Some devices (e.g. android phones running pmOS) cannot have their OEM
partition table altered without breaking the firmware, so the distros's
partitions live inside a nested GPT carved into one of the OEM
partitions. Exposing these subpartitions requires wrapping the outer
partition in a loop device with partscan enabled, since the kernel does
not go into nested partition tables.
systemd already detects this case in udev-builtin-blkid
(ID_PART_GPT_AUTO_ROOT_DISK_NEEDS_LOOP) and acts on with
systemd-loop@.service, but this fails towards the end.
loop_device_make_internal has an optimization where if the input is
already a block device with a matching sector size, it skips creating
a loop and just hands back the original fd. That's fine for whole disks
but wrong for partitions, which don't support partscan, so this causes
dissect_image to fail with EPROTONOSUPPORT.
This patch changes the behavior to only take the shortcut when the input
is a whole disk, or when partscan was not requested.
Co-Authored-By: Clayton Craft <clayton@craftyguy.net>
Previously, loop_device_make_internal() always used the block device
directly (via loop_device_open_from_fd()) for whole-device access,
regardless of sector size. This is incorrect when the GPT partition
table was written with a different sector size than the device reports,
as happens with CD-ROM/ISO boot via El Torito: the device has
2048-byte blocks but the GPT uses 512-byte sectors.
Restructure the sector size handling in loop_device_make_internal():
- Move GPT sector size probing (UINT32_MAX case) before the
block-vs-regular-file split so both paths share the same logic and
O_DIRECT handling. Check f_flags instead of loop_flags for O_DIRECT
detection, since we're probing the original fd before any reopening.
- For block devices, get the device sector size and compare it against
the resolved sector_size. Only use the block device directly when
sector sizes match. When they differ (probed GPT mismatch or explicit
sector size request), fall through to create a real loop device with
the correct sector size.
- Default sector_size=0 to the device sector size for block devices
(instead of always 512), so "no preference" correctly matches the
device's sector size.
Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>
While we're at it, rename to test-loop-util.c so it matches its
source file.
We also drop the root check and solely check for CAP_SYS_ADMIN since
that's sufficient to run the tests.
Clayton Craft
Daan De Meyer