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Upcomming trouble with grub-mkrescue ISOs and libblkid

From: Thomas Schmitt
Subject: Upcomming trouble with grub-mkrescue ISOs and libblkid
Date: Sat, 14 Dec 2019 11:58:12 +0100


there is trouble at the horizon by

which disables the recognition of ISO 9660 filesystems on the base device
in favor of potential partitions which may or may not mark the range of
the ISO filesystem.

This causes Fedora ISOs to fail booting from /dev/srN
because the /dev/disk/by-label/ link is not created by udev.

The commit shall obviously heal a shortcomming of the old questionable
udev rule
  # for partitions import parent information
  ENV{DEVTYPE}=="partition", IMPORT{parent}="ID_*"
when it meets a USB stick with an ISO on the base device and a partition
which has no own filesystem. The latter is a rare situation.

The effect on /dev/srN is obviously unintended.
To my theory, libblkid detects the partition table on the DVD and bets
that the Linux kernel will create partition devices.
But with /dev/srN there will no partition devices be created.

I expect grub-mkrescue x86 ISOs to show no ISO 9660 at all, because they get
a MBR partition starting at disk block 1 in the BIOS-only case, and a GPT
with no ISO 9660 partition in the EFI-case.
The only way to access their ISO filesystems is via the base device.

This was a design decision by Vladimir Serbinenko about 10 years ago.

It has to be expected that payload operating systems of grub-mkrescue,
which use the new blkid, will not find their origin device after start-up,
similar to what happens to Fedora ISOs now.

Workarounds to let grub-mkrescue ISO 9660 filesystems show up for the new
blkid could be the modifications made by
MKRESCUE_SED_MODE "mbr_only", "mbr_hfs",
or "gpt_appended" with -partition_offset 16 will create partitions which
mark the ISO 9660 filesystem. (All against Vladimir's idea of partition
layout, of course.)

Best solution would be to convince util-linux to not ignore filesystems
and to rather urge udev to fix its rule.
(Does anybody know why udev attributes the filesystem of the base device
 to any partition which has no own filesystem ?)

Have a nice day :)


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