Script for printing MBR headers to stdout

[PizieDust]

part of #14

• Inspect a MBR header's content. For example printing out what partions it contains, their size and type etc.
• Retrieve contents of partitions for example to stdout

Using this script:

./_build/default/bin/mbr_inspect.exe [file_name]

replace [file_name] with the name to any disk image you want to read the MBR Header.
This will print the headers of an MBR to stdout including information about the partitions it contains.
Sample output looks like this:

to test, create a disk image:

fallocate -l 2M test.img
parted --align none test.img mklabel msdos
parted --align none test.img mkpart primary 1s 33s
parted --align none test.img mkpart primary 34s 65s
parted --align none test.img mkpart primary 66s 97s

the commands above will create an MBR with three partitions

to make any of the partitions bootable, you can use the parted command.

parted test.img
set 1 boot on

cc @reynir

[PizieDust]

The reason for moving the print_mbr_fields and read_mbr functions to the main mbr.ml file is so that these functions can be used in other use cases.
For instance, to resize partitions, we can use the read_mbr function to read the mbr, then resize the partitions as needed and then use the print_mbr_fields function to print the new MBR structure with the resized partition.
Including them in the main mbr.ml file ensures we can use the functions accros different implementations without needing to rewrite the same code in every file.

[reynir]

I'm quite pleased overall with the code. There are some minor issues with how some values are printed and I would like the read_mbr function moved back out of the mbr library. With those changes I would be happy to merge.

Another task on top of this could be to add flags to toggle printing the boot code for example. For this I like to use the library cmdliner.

[reynir]

mbr.bootstrap_code contains binary (executable) data. In OCaml you can use the %S printf modifier to print it as an escaped string, but even better would be to print it as hexadecimal.

[PizieDust]

Thank you. I will see how to implement the cmdliner library. It looks great with good documentation.

[PizieDust]

@reynir concerning this, I'll open a new Issue for it to keep as a reminder so that when cstruct 6.2.0 is released we can implement the command line flags to toggle display for the bootstrap_code. so if that's okay by you then we could merge .

[reynir]

Sounds good! Please go ahead and open an issue.

[reynir]

It would be great to print the disk signature as hexadecimal. This is what fdisk -l test.img does.

[PizieDust]

for this, I was exploring the Cstruct library and found a useful function to_hex_string, however this function exist only in v6.2.0 while the latest release of Cstruct on Opam is v6.1.1

The PR to publish the new release is still pending.
ocaml/opam-repository#23374

I am unsure if it's wise to wait or find some other way to convert the Cstruct.t value to a hex string.

[reynir]

For disk_signature you can use %lx in printf.

Good question about bootstrap_code. There is a 'hex' library that can be used. That would introduce another dependency for the binary, but that is maybe alright. Another way forward could be to print "" since I think that's rarely what you're interested in. And then later when cstruct.6.2.0 is released we can add a flag to print the bootstrap code.

[PizieDust]

I think the best way will be to print an empty string and update later when cstruct 6.2.0 is released.
I think adding another dependency (hex library) will be too much given that we are using it just for this purpose.

[PizieDust]

:-) Thanks for the %lx. It's just what I needed.

[reynir]

Thanks! I merged and pushed a commit that makes the usage message use the 'basename' of the command so as to not print the whole path to the command in the usage message.

[PizieDust]

that's great. thank you. I'll go over it to learn how you did that.