Re: [PATCH v7 3/9] block: blk-crypto-fallback for Inline Encryption

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Feb 21, 2020 at 03:50:44AM -0800, Satya Tangirala wrote:
> Blk-crypto delegates crypto operations to inline encryption hardware when
> available. The separately configurable blk-crypto-fallback contains a
> software fallback to the kernel crypto API - when enabled, blk-crypto
> will use this fallback for en/decryption when inline encryption hardware is
> not available. This lets upper layers not have to worry about whether or
> not the underlying device has support for inline encryption before
> deciding to specify an encryption context for a bio, and also allows for
> testing without actual inline encryption hardware. For more details, refer
> to Documentation/block/inline-encryption.rst.
> 
> Signed-off-by: Satya Tangirala <satyat@google.com>

In v7, only blk_mq_make_request() actually calls blk_crypto_bio_prep().
That will make the crypto contexts be silently ignored (no fallback) if
q->make_request_fn != blk_mq_make_request.

In recent kernels that *hopefully* won't matter in practice since almost
everyone is using blk_mq_make_request.  But it still seems like a poor design.
First, it's super important that if someone requests encryption, then they
either get it or get an error; it should *never* be silently ignored.  Second,
part of the goal of blk-crypto-fallback is that it should always work, so that
in principle users don't have to implement the encryption twice, once via
blk-crypto and once via fs or dm-layer crypto.

So is there any reason not to keep the blk_crypto_bio_prep() call in
generic_make_request()?

I think performance can't be much of a complaint, since if almost everyone is
using blk_mq_make_request() then they are making the function call anyway...

- Eric