Kernel 4.14: Using dm-verity with squashfs rootfs - mounting issue

Pintu Agarwal
2021-07-20T09:39:14Z

Hi,

Our ARM32 Linux embedded system consists of these:
* Linux Kernel: 4.14
* Processor: Qualcomm Arm32 Cortex-A7
* Storage: NAND 512MB
* Platform: Simple busybox
* Filesystem: UBIFS, Squashfs
* Consists of nand raw partitions, squashfs ubi volumes.

My requirement:
We wanted to use dm-verity at boot time to check the integrity of
squashfs-rootfs before mounting.

Problem:
dm-0 is not able to locate and mount the squash fs rootfs block.
The same approach is working when emulating with ext4 but fails with squashfs.

Logs:
[....]
[ 0.000000] Kernel command line: [...] verity="96160 12020
d7b8a7d0c01b9aec888930841313a81603a50a2a7be44631c4c813197a50d681 0 "
rootfstype=squashfs root=/dev/mtdblock34 ubi.mtd=30,0,30 [...]
root=/dev/dm-0 dm="system none ro,0 96160 verity 1 /dev/mtdblock34
/dev/mtdblock39 4096 4096 12020 8 sha256
d7b8a7d0c01b9aec888930841313a81603a50a2a7be44631c4c813197a50d681
aee087a5be3b982978c923f566a94613496b417f2af592639bc80d141e34dfe7"
[....]
[ 4.693620] vreg_conn_pa: disa▒[ 4.700662] md: Skipping
autodetection of RAID arrays. (raid=autodetect will force)
[ 4.700713] device-mapper: init: attempting early device configuration.
[ 4.708224] device-mapper: init: adding target '0 96160 verity 1
/dev/mtdblock34 /dev/mtdblock39 4096 4096 12020 8 sha256
d7b8a7d0c01b9aec888930841313a81603a50a2a7be44631c4c813197a50d681
aee087a5be3b982978c923f566a94613496b417f2af592639bc80d141e34dfe7'
[ 4.714979] device-mapper: verity: sha256 using implementation
"sha256-generic"
[ 4.737808] device-mapper: init: dm-0 is ready
[....]
[ 5.278103] No filesystem could mount root, tried:
[ 5.278107] squashfs
[ 5.280477]
[ 5.287627] Kernel panic - not syncing: VFS: Unable to mount root
fs on unknown-block(253,0)
[...]

Not sure, why is it still locating block "253" here which seems like a
MAJOR number ?

Working logs on ext4:
[....]
[ 4.529822] v▒[ 4.534035] md: Skipping autodetection of RAID
arrays. (raid=autodetect will force)
[ 4.534087] device-mapper: init: attempting early device configuration.
[ 4.550316] device-mapper: init: adding target '0 384440 verity 1
/dev/ubiblock0_0 /dev/ubiblock0_0 4096 4096 48055 48063 sha256
a02e0c13afb31e99b999c64aae6f4644c24addbc58db5689902cc5ba0be2d15b
aee087a5be3b982978c923f566a94613496b417f2af592639bc80d141e34dfe7 10
restart_on_corruption ignore_zero_blocks use_fec_from_device
/dev/ubiblock0_0 fec_roots 2 fec_blocks 48443 fec_start 48443'
[ 4.572215] device-mapper: verity: sha256 using implementation
"sha256-generic"
[ 4.610692] device-mapper: init: dm-0 is ready
[ 4.720174] EXT4-fs (dm-0): mounted filesystem with ordered data
mode. Opts: (null)
[ 4.720438] VFS: Mounted root (ext4 filesystem) readonly on device 253:0.
[ 4.737256] devtmpfs: mounted
[....]

Questions:
a) Is dm-verity supposed to work on squashfs block devices ?
b) Are there any known issues with dm-verity on Kernel 4.14 ?
c) Are there any patches that we are missing ?

Thanks,