On 7/9/25 9:09 PM, Eric Biggers wrote:
The support for asynchronous hashes in dm-verity has outlived its
usefulness. It adds significant code complexity and opportunity for
bugs. I don't know of anyone using it in practice. (The original
submitter of the code possibly was, but that was 8 years ago.) Data I
recently collected for en/decryption shows that using off-CPU crypto
"accelerators" is consistently much slower than the CPU
(https://lore.kernel.org/r/20250704070322.20692-1-ebiggers@xxxxxxxxxx/),
even on CPUs that lack dedicated cryptographic instructions. Similar
results are likely to be seen for hashing.
I already removed support for asynchronous hashes from fsverity two
years ago, and no one ever complained.
Moreover, neither dm-verity, fsverity, nor fscrypt has ever actually
used the asynchronous crypto algorithms in a truly asynchronous manner.
The lack of interest in such optimizations provides further evidence
that it's only the CPU-based crypto that actually matters.
Historically, it's also been common for people to forget to enable the
optimized SHA-256 code, which could contribute to an off-CPU crypto
engine being perceived as more useful than it really is. In 6.16 I
fixed that: the optimized SHA-256 code is now enabled by default.
Therefore, let's drop the support for asynchronous hashes in dm-verity.
Tested with verity-compat-test.
Hi,
I shortly tested it with veritysetup too, also on 32bit.
And I like this patch (I wish we can remove the async thing from the dmcrypt too...)
Just one nitpick - could you please increase minor version of dm-verity target,
so we have information in debug log that it is a patched version?
Thanks,
Milan
Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
Changed in v2:
- Removed the now-unused 'may_sleep' parameter from verity_hash()
- Fixed a typo in commit message
- Added comment in verity_setup_salt_and_hashstate()
- Keep SHASH_DESC_ON_STACK in existing place in
verity_setup_salt_and_hashstate(), to reduce the diff slightly
- Added Ard's Acked-by
drivers/md/dm-verity-fec.c | 4 +-
drivers/md/dm-verity-target.c | 183 ++++++----------------------------
drivers/md/dm-verity.h | 22 ++--
3 files changed, 37 insertions(+), 172 deletions(-)
