[PATCH v2 05/11] fscrypt: split fscrypt_dummy_context_enabled() into supp/notsupp versions

[Eric Biggers]

On Fri, Jan 05, 2018 at 12:40:09PM -0800, Jaegeuk Kim wrote:
> On 01/05, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers at google.com>
> > 
> > fscrypt_dummy_context_enabled() accesses ->s_cop, which now is only set
> > when the filesystem is built with encryption support.  This didn't
> > actually matter because no filesystems called it.  However, it will
> > start being used soon, so fix it by moving it from fscrypt.h to
> > fscrypt_supp.h and stubbing it out in fscrypt_notsupp.h.
> 
> Ted, do we have a chance to get rid of this dummy_context? If there exists
> backward compatibility issue, please never mind tho.
> 

It's used to implement the test_dummy_encryption mount option for ext4, which is
used by the 'ext4/encrypt' config for gce-xfstests.  Its purpose is to cause all
new files (directories, regular files, and symlinks) to be automatically
encrypted with a default key, so that the encrypted I/O paths are tested more
thoroughly than by just running the 'encrypt' group tests.

There are no backward compatibility concerns with changing or removing the
test_dummy_encryption mount option; we just don't have a better solution yet.

Ideally, instead of using test_dummy_encryption we would encrypt the root
directory of the filesystem immediately after it is formatted.  However, that
doesn't work for ext4 because the lost+found directory has to be located in the
root directory, and must be unencrypted, and the lost+found directory entry must
be unencrypted.

So I think getting rid of test_dummy_encryption depends on a solution to the
lost+found problem.

An alternative would be to add support for "inherit-only" encryption policies,
then set such a policy on the root directory of the filesystem.  But, there
haven't been any requests for such a feature yet outside of this specific
testing-only use case, so I've been hesitant to add it.

Eric