api: Add method to add logon keys

[rcatolino]

The keyring has a "logon" key type for keys that userspace can create and update but not read. This method allows to add such keys.

[mathstuf]

Thanks. Where is this documented? Is it a freedesktop.org thing?

[rcatolino]

It's documented in the kernel doc (search for 'logon'). I don't think it has anything to do with freedesktop.org, personally I need it to use the ext4 encryption feature.

[mathstuf]

Thanks. The libkeyutils docs don't mention it (which where I usually look). Could the subclass be added as a separate argument since it seems required as well?

[rcatolino]

It would probably be better to only have an additional argument to add_key but I didn't want to break backward compatibility by changing add_key signature. If you prefer I can update the pull request to use

pub fn add_key(&mut self, keytype: KeyType, description: &str, payload: &[u8] -> Result<Key>

Withg KeyType a suitable enum

[mathstuf]

That sounds good to me. I think once this and #7 land, I'll do 0.2 in preparation for 1.0.

[rcatolino]

I've updated my pull request. And I just realized you were talking about the subclass prefix in the key description, not the key type...
If we add the subclass as a separate argument, add_logon_key does needs to be a separate method since the subclass isn't used for user keys.

I initially thought of using a separate subclass argument but if user gets a key description string already containing the subclass from somewhere else, they would have to split it on ":" and pass both pieces separately. Although I'm not sure how likely this is to happen. I don't feel too strongly about it anyway, so if you want I can create an add_logon_key method with a subclass instead.