I kinda wish some of the FS utilities were better. Despite FUSE, mounting a block device (like, disk image) as a non-root user is tough, so writing to an image with actual partitions/FS on it is difficult, especially to script, especially if you don't want to sudo in a build script. losetup on a disk image doesn't (to my knowledge) detect partitions… for reasons unknown to me.

Bootsectors are similar. You can't just install grub to a disk image. (You have to losetup it, at the very least, which implies root. Why can't I just install to a file?)

You want a script/build system that allows you, at the very least, to:

1. Code. 2. Run build system/script. 3. Fire up qemu or similar.

You can't be rebooting. Ideally, it'd be great to do this in userspace.

That said, if you're starting out, just do [boot sector] + [kernel] = tada image until you need to do otherwise. (Really, do whatever works and is easy.)

That said, I've found a few somewhat helpful tools.

fuseloop[1] takes a file and offset/size, and exposes a single file. If you have a partitioned disk image, then you can feed it the partition offset, and it gives you back something you can format as an FS. (e.g., you can run ext2fs on.)

Then there's fuse.ext2[2], which mounts an ext2 FS on FUSE, so non-root usable again. Note that I'm linking to my fork of it, since the original didn't build for me, but I didn't write it. (Which I fixed, and sent a pull request, but never heard back.)

Finally — and sorry to peddle my own stuff again — I wrote a Python library for dealing with the MBR.[3] I use it to figure out offsets and sizes in a disk image.

I've had a bit of fun writing a boot loader, and I've managed to get it to load up its stage 2 and switch to 32-bit pmode. Had a fun error where a division instruction was throwing things into a triple fault; see [4] if you want to see how a division instruction can fail without dividing by zero (which was the first thing I checked). The disk layout is currently:

  [boot sector] [stage 2] [kernel] [ partitions, FS, real data, etc. ]

stage2's size is hard coded into the first sector of stage 2 (into itself), and the kernel's location and size will be similarly hard coded into it as well. (When I get there. Disks in pmode are different, as you can't just have the BIOS do all the work for you, sadly!) And by hard-coded, a build script calculates and just re-writes a few bytes.

[1]: https://github.com/jmattsson/fuseloop

[2]: https://github.com/thanatos/fuse-ext2-fakeFS

[3]: https://github.com/thanatos/pymbr

[4]: http://stackoverflow.com/questions/21212174/why-cant-i-step-...