[Pkg-raspi-maintainers] Bug#984691: /etc/kernel/postinst.d/z50-raspi-firmware: generated cmdline.txt root parameter broken when using Btrfs

[João Ricardo Sares Teles de Matos]

Package: raspi-firmware
Version: 1.20210111+ds-2
Severity: important
File: /etc/kernel/postinst.d/z50-raspi-firmware

Dear Maintainer,

(Note: a solution is provided below)

When using a Btrfs filesystem as /, the root= parameter generated by the 
z50-raspi-firmware script is incorrect due to the output of findmnt, as 
used on line 52.

# findmnt -n --output=source /
/dev/mapper/raspberry-btrfs[/bullseye/root/@]

This can be somewhat corrected with the --nofsroot flag:

# findmnt -n --output=source --nofsroot /
/dev/mapper/raspberry-btrfs

I wrote "somewhat" because Btrfs filesystems can span multiple devices 
and should therefore be referenced by UUID.

The part inside the square brackets above (/bullseye/root/@) is the 
btrfs subvolume, which is passed in an additional cmdline parameter:

rootflags=subvol=/bullseye/root/@

It's also possible to reference the subvolume by ID, but I'm unfamiliar 
with the use cases for that:

rootflags=subvolid=324


-- Scope

This bug should cause any Raspberry Pi system with a Btrfs root 
filesystem to be unable to boot whenever cmdline.txt is re-generated by 
z50-raspi-firmware, unless the workaround mentioned below is used.


-- Rescue

If this bug renders a system unbootable, you can either edit the 
cmdline.txt file on another system or manually mount the root filesystem 
on /root using the initramfs emergency shell, which is currently 
available by default via the serial terminal.


-- Workaround

I'm working around this problem by configuring ROOTPART in 
/etc/default/raspi-firmware, as such:

ROOTPART=/dev/mapper/raspberry-btrfs

And creating the /etc/default/raspi-extra-cmdline file, with the 
following line:

rootflags=subvol=/bullseye/root/@


-- Solution

The grub-common package has some interesting special handling of Btrfs 
(and ZFS) root filesystems in /etc/grub.d/10_linux that can be 
interesting to look at as a reference.
Ultimately, they get the filesystem UUID using grub-probe and the 
subvolume path using grub-mkrelpath.

We can get the same information using findmnt:

device="$(findmnt --noheadings --first-only --nofsroot --output=source /)"
subvolume="$(findmnt --noheadings --first-only --output=fsroot /)"
uuid="$(findmnt --noheadings --first-only --output=uuid /)"
fstype="$(findmnt --noheadings --first-only --output=fstype /)"

Then it's just a matter of testing when fstype equals btrfs and setting 
the cmdline parameters accordingly:

root=UUID=$uuid rootflags=subvol=$subvolume

Note: I tried guarding rootflags against whitespace in $subvolume using 
double quotes, but initramfs just blindly wordsplits the cmdline, so it 
can't be helped.

ZFS will likely need something similar, but I don't have any experience 
with it.


-- Hardware and storage layout tested

I'm using a Raspberry Pi 4 Model B with a Micro SD card and an USB HDD

The examples above are with the following storage configuration:

Micro SD Card
└─ FAT partition (mounted at /boot/firmware)

USB Hard Drive
└─ LVM Volume Group named "raspberry"
     └─ LVM Logical Volume named "btrfs" (/dev/mapper/raspberry-btrfs)
         └─ Btrfs filesystem
              └─ Btrfs subvolume at /bullseye/root/@ (mounted at /)

-- System Information:
Debian Release: bullseye/sid
APT prefers testing
APT policy: (500, 'testing')
Architecture: arm64 (aarch64)

Kernel: Linux 5.10.0-3-arm64 (SMP w/4 CPU threads)
Kernel taint flags: TAINT_CRAP
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE 
not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages raspi-firmware depends on:
ii dosfstools 4.2-1
ii dpkg 1.20.7.1

raspi-firmware recommends no packages.

raspi-firmware suggests no packages.

-- Configuration Files:
/etc/default/raspi-firmware changed [not included]

-- no debconf information

Best regards, João Matos