Re: [PATCH v5 7/9] fscrypt: add inline encryption support

Eric Biggers
2019-11-05T01:03:19Z

On Mon, Nov 04, 2019 at 04:15:54PM -0800, Christoph Hellwig wrote:
> > I don't think combining these things is a good idea because it would restrict
> > the use of inline encryption to filesystems that allow IV_INO_LBLK_64 encryption
> > policies, i.e. filesystems that have stable inode numbers, 32-bit inodes, and
> > 32-bit file logical block numbers.
> >
> > The on-disk format (i.e. the type of encryption policy chosen) and the
> > implementation (inline or filesystem-layer crypto) are really two separate
> > things. This was one of the changes in v4 => v5 of this patchset; these two
> > things used to be conflated but now they are separate. Now you can use inline
> > encryption with the existing fscrypt policies too.
> >
> > We could use two separate SB_* flags, like SB_INLINE_CRYPT and
> > SB_IV_INO_LBLK_64_SUPPORT.
>
> Yes, I think that is a good idea.
>
> > However, the ->has_stable_inodes() and
> > ->get_ino_and_lblk_bits() methods are nice because they separate the filesystem
> > properties from the question of "is this encryption policy supported".
> > Declaring the filesystem properties is easier to do because it doesn't require
> > any fscrypt-specific knowledge. Also, fs/crypto/ could use these properties in
> > different ways in the future, e.g. if another IV generation scheme is added.
>
> I don't really like writing up method boilerplates for something that
> is a simple boolean flag.

fs/crypto/ uses ->has_stable_inodes() and ->get_ino_and_lblk_bits() to print an
appropriate error message. If we changed it to a simple flag we'd have to print
a less useful error message. Also, people are basically guaranteed to not
understand what "SB_IV_INO_LBLK_64_SUPPORT" means exactly, and are likely to
copy-and-paste it incorrectly when adding fscrypt support to a new filesystem.
Also it would make it more difficult to add other fscrypt IV generation schemes
in the future as we'd then need to add another sb flag (e.g. SB_IV_INO_LBLK_128)
and make filesystem-specific changes, rather than change fs/crypto/ only.

So personally I'd prefer to keep ->has_stable_inodes() and
->get_ino_and_lblk_bits() for now.

Replacing ->inline_crypt_enabled() with SB_INLINE_CRYPT makes much more sense
though.

- Eric