Re: [Qemu-devel] [RFC][BROKEN] rbd: Allow configuration of authentication scheme

[Eric Blake]

On 04/06/2018 03:04 AM, Kevin Wolf wrote:
> Am 05.04.2018 um 19:06 hat Kevin Wolf geschrieben:
>> The legacy command line syntax supports a "password-secret" option that
>> allows to pass an authentication key to Ceph. This was not supported in
>> QMP so far.
>>
>> This patch introduces authentication options in the QAPI schema, makes
>> them do the corresponding rados_conf_set() calls and adds compatibility
>> code that translates the old "password-secret" option both for opening
>> and creating images to the new set of options.
>>
>> Note that the old option didn't allow to explicitly specify the set of
>> allowed authentication schemes. The compatibility code assumes that if
>> "password-secret" is given, only the cephx scheme is allowed. If it's
>> missing, both none and cephx are allowed because the configuration file
>> could still provide a key.
> 
> There is another problem here that suggests that maybe this is not the
> right QAPI schema after all: The defaults needed for command line
> compatibility and those promised in the QAPI schema are conflicting.
> 
> The required command line behaviour is as described above:
> 
> * password-secret given: only cephx
> * no options given: cephx, none
> 
> The desired QMP default behaviour is:
> 
> * auth-cephx given: allow cephx
> * auth-none given: allow none
> * both given: allow both
> * no options given: error
> 
> In .bdrv_open() there is no way to distinguish the "no options given" of
> the command line from that of QMP. The current implementation allows
> everything if no options are given, i.e. it keeps existing command lines
> working, but it doesn't correctly implement the behaviour described in
> the QAPI schema.

Can we use a QAPI alternate with an explicit JSON null for the command
line 'no options given' case?

> 
> I don't think changing the description of the QAPI schema would be a
> good idea, it would be a rather surprising interface.
> 
>> Signed-off-by: Kevin Wolf <address@hidden>
>> ---
>>
>> This doesn't actually work correctly yet because the way that options
>> are passed through the block layer (QAPI -> QemuOpts -> QDict). Before
>> we fix or hack around this, let's make sure this is the schema that we
>> want.

Can we skip the intermediate QemuOpts?  If we can go straight from
command line to QDict using just QAPI, won't that give us what we really
need?

>>
>> The two known problems are:
>>
>> 1. QDict *options in qemu_rbd_open() may contain options either in their
>>    proper QObject types (if passed from blockdev-add) or as strings
>>    (-drive). Both forms can be mixed in the same options QDict.
>>
>>    rbd uses the keyval visitor to convert the options into a QAPI
>>    object. This means that it requires all options to be strings. This
>>    patch, however, introduces a bool property, so the visitor breaks
>>    when it gets its input from blockdev-add.
>>
>>    Options to hack around the problem:
>>
>>    a. Do an extra conversion step or two and go through QemuOpts like
>>       some other block drivers. When I offered something like this to
>>       Markus a while ago in a similar case, he rejected the idea.
>>
>>    b. Introduce a qdict_stringify_entries() that just does what its name
>>       says. It would be called before the running keyval visitor so that
>>       only strings will be present in the QDict.
>>
>>    c. Do a local one-off hack that checks if the entry with the key
>>       "auth-none" is a QBool, and if so, overwrite it with a string. The
>>       problem will reappear with the next non-string option.
>>
>>    (d. Get rid of the QDict detour and work only with QAPI objects
>>        everywhere. Support rbd authentication only in QEMU 4.0.)

Oh, even one step further than just avoiding QemuOpts, but using REAL
types everywhere in the block layer!  It might be a nice cleanup, but it
would probably take a lot of effort in the meantime to get to that point?

>>
>> 2. The proposed schema allows 'auth-cephx': {} as a valid option with
>>    the meaning that the cephx authentication scheme is enabled, but no
>>    key is given (e.g. it is taken from the config file).
>>
>>    However, an empty dict cannot currently be represented by flattened
>>    QDicts. We need to find a way to enable this. I think this will be
>>    externally visible because it directly translates into the dotted
>>    syntax of -blockdev, so we may want to be careful.

Can we just require users to give -blockdev with the JSON format (rather
than dotted format) when they need to use that particular feature on the
command line?

>>
>> Any thoughts on the proposed QAPI schema or the two implementation
>> problems are welcome.
> 
> Kevin
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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