Re: [PATCH 1/2] ext4: Handle casefolding with encryption

On Feb 9, 2021, at 4:22 PM, Theodore Ts'o [off-list ref] wrote:

On Wed, Feb 03, 2021 at 11:31:28AM -0500, Theodore Ts'o wrote:

quoted

On Wed, Feb 03, 2021 at 03:55:06AM -0700, Andreas Dilger wrote:

quoted

It looks like this change will break the dirdata feature, which is similarly
storing a data field beyond the end of the dirent. However, that feature also
provides for flags stored in the high bits of the type field to indicate
which of the fields are in use there.
The first byte of each field stores
the length, so it can be skipped even if the content is not understood.

Daniel, for context, the dirdata field is an out-of-tree feature which
is used by Lustre, and so has fairly large deployed base.  So if there
is a way that we can accomodate not breaking dirdata, that would be
good.

Did the ext4 casefold+encryption implementation escape out to any
Android handsets?

So from an OOB chat with Daniel, it appears that the ext4
casefold+encryption implementation did in fact escape out to Android
handsets.  So I think what we will need to do, ultiumately, is support
one way of supporting the casefold IV in the case where "encryption &&
casefold", and another way when "encryption && casefold && dirdata".

That's going to be a bit sucky, but I don't think it should be that
complex.  Daniel, Andreas, does that make sense to you?

I was just going to ping you about this, whether it made sense to remove
this feature addition from the "maint" branch (i.e. make a 1.45.8 without
it), and keep it only in 1.46 or "next" to reduce its spread?

Depending on the size of the "escape", it probably makes sense to move
toward having e2fsck migrate from the current mechanism to using dirdata
for all deployments.  In the current implementation, tools don't really
know for sure if there is data beyond the filename in the dirent or not.

I guess it is implicit with the casefold+encryption case for dirents in
directories that have the encryption flag set in a filesystem that also
has casefold enabled, but it's definitely not friendly to these features
being enabled on an existing filesystem.

For example, what if casefold is enabled on an existing filesystem that
already has an encrypted directory?  Does the code _assume_ that there is
a hash beyond the name if the rec_len is long enough for this?  There will
definitely be some pre-existing dirents that will have a large rec_len
(e.g. those at the end of the block, or with deleted entries immediately
following), that do *not* have the proper hash stored in them.  There may
be random garbage at the end of the dirent, and since every value in the
hash is valid, there is no way to know whether it is good or bad.

With the dirdata mechanism, there would be a bit set in the "file_type"
field that will indicate if the hash was present, as well as a length
field (0x08) that is a second confirmation that this field is valid.

Cheers, Andreas