Re: QAPI schema for desired state of LUKS keyslots

[Max Reitz]

On 25.02.20 17:48, Markus Armbruster wrote:
> Max Reitz <address@hidden> writes:
> 
>> On 15.02.20 15:51, Markus Armbruster wrote:
>>> Review of this patch led to a lengthy QAPI schema design discussion.
>>> Let me try to condense it into a concrete proposal.
>>>
>>> This is about the QAPI schema, and therefore about QMP.  The
>>> human-friendly interface is out of scope.  Not because it's not
>>> important (it clearly is!), only because we need to *focus* to have a
>>> chance at success.
>>>
>>> I'm going to include a few design options.  I'll mark them "Option:".
>>>
>>> The proposed "amend" interface takes a specification of desired state,
>>> and figures out how to get from here to there by itself.  LUKS keyslots
>>> are one part of desired state.
>>>
>>> We commonly have eight LUKS keyslots.  Each keyslot is either active or
>>> inactive.  An active keyslot holds a secret.
>>>
>>> Goal: a QAPI type for specifying desired state of LUKS keyslots.
>>>
>>> Proposal:
>>>
>>>     { 'enum': 'LUKSKeyslotState',
>>>       'data': [ 'active', 'inactive' ] }
>>>
>>>     { 'struct': 'LUKSKeyslotActive',
>>>       'data': { 'secret': 'str',
>>>                 '*iter-time': 'int } }
>>>
>>>     { 'struct': 'LUKSKeyslotInactive',
>>>       'data': { '*old-secret': 'str' } }
>>>
>>>     { 'union': 'LUKSKeyslotAmend',
>>>       'base': { '*keyslot': 'int',
>>>                 'state': 'LUKSKeyslotState' }
>>>       'discriminator': 'state',
>>>       'data': { 'active': 'LUKSKeyslotActive',
>>>                 'inactive': 'LUKSKeyslotInactive' } }
>>
>> Looks OK to me.  The only thing is that @old-secret kind of works as an
>> address, just like @keyslot,
> 
> It does.
> 
>>                              so it might also make sense to me to put
>> @keyslot/@old-secret into a union in the base structure.
> 
> I'm fine with state-specific extra adressing modes (I better be, I
> proposed them).
> 
> I'd also be fine with a single state-independent addressing mode, as
> long as we can come up with sane semantics.  Less flexible when adding
> states, but we almost certainly won't.
> 
> Let's see how we could merge my two addressing modes into one.
> 
> The two are
> 
> * active
> 
>   keyslot     old-secret      slot(s) selected
>   absent      N/A             one inactive slot if exist, else error
>   present     N/A             the slot given by @keyslot

Oh, I thought that maybe we could use old-secret here, too, for
modifying the iter-time.  But if old-secret makes no sense for
to-be-active slots, then there’s little point in putting old-secret in
the base.

(OTOH, specifying old-secret for to-be-active slots does have a sensible
meaning; it’s just that we won’t support changing anything about
already-active slots, except making them inactive.  So that might be an
argument for not making it a syntactic error, but just a semantic error.)

[...]

> Note we we don't really care what "inactive, both absent" does.  My
> proposed semantics are just the most regular I could find.  We can
> therefore resolve the conflict by picking "active, both absent":
> 
>   keyslot     old-secret      slot(s) selected
>   absent      absent          one inactive slot if exist, else error
>   present     absent          the slot given by @keyslot
>   absent      present         all active slots holding @old-secret
>   present     present         the slot given by @keyslot, error unless
>                               it's active holding @old-secret
> 
> Changes:
> 
> * inactive, both absent: changed; we select "one inactive slot" instead of
>   "all slots".
> 
>   "All slots" is a no-op when the current state has no active keyslots,
>   else error.
> 
>   "One inactive slot" is a no-op when the current state has one, else
>   error.  Thus, we no-op rather than error in some states.
> 
> * active, keyslot absent or present, old-secret present: new; selects
>   active slot(s) holding @old-secret, no-op when old-secret == secret,
>   else error (no in place update)
> 
> Can do.  It's differently irregular, and has a few more combinations
> that are basically useless, which I find unappealing.  Matter of taste,
> I guess.
> 
> Anyone got strong feelings here?

The only strong feeling I have is that I absolutely don’t have a strong
feeling about this. :)

As such, I think we should just treat my rambling as such and stick to
your proposal, since we’ve already gathered support for it.

Max

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