Resize partitions in a disk image.

[PizieDust]

part of #14

Resizing partitions in a disk image

Outline

This PR introduces a new binary resize_partition which resizes a partition to a new size.
We are using the cmdliner package to read arguments from the command line. It also provides a helpful --help flag which can displays a user manual.
Basically we read in a disk image, unmarshal it's MBR header to get the partition table, grab the specific partition we want to resize. After getting this partition, we calculate a new end_sector and create a new partition changing only this end_sector. Then we swap this new partition with it's counterpart in the original partition table and create a new mbr header with the updated partition table. The new mbr header is then written back to the disk image, overwriting just the MBR header.

Usage

After building, the command can be called with:

resize_partition test.img 2 4096

• test.img : The disk image which contains the partition.
• 2 : The partition number we want to resize.
• 4096: the new size of the partition

Outputs

Read initial disk image

dune exec -- bin/mbr_inspect.exe disk.img
MBR fields:
  original_physical_drive: 0
  seconds: 0
  minutes: 0
  hours: 0
  disk_signature: 3153ef95
  Partition 1:
    bootable: false
    chs_begin: (cylinders: 8, heads: 0, sectors: 2)
    ty: 83
    chs_end: (cylinders: 95, heads: 2, sectors: 23)
    lba_begin: 1
    size_sectors: 2006
  Partition 2:
    bootable: false
    chs_begin: (cylinders: 143, heads: 3, sectors: 32)
    ty: 83
    chs_end: (cylinders: 123, heads: 0, sectors: 26)
    lba_begin: 2047
    size_sectors: 1435
  Partition 3:
    bootable: false
    chs_begin: (cylinders: 31, heads: 2, sectors: 3)
    ty: 83
    chs_end: (cylinders: 159, heads: 3, sectors: 32)
    lba_begin: 3522
    size_sectors: 574

Resize 3rd partition

dune exec -- bin/resize_partition.exe disk.img 3 307200

Verify 3rd partition has been resized

MBR fields:
  original_physical_drive: 0
  seconds: 0
  minutes: 0
  hours: 0
  disk_signature: 3153ef95
  Partition 1:
    bootable: false
    chs_begin: (cylinders: 0, heads: 0, sectors: 0)
    ty: 83
    chs_end: (cylinders: 0, heads: 0, sectors: 0)
    lba_begin: 1
    size_sectors: 2006
  Partition 2:
    bootable: false
    chs_begin: (cylinders: 0, heads: 0, sectors: 0)
    ty: 83
    chs_end: (cylinders: 0, heads: 0, sectors: 0)
    lba_begin: 2047
    size_sectors: 1435
  Partition 3:
    bootable: false
    chs_begin: (cylinders: 0, heads: 0, sectors: 0)
    ty: 83
    chs_end: (cylinders: 0, heads: 0, sectors: 0)
    lba_begin: 3522
    size_sectors: 4122

Resizing a partition which may overlap

dune exec -- bin/resize_partition.exe disk.img 1 5000000
resize_partition: internal error, uncaught exception:
                  Failure("Partitions overlap")

cc @reynir

[PizieDust]

Hello @reynir
Could you review this?

[reynir]

This looks good. Besides my one comment could you as well check that the resize doesn't result in the partitions overlapping before writing the new MBR header?

Sorry for the long wait - I was on Easter holiday.

[PizieDust]

This looks good. Besides my one comment could you as well check that the resize doesn't result in the partitions overlapping before writing the new MBR header?

While making the new MBR header, we have a check already in the Mbr.make constructor that checks if the partitions overlap.
https://github.com/mirage/ocaml-mbr/blob/master/lib/mbr.ml#L188

If the partitions overlap, it will fail without trying to write the new MBR header. The MBR header is only overwritten if the new MBR header is created successfully.

[reynir]

While making the new MBR header, we have a check already in the Mbr.make constructor that checks if the partitions overlap.

Nice. In that case it would be nice to write a comment on this.

[reynir]

Thanks! I pushed a small change to the command help text.

[PizieDust]

amazing. thank you