[systemd-devel] About a problem with GRUB2 rescue images and /dev/disk/by-label

[Thomas Schmitt]

Hi,

the content of /dev/disk/by-label does not look right with
USB sticks which carry bootable ISO 9660 images produced
by GRUB2 script grub-mkrescue.

The maintainer of Debian's udev package advised me to report
the problem here:

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A not mountable partition becomes link target in /dev/disk/by-label
under the name of the filesystem on the mountable parent device.

grub-mkrescue ISO images are suitable for booting from CD/DVD
and from USB stick. They begin by an MBR which marks a single
partition beginning at LBA 1. The ISO filesystem has to begin
at LBA 0, because else it would not be bootable from CD/DVD.

So the partition is not mountable, intentionally. It just protects
the ISO filesystem from partition editors and alike, but leaves
the MBR unclaimed in order to please some boot firmware.

I verified that the problem still exists in
  http://cgit.freedesktop.org/systemd/systemd/plain/rules/60-persistent-storage.rules
as of today, by experimenting with archlinux-2013.11.01-dual.iso
which has an identical file 60-persistent-storage.rules .

---------------------------------------------------------------

When i discussed this issue at grub-devel, Andrey Borzenkov pointed
me to this rule in 60-persistent-storage.rules

# for partitions import parent information
ENV{DEVTYPE}=="partition", IMPORT{parent}="ID_*"

The properties ID_FS_USAGE and ID_FS_LABEL_ENC get imported
and later cause the wrong link:

ENV{ID_FS_USAGE}=="filesystem|other", ENV{ID_FS_LABEL_ENC}=="?*", SYMLINK+="disk/by-label/$env{ID_FS_LABEL_ENC}"

This happens because the blkid rule does not overwrite the
imported ID_FS_LABEL_ENC and ID_FS_TYPE by own findings:

# probe filesystem metadata of disks
KERNEL!="sr*", IMPORT{builtin}="blkid"


To prove my assessment, i booted archlinux-2013.11.01-dual.iso,
verified that the link in /dev/disk/by-label is wrong, and then
set some properties to empty text. I picked the names from the
output of blkid -o udev:

# for partitions import parent information
ENV{DEVTYPE}=="partition", IMPORT{parent}="ID_*", \
        ENV{ID_FS_LABEL}="" , ENV{ID_FS_LABEL_ENC}="" , \
        ENV{ID_FS_TYPE}="" , ENV{ID_FS_USAGE}=""

After this change, udevadm control --reload, and replugging
the USB stick, i get in /dev/disk/by-label a mountable link
to /dev/sdb.

So i assume that with the original rules file, the link to
the parent device was made, but immediately overwritten by the
inappropriate link to the partition.

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I fail to imagine a use case where a filesystem ID should
propagate from mountable parent to not mountable partition.
Is there any ?


Have a nice day :)

Thomas