Virtual block devices are a bit weird: they have no parent device, and
thus cannot be related to the subsystem they belong to, except by
pattern matching their name. This is OK to do if one knows what to look
for. However for tools that do not want to carry a list of known
subsystems with their appropriate matching patters this sucks. Let's
introduce a new ID_BLOCK_SUBSYSTEM property we can set on block devices
that carries an explicit string for this. Do so for a small number of
key subsystems: DM, loopback and zram.
Loopback block devices are agressively reused, without being removed in
between. This means various inode attributes on their device nodes will
– so far – remain in effect between uses of the devices. Since there are
applications which change access mode/ownership of such devices after
attaching files to them, let's undo this again when we detect them to be
unused again.
Fixes: #37745
Tools such as lsblk which query the udev database instead of probing
devices directly fail when run on ublk devices. For instance, in the
following commands, the partition type is missing, despite the fact that
/dev/ublkb0 was just partitioned with a single Linux filesystem type
partition.
$ lsblk /dev/ublkb0
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
ublkb0 259:0 0 31.3G 0 disk
└─ublkb0p1 259:1 0 31.2G 0 part
$ lsblk -o pkname,parttype /dev/ublkb0
PKNAME PARTTYPE
ublkb0
This happens because ublk devices are missing from a couple of
whitelists in the udev rules which are responsible for populating the
database with the data lsblk is looking for. Add the ublk devices to
these whitelists.
The /dev/zramN devices can be used as regular block devices. They are
typically used for swap areas, but it would be beneficial to have
LABEL and UUID in the udev database to make it more user-friendly for
tools such as lsblk or mount (if used with other filesystems).
The RADOS Block Device (rbd) can be used as any other block device with
further layers on top of it, hence allow the common persistent storage
rules to apply, including watching for changes.
This change is only about the source tree. We have tmpfiles.d/, modprobe.d/,
sysctl.d/, and sysusers.d/, but for historical reasons, rules/ didn't fit this
pattern. We also *install* it as rules.d/. Let's rename to be consistent.
