[dm-crypt] veritysetup forward error correction failure

[Tom Eccles <tom.eccles@codethink.co.uk>]

Hello,

I have a question about veritysetup using forward error correction.

I created my dm-verity volume using

veritysetup --check-at-most-once --fec-device /dev/vdc --fec-roots 24 format
/dev/vdb1 /dev/vdd

Then I introduce a single bit error into /dev/vdb1.

If I then open the dm-verity device using

veritysetup --fec-device /dev/vdc --fec-roots 24 open /dev/vdb1 vroot
/dev/vdd <hash>

When I mount the device and inspect the corrupted file, the single bit error
cannot be corrected:
[  108.994804] device-mapper: verity-fec: 254:17: FEC 19300352: failed to
correct: -74
[  108.994834] device-mapper: verity: 254:17: data block 273411 is corrupted

However, if I verify the whole device using veritysetup it succeeds:

veritysetup --fec-device /dev/vdc --fec-roots 24 verify /dev/vdb1 /dev/vdd
<hash>
Verification failed at position 1119891456.
Verification of data area failed.
Found 1 repairable errors with FEC device

So I suspect that I have the wrong flags to veritysetup open. Can anybody
see where I am going wrong?

Thanks,
Tom