Re: [PATCH v4 3/7] iomap: support direct I/O with fscrypt using blk-crypto

[Eric Biggers <ebiggers@kernel.org>]

On Sat, Jul 25, 2020 at 07:42:11PM -0700, Eric Biggers wrote:
> > Exactly my point. Requiring infrastructure and storage layers to
> > obey completely new, undefined, undiscoverable, opaque and variable
> > definition of the block devices' "atomic unit of IO", then that's
> > simply a non-starter. That requires a complete re-architecture of
> > the block layers and how things interface and transmit information
> > through them. At minimum, high level IO alignment constraints must
> > be generic and not be hidden in context specific crypto structures.
> 
> Do you have any specific examples in mind of where *encrypted* I/O may broken at
> only a logical_block_size boundary?  Remember that encrypted I/O with a
> particular data_unit_size is only issued if the request_queue has declared that
> it supports encryption with that data_unit_size.  In the case of a layered
> device, that means that every layer would have to opt-into supporting encryption
> as well as the specific data_unit_size.
> 
> Also, the alignment requirement is already passed down the stack as part of the
> bio_crypt_ctx.  If there do turn out to be places that need to use it, we could
> easily define generic helper functions:
> 
> unsigned int bio_required_alignment(struct bio *bio)
> {
>         unsigned int alignmask = queue_logical_block_size(bio->bi_disk->queue) - 1;
> 
> #ifdef CONFIG_BLK_INLINE_ENCRYPTION
>         if (bio->bi_crypt_context)
>                 alignmask |= bio->bi_crypt_context->bc_key->crypto_cfg.data_unit_size - 1;
> #endif
> 
>         return alignmask + 1;
> }
> 
> unsigned int rq_required_alignment(struct request *rq)
> {
>         unsigned int alignmask = queue_logical_block_size(rq->q) - 1;
> 
> #ifdef CONFIG_BLK_INLINE_ENCRYPTION
>         if (rq->crypt_ctx)
>                 alignmask |= rq->crypt_ctx->bc_key->crypto_cfg.data_unit_size - 1;
> #endif
> 
>         return alignmask + 1;
> }
> 
> Sure, we could also add a new alignment_required field to struct bio and struct
> request, but it would be unnecessary since all the information is already there.
> 
> > > Is it your opinion that inline encryption should only be supported when
> > > data_unit_size <= logical_block_size?  The problems with that are
> > 
> > Pretty much.
> > 
> > >     (a) Using an unnecessarily small data_unit_size degrades performance a
> > > 	lot -- for *all* I/O, not just direct I/O.  This is because there are a
> > > 	lot more separate encryptions/decryptions to do, and there's a fixed
> > > 	overhead to each one (much of which is intrinsic in the crypto
> > > 	algorithms themselves, i.e. this isn't simply an implementation quirk).
> > 
> > Performance is irrelevant if correctness is not possible.
> > 
> 
> As far as I know, data_unit_size > logical_block_size is working for everyone
> who has used it so far.
> 
> So again, I'm curious if you have any specific examples in mind.  Is this a
> real-world problem, or just a theoretical case where (in the future) someone
> could declare support for some data_unit_size in their 'struct request_queue'
> (possibly for a layered device) without correctly handling alignment in all
> cases?
> 
> I do see that logical_block_size is used for discard, write_same, and zeroout.
> But that isn't encrypted I/O.
> 
> BTW, there might very well be hardware that *only* supports
> data_unit_size > logical_block_size.

I found get_max_io_size() in block/blk-merge.c.  I'll check if that needs to be
updated.

Let me know if you have any objection to the fscrypt inline encryption patches
*without direct I/O support* going into 5.9.  Note that fscrypt doesn't directly
care about this block layer stuff at all; instead it uses
blk_crypto_config_supported() to check whether inline encryption with the
specified (crypto_mode, data_unit_size, dun_bytes) combination is supported on
the filesystem's device(s).  Only then will fscrypt use inline encryption
instead of the traditional filesystem-layer encryption.

So if blk_crypto_config_supported() is saying that some crypto configuration is
supported when it isn't, then that's a bug in the blk-crypto patches that went
into the block layer in 5.8, which we need to fix there.  (Ideally by fixing any
cases where encrypted I/O may be split in the middle of a data unit.  But in the
worst case, we could easily make blk_crypto_config_supported() return false when
'data_unit_size > logical_block_size' for now.)

- Eric