EXT Filesystem Components

[rbs-jacob]

• [x] I have reviewed the OFRAK contributor guide and attest that this pull request is in accordance with it.

One sentence summary of this PR (This should go in the CHANGELOG!)

Add packers and unpackers for EXT filesystems (versions 2 through 4).

Anyone you think should look at this, specifically?

@whyitfor @EdwardLarson

Outstanding tasks:

• [ ] Packer
• [x] Tests

[rbs-jacob]

Is there any dependencies that need to be installed? That is, is a change to the Dockerstub missing from this PR?

IIRC this doesn't work for Windows. I feel it's alright that this only works on Linux/Mac systems, if not ideal. The ComponentExternalTool being there can protect against trying to run it on Windows. I do think there should be a TODO comment somewhere in here explicitly saying Windows support is limited.

On macOS, there is a dependency that needs to be installed via brew, and is included as a ComponentExternalTool. There are no such external dependencies on Linux, since I believe Debian bundles the required tools by default.

jacob@blade:~$ docker run --rm -it debian:latest bash
root@25be021f7df1:/# which debugfs
/usr/sbin/debugfs

While it's true that I haven't been able to find a version that works for Windows, I suspect that the 7z command line tool can at least extract EXT filesystems. It may not be able to rebuild them, though. It seems like we agree that not having a Windows version of the tool should block merging this.

[rbs-jacob]

In the interest of getting this merged in, we're omitting a packer for now. A packer is considerably more complicated to add because of cross-platform concerns with the normal workflow for making a populated EXT filesystem. Whereas for unpacking, the debugfs utility can easily be used cross-platform.

The workflow for packing usually involves:

• Making an empty file with some dd incantation
• Using some variation of mkfs.ext4 to make the file into a filesystem
• Mounting the filesystem with mount
• Populating the filesystem
• Unmounting it

This can all be done fairly easily from a Linux system, but for other OSes, the kernels lack support for EXT filesystems, so they cannot easily be mounted. The packer will likely have to involve some alternative, cross platform way to build and populate them. As such, we're waiting for a separate PR for the packer so that this work doesn't stall in the short-term.