WIP: Protective/fake/overlapping MBR/partition table support

[tbzatek]

(this is a WIP proposal and will be updated along my thought process)

Some mkfs tools as called via org.freedesktop.UDisks2.Block.Format() create a protective partition table with a partition that overlaps the partition table area (i.e. a compat/protective/fake partition table that is part of the filesystem superblock itself). That is typically detected by the kernel and extra block devices are exposed by udev.

Requirements:

• keep the UDisks API stable, expect that the current clients will continue using the original block device for mounting or further usage

Open questions:

• consider adding a new org.freedesktop.UDisks2.Block.Format() option indicating that the client understands the protective partition table and switches to a child block device usage instead of whole-disk

Suggestions:

• either add new property ProtectiveTable to the org.freedesktop.UDisks2.PartitionTable interface or add new type protective-mbr in the org.freedesktop.UDisks2.PartitionTable.Type enum when one of the partitions start at 0 sector boundary. Currently with util-linux-2.37 the value is empty for VFAT.
• deny any calls to org.freedesktop.UDisks2.Block.Format() on the partitions in such case, allow only on the whole-disk object. This will also make sure wipefs is called on the right device to erase the remains of such partition table.
• when mkfs is called on an unpartitioned block device (no org.freedesktop.UDisks2.PartitionTable or org.freedesktop.UDisks2.Partition interfaces present on the block object), issue BLKRRPART and trigger uevent sync in case the filesystem type hints indicate a possible protective partition table presence (will be provided by libblockdev 3.x filesystem definition flags). Then check for presence of the org.freedesktop.UDisks2.PartitionTable interface and trigger uevents on all the partitions if any. All this within the actual method call.

Related:

• add simple tests to expect a child block object
• multiple-fs-over-format tests: #858
• (optional) reuse some of the exotic filesystem scenario images from util-linux: https://github.com/karelzak/util-linux/tree/master/tests/ts/blkid/images-fs

[tbzatek]

Updated the description with implemented features in #901. Still some work left to do...