalled-server-armhf+raspi-modified.img
Type = MBR
Physical Size = 4363124736
Sector Size = 512
ID = 699253995

   Date      Time    Attr         Size   Compressed  Name
------------------- ----- ------------ ------------  ------------------------
                    .....    536870912    536870912  0.fat
                    .....   3807518720   3807518720  1.img
                    .....     17686528     17686528  2
------------------- ----- ------------ ------------  ------------------------
                            4362076160   4362076160  3 files

It would be great, if 7z could parse these sort of images considering that the LBA is there.

[Jakub Kuczys]

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