[dm-devel] [RFC PATCH 0/2] dm crypt: Allow unaligned buffer lengths for skcipher devices

[Eric Biggers]

On Wed, Sep 23, 2020 at 09:27:32PM -0400, Mike Snitzer wrote:
> You've clearly done a nice job with these changes.  Looks clean.
> 
> BUT, I'm struggling to just accept that dm-crypt needs to go to these
> extra lengths purely because of one bad apple usecase.
> 
> These alignment constraints aren't new.  Are there other portions of
> Linux's crypto subsystem that needed comparable fixes in order to work
> with Microsfot OS initiated IO through a guest?
> 
> You forecast that these same kinds of changes are needed for AEAD and
> dm-integrity... that's alarming.
> 
> Are we _certain_ there is no other way forward?
> (Sorry I don't have suggestions.. I'm in "fact finding mode" ;)
> 

I don't understand why this is needed, since dm-crypt already sets its
logical_block_size to its crypto sector_size.  Isn't it expected that I/O that
isn't aligned to logical_block_size fails?  It's the I/O submitter's
responsibility to ensure logical_block_size alignment of all I/O segments.
Exactly how is the misaligned I/O actually being submitted here?

- Eric