Grub issues on a Macbook running Debian stable and Mac OS High Sierra

context.md

Setup

The machine contains one SSD drive which is partitionned as following:

• EFI partition, vfat
• macos partition, apfs (recently converted from something else, pre high sierra)
• linux swap
• linux root, ext4

Boot setup

• grub is installed on the linux root and grub.efi was copied to the EFI fs
• the grub,efi was blessed by macosx so it could appear in the mac boot list (pressing Option at boot)
• grub then set its root to (hd0,gpt4) and run the linux kernel

Issue

Since the high sierra update, grub enters rescue mode and stops with a no such partition error. The system cannot be booted.

Indeed, typing at the prompt :

• ls shows no (hd0,gpt4) but a list of unexpected devices (hd0), (hd1), (hd2), (hd3), (hd4), (hd5), (hd5,gpt4), (hd5,gpt3),(hd5,gpt2), (hd5,gpt1)
• set shows root and prefix set to the wrong devices : (hd0,gpt4) and (hd0,gpt5)/boot/grub

Workarounds

Rescue

From the rescue prompt:

set prefix=(hd5,gpt4)/grub
set root=(hd5,gpt4)
insmod normal
normal

Grub menu appears and linux can boot if /boot/grub/grub.cfg is correctly set.

Device map

Obviousy, grub doesn't detect the same disk order at the EFI stage and when ran on linux.

grub-mkdevicemap creates a /boot/grub/device.map that can be altered to return (hd5) in place of (hd0).

cat /boot/grub/device.map
(hd5)   /dev/disk/by-id/ata-APPLE_SSD_SM0256F_S1K4NYBDB79405
(hd1)   /dev/disk/by-id/usb-USB_Flash_Disk_FBF1101121103781-0:0

Reinstall grub on this hd5 device

... hopefully setting prefix and root to the correct device

grub-install '(hd5,gpt4)'