Read data in partition, output to stdout or file

[PizieDust]

part of #14

Outline

• This PR introduces a new executable read_partition.ml which reads content stored in a partition.
• This executable uses the cmdliner package to parse and read arguments passed when calling this executable and also generates a lovely manual which can be called by adding the --help flag (I love this package)
• Basically we read in a disk image, unmarshal it's MBR header to get the partition table, grab the specific partition we want to read from. After getting this partition, we calculate information about it's sectors and then read the data from these sectors.

Usage

After building, the command can be called with:

read_partition.exe test.img 2

• test.img : The disk image which contains the partition.
• 2 : The partition number we want to read from.
  Running this command will print the contents of partition 2 to stdout.

To output the contents to a file:

read_partition.exe -f file.txt test.img 2

This will output the contents to a file called file.txt. If this file doesn't exist, the command will create the file.

Simple Test

• Create a new text file with some text:

echo "Simple file for testing" > sample.txt

• Create a new disk image with 3 partitions:

fallocate -l test.img 2M`
parted --align none test.img mklabel msdos
parted --align none test.img mkpart primary 1s 2s
parted --align none test.img mkpart primary 3s 4s
parted --align none test.img mkpart primary 5s 6s

• Add the text file to the partition

dd if=sample.txt of=test.img bs=512 seek=1 conv=notrunc

• Run the command now to read file from the partition:

read_partition.exe test.img 1`

cc @reynir

[reynir]

Nice!

$ fallocate -l test.img 2M
$ parted --align none test.img mklabel msdos
$ parted --align none test.img mkpart primary 1s 20s
$ tar -cf tmp.tar LICENSE
$ dd if=test.tar of=test.img seek=1 conv=notrunc
20+0 records in
20+0 records out
10240 bytes (10 kB, 10 KiB) copied, 0.000348219 s, 29.4 MB/s
$ dune exec -- bin/read_partition.exe test.img  1 | tar -t
LICENSE

[reynir]

We should incrementally read and write the data as the partition could potentially be larger than what we can fit in memory.

[PizieDust]

I've updated it to use a chunk size of 4096 bytes. let me know if to use a bigger chunk size.

[reynir]

Thanks!