Re: QAPI schema for desired state of LUKS keyslots

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2020-02-26 at 08:28 +0100, Markus Armbruster wrote:
> Max Reitz <mreitz@redhat.com> writes:
> 
> > On 25.02.20 17:48, Markus Armbruster wrote:
> > > Max Reitz <mreitz@redhat.com> 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.
> 
> Update in place is unsafe.
> 
> >                           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.)
> 
> Matter of taste.  I like to keep simple things syntactic, and thus
> visible in introspection.
> 
> > [...]
> > 
> > > 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.
> 
> Thanks!

So in summary, do I have the green light to implement the Markus's proposal as is?

Best regards,
	Maxim Levitsky