Re: [PATCH v4 1/8] crypto: shash - add support for finup_mb

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Jun 04, 2024 at 08:55:48PM +0200, Ard Biesheuvel wrote:
> >
> > This patch takes a new approach of just adding an API
> > crypto_shash_finup_mb() that synchronously computes the hash of multiple
> > equal-length messages, starting from a common state that represents the
> > (possibly empty) common prefix shared by the messages.
> >
> 
> This is an independent optimization, right? This could be useful even
> more sequential hashing, and is not a fundamental aspect of parallel
> hashing?

If you're referring to the part about using a common starting state, that's not
an independent optimization.  Only multibuffer hashing processes multiple
messages in one call and therefore has an opportunity to share a starting
shash_desc for finup.  This isn't just an optimization but it also makes the
multibuffer hashing API and its implementation much simpler.

With single-buffer there has to be one shash_desc per message as usual.

If you're asking if crypto_shash_finup_mb() can be used even without multibuffer
hashing support, the answer is yes.  This patchset makes crypto_shash_finup_mb()
fall back to crypto_shash_finup() as needed, and this is used by fsverity and
dm-verity to have one code path that uses crypto_shash_finup_mb() instead of
separate code paths that use crypto_shash_finup_mb() and crypto_shash_finup().
This just makes things a bit simpler and isn't an optimization; note that the
fallback has to copy the shash_desc for each message beyond the first.

- Eric