Re: [PATCH v3 2/3] btrfs: add authentication support

[David Sterba <dsterba@suse.cz>]

On Wed, May 27, 2020 at 06:04:57PM +0000, Johannes Thumshirn wrote:
> On 27/05/2020 15:25, David Sterba wrote:
> > The key for all userspace commands needs to be specified the same way as
> > for kernel, ie. "--auth-key btrfs:foo" and use the appropriate ioctls to
> > read the key bytes.
> 
> Up to now I haven't been able to add a key to the kernel's keyring which 
> can be read back to user-space.

This needs permissions on the key and I think keys from some keyrings
cannot be read back even with the permissions set. There's an ioctl
equivalent of 'keyctl read' which I used to emulate the reading. Setting
the permissions is cumbersome, as it needs to manually craft the hexa
value, but otherwise would seems a better way than either specifying the
key payload on command line or storing it in a file.

I've looked at other projects using keys, eg. ecryptfs-utils, it uses
keyctl_read_alloc, so that seems to be the preferred way.

> How about passing in a key file, like it is done in UBIFS? Should be doable
> both with libsodium and libgcrypt.

Keyfile would be better, that's what dm-crypt uses, but still.