linux-kernel - Re: Chaining is dead

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Message-ID: <20250401033303.GA56851@sol.localdomain>
Date: Mon, 31 Mar 2025 20:33:03 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>
Subject: Re: Chaining is dead

On Tue, Apr 01, 2025 at 10:44:34AM +0800, Herbert Xu wrote:
> Thanks for the pointer, I wasn't aware of its existence.  Yes
> there should definitely be only one code path for this.  So
> what's stopping you from removing fscrypt right now? IOW what's
> missing from blk-crypto?

Well, fscrypt (ext4/f2fs/ubifs/ceph encryption) wouldn't be removed; its
implementation would just change on ext4 and f2fs.  Instead of providing a
choice of whether to use blk-crypto or fs-layer crypto for file contents
en/decryption, ext4 and f2fs would always use blk-crypto.  Supporting code such
as fscrypt_decrypt_bio() that would become unused by that would be removed.

A few reasons I've waited so long:

- The fs-layer file contents en/decryption code was there first, and there
  hasn't been a strong need to remove it yet
- Much of the file contents en/decryption code in fs/crypto/ would still be
  needed, since ubifs and ceph still use it as they are not block-based
- It would make CONFIG_BLK_INLINE_ENCRYPTION, which adds a field to struct bio,
  required on more systems
- It would add the overhead of keyslot management to software crypto
- blk-crypto currently always uses inline encryption hardware when it's
  available; but, I'd like to preserve ext4's and f2fs's existing behavior where
  the use of inline encryption hardware is opt-in via a mount option.

But I'm thinking it's finally time, especially with the conversions of
filesystems to operate on folios that's going on.

dm-crypt could of course use blk-crypto too, but the dm people haven't been
super comfortable so far with delegating en/decryption to the block layer.

- Eric