Re: FAQ / encryption / error handling?

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2017-11-27 05:06, Dmitrii Tcvetkov wrote:
> On Mon, 27 Nov 2017 09:06:12 +0100
> Daniel Pocock <daniel@pocock.pro> wrote:
> 
>> Hi all,
>>
>> The FAQ has a couple of sections on encryption (general and dm-crypt)
>>
>> One thing that isn't explained there: if you create multiple encrypted
>> volumes (e.g. using dm-crypt) and use Btrfs to combine them into
>> RAID1, how does error recovery work when a read operation returns
>> corrupted data?
>>
>> Without encryption, reading from one disk would give a checksum
>> mismatch and Btrfs would read from the other disk to (hopefully) get
>> a good copy of the data.
>>
>> With this encryption scenario, the failure would potentially be
>> detected in the decryption layer code and instead of returning bad
>> data to Btrfs, it would return some error code. In that case, will
>> Btrfs attempt to read from the other volume and allow the application
>> to proceed as if nothing was wrong?
>>
>> Regards,
>>
>> Daniel
> 
> Default (aes-xts-plain64) dm-crypt setup can't verify integrity
> of encrypted block and in case of silent corruption will decrypt it to
> garbage which btrfs will catch. In case of AEAD encryption
> (dm-crypt plus dm-integrity) it can verify integrity itself but I'm not
> sure right now which exact error it returns to upper layer as I didn't
> used it yet.
The exact error shouldn't matter, provided that BTRFS perceives it as a 
read error from the 'device' (in reality the virtual DM device). 
Provided that condition is met, the error is handled pretty much the 
same regardless of the exact error code.
> 
> I use btrfs raid1 on top of LVM on top of dm-crypt devices and
> it handled bad blocks on physical devices normally (there was a burst of
> about 900 reallocates on one device which btrfs caught and fixed).Same here, and I've also tested it on top of dm-integrity, where BTRFS 
will correctly handle errors passed up from dm-integrity failing to 
verify blocks.