GRUB, ancient and modern GRUB Legacy Of course originally just GRUB - - PowerPoint PPT Presentation

GRUB, ancient and modern

GRUB Legacy

• Of course originally just GRUB
• Started in 1995 by Erich Boleyn
• Multiboot
• Editable menus, reasonable FS interface
• BALGE

Multiboot

https://xkcd.com/927/

GRUB Legacy: stages

• Stage 1: MBR, just reads first sector of

Stage 1.5 and jumps to it

• Stage 1.5: enough file system code to read

Stage 2 from an ordinary file system

• Stage 2: rest of loader, e.g. Linux boot code

GRUB Legacy: problems

• Ad-hoc FS abstraction, multiple stage1.5s
• Not much other abstraction worth mentioning
• Difficult to extend safely
• Lots of PC/BIOS assumptions
• Divergence

GRUB 2 rewrite

New design

• Small kernel, almost everything in modules
• grub-install detects modules needed to

read /boot/grub/ and builds a core image

• Heavy use of abstraction layers; highly

composable

• Userspace tools built from same code
• 10 kLoC asm, 250 kLoC C (excluding

libgcrypt)

– Includes all arches; i386-pc 4.5 kLoC asm

Architecture support

• x86: BIOS, UEFI, Coreboot, IEEE1275 (!),

Xen

• PowerPC, SPARC: IEEE1275
• IA/64: UEFI
• MIPS: ARC, Yeeloong, Fuloong, Loongson
• ARM: U-Boot, UEFI
• ARM64: UEFI

Normal hacking routine

• Hardest Legacy problems were with reading

files from disk

• Usually stuck with emulator + printf or (in

extremis) gdb

• grub-probe and grub-fstest
• Spin-off benefits: grub-mount

When userspace doesn't help

• bash-style interactive shell; set debug=foo
• grub-mkrescue
• Easy to run configuration files or even plug

in new commands

Configuration

• Debian update-grub → upstream, with

clearer input/output separation

• Now /etc/default/grub and /etc/grub.d/
• (Almost) all customisable
• … but some customisation is hard

PCs suck

• MBR: no formal space for boot code

– Trivial loader in 446 bytes – Embed rest of brain in file system – Skate edge of (non-)spec: use boot track

• Unpredictable with multiple disks

PCs suck (2)

• GPT much better, but different problems
• Variant MBR interpretations

– Apple – boot flag on PMBR

UEFI

• New big thing on PCs; bye-bye legacy BIOS
• Core support basically fine
• “Secure Boot” works in Ubuntu, but needs

archive signing set up in Debian

• Can require signed kernels without

impinging on freedom once MokManager is in place

• ESP layout pain

Non-x86

• powerpc/sparc basically work fine, though

defaults not switched

• Some mipsen work well, e.g. Yeeloong
• Trend towards new architectures including

GRUB port early on

• Most recent port (arm64) was about 2000

lines

Debian pros/cons

• Other loaders: LILO, syslinux and friends,

yaboot, etc.

• Some pretty good, all smaller/simpler
• … but result is complexity elsewhere
• Any architecture, any disk layout
• Portable loader allows common boot UI

across architectures and common tools

Help wanted

• Mostly just me right now in Debian; BUGS
• Everyone's boot problem is critical for them
• Overhaul configuration
• MBR robustness
• Rest of UEFI
• Xen
• Take better advantage of ports

Discussion

Colin Watson <cjwatson@debian.org>