Re: [PATCH 1/1] block: Add support for setting inline encryption key per block device

[Christoph Hellwig <hch@lst.de>]

On Wed, Jul 20, 2022 at 11:49:07PM -0700, Eric Biggers wrote:
> On the other hand, I'd personally be fine with saying that this isn't actually
> needed, i.e. that allowing arbitrary overriding of the default key is fine,
> since userspace should just set up the keys properly.  For example, Android
> doesn't need this at all, as it sets up all its keys properly.  But I want to
> make sure that everyone is generally okay with this now, as I personally don't
> see a fundamental difference between this and what the dm-crypt developers had
> rejected *very* forcefully before.  Perhaps it's acceptable now simply because
> it wouldn't be part of dm-crypt; it's a new thing, so it can have new semantics.

I agree with both the dm-crypt maintainer and you on this.  dm-crypt is
a full disk encryption solution and has to provide guarantees, so it
can't let upper layers degrade the cypher.  The block layer support on
the other hand is just a building block an can as long as it is carefully
documented.

> I'm wondering if anyone any thoughts about how to allow data_unit_size >
> logical_block_size with this patch's approach.  I suppose that the ioctl could
> just allow setting it, and the block layer could fail any I/O that isn't
> properly aligned to the data_unit_size.

We could do that, but we'd need to comunicate the limit to the upper
layers both in the kernel an user space.  Effectively this changes the
logical block size for all practical purposes.