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Writing an GRUB module

Recently I’m writing a GRUB module. And to be honest, the GRUB documentation is really bad. Usually OSS project like GCC would have detailed compiling/debugging notes for other developers. But in GRUB I can find nothing. Googling shows almost NOTHING. But eventually I finished my module, so I’m here leaving some notes for others.

Module template

Usually grub module are compiled in-the-tree, so you’ll need to have a grub code base and bootstrap it. However there’re some helper scripts to simplify the process: https://github.com/jesusdf/grub-msr

Noticing grub-msr is already being merged into upstream, so that module is surely of high quality. Using those scripts will simply give you a GRUB building environment. And then you can simply use that repo as a template to implement your own feature

Caveats

A good module name is very crucial. If the module name is too long, or contains hyphen( “-” ),  you may have successful builds, but you will then experience many random crashes.

Debugging

Debugging is quite easy. First we need to setup the grub installtion

  1. make install
  2. dd if=/dev/zero of=testgrub bs=1M count=100
  3. fdisk testgrub # init MS-DOS partition map and create a partition
  4. sudo kpartx -c -v testgrub
  5. sudo mkfs.ext4 /dev/mapper/loopXXp1
  6. sudo mount /dev/mapper/loopXXp1 tt
  7. grub-install -v –target=i386-pc –boot-directory=tt/boot testgrub

  8. sudo umount tt

  9. sudo kpartx -d testgrub
  10. qemu-img convert testgrub testgrub.img -O qcow2

Then you can run it in the qemu using  qemu-system-i386 testgrub.img -s -S.

After the qemu boots up, cd into the grub/grub-core directory, and execute  gdb -x gdb_grub , the gdb will automatically hooks up something and loads symbol when a module is loaded.