dm-verity: How to exactly use the dm-mod.create with verity-metadata append

[Pintu Agarwal]

Hi,

For rootfs dm-verity I am trying to pass dm-mod.create from our
bootloader but it seems not working for me.
So, I need some guidance on the parameters that we pass here.
The documentation also does not seem to help much.

Kernel: 4.14 (with dm-init patch backported)
Target: Arm-32 / NAND / Simple Busybox / Bootloader (edk2)
Build: Ubuntu-18.04 / Yocto 2.6

Steps I followed:
1) First I am trying to generate the root hash for our rootfs using
the veritysetup command:
$ ls -l system.img
64172032 ==> IMAGE_SIZE
$ veritysetup format system.img dm-init-verity.img
UUID:                   eca62b73-b66a-4249-834b-471e83fc382c
Hash type:              1
Data blocks:            15667
Data block size:        4096
Hash block size:        4096
Hash algorithm:         sha256
Salt:
8b66f42c07f576429109cf4e5d12ec072b23d242a9e653ac3423e49647339f5b
Root hash:
10d9036f6efdd48dd49f09c8ece016a36a2c4d9a01a1f77f01485c65cf0e78af

2) Then I am trying to append the verity with the system image itself:
$ cat dm-init-verity.img >> system.img

3) After that I am trying to pass dm-mod.create parameter like this:
dm-mod.create=\"system,,,ro, 0 IMAGE_SIZE/512 verity 1
/dev/ubiblock0_0 /dev/ubiblock0_0 4096 4096 DATA_BLOCKS 1 sha256
10d9036f6efdd48dd49f09c8ece016a36a2c4d9a01a1f77f01485c65cf0e78af
8b66f42c07f576429109cf4e5d12ec072b23d242a9e653ac3423e49647339f5b\"

4) The Kernel command line seems to be updated properly:
[    0.000000] Kernel command line:.. rootfstype=squashfs
ubi.mtd=40,0,30 ubi.block=0,0 root=/dev/ubiblock0_0
dm-mod.create="system,,,ro, 0 125336 verity 1 /dev/ubiblock0_0
/dev/ubiblock0_0 4096 4096 15667 1 sha256
10d9036f6efdd48dd49f09c8ece016a36a2c4d9a01a1f77f01485c65cf0e78af
8b66f42c07f576429109cf4e5d12ec072b23d242a9e653ac3423e49647339f5b" ....

But it does not seem to work as expected.
It gives below errors:
....
[    4.747708] block ubiblock0_0: created from ubi0:0(system)
[    4.752313] device-mapper: init: waiting for all devices to be
available before creating mapped devices
[    4.752313]
[    4.766061] device-mapper: verity: sha256 using implementation
"sha256-generic"
[    4.776178] device-mapper: ioctl: dm-0 (system) is ready
[    4.848886] md: Skipping autodetection of RAID arrays.
(raid=autodetect will force)
[    4.849288] VFS: Cannot open root device "ubiblock0_0" or
unknown-block(252,0): error -16
....

I followed almost the same example from dm-init document:
"verity":
  dm-verity,,4,ro,
    0 1638400 verity 1 8:1 8:2 4096 4096 204800 1 sha256
    fb1a5a0f00deb908d8b53cb270858975e76cf64105d412ce764225d53b8f3cfd
    51934789604d1b92399c52e7cb149d1b3a1b74bbbcb103b2a0aaacbed5c08584

But this seems only refer to system and verity on a different blocks.
I am not sure what parameter should be changed if my verity metadata
is part of system image itself.
Also, I don't know how 1638400;204800;1 is calculated here based on image size ?

So, people who have made this working successfully, please share the
correct parameter to be used for the same block device.

Thanks,
Pintu