Re: [PATCH v6 11/11] blksnap: prevents using devices with data integrity or inline encryption

Eric Biggers
2023-11-28T17:18:30Z

On Tue, Nov 28, 2023 at 12:00:17PM +0100, Sergei Shtepa wrote:
> But I haven't tested the code on a device where hardware inline encryption is
> available. I would be glad if anyone could help with this.
> >
> > Anyway, this patch is better than ignoring the problem. It's worth noting,
> > though, that this patch does not prevent blksnap from being set up on a block
> > device on which blk-crypto-fallback is already being used (or will be used).
> > When that happens, I/O will suddenly start failing. For usability reasons,
> > ideally that would be prevented somehow.
>
> I didn't observe any failures during testing. It's just that the snapshot
> image shows files with encrypted names and data. Backup in this case is
> useless. Unfortunately, there is no way to detect a blk-crypto-fallback on
> the block device filter level.

Huh, I thought that this patch is supposed to exclude blk-crypto-fallback too.
__submit_bio() calls bio->bi_bdev->bd_filter->ops->submit_bio(bio) before
blk_crypto_bio_prep(), so doesn't your check of ->bi_crypt_context exclude
blk-crypto-fallback?

I think you're right that it might actually be fine to use blksnap with
blk-crypto-fallback, provided that the encryption is done first. I would like
to see a proper explanation of that, though. And we still have this patch which
claims that it doesn't work, which is confusing.

- Eric