Interceptor for org.freedesktop.DBus.Properties.Set

[Patrick Williams]

On Tue, Sep 13, 2016 at 02:11:04PM +0530, vishwa wrote:
> /org/openbmc/settings/host0 provides the dbus properties that are 
> settable via org.freedesktop.DBus.properties.Set.
> The properties are treated as user settings and are consumed by 
> respective daemons by having a signal handler registered for 
> PropertyChanged method.
> 
> The challenge at the hand is "How to make sure the users that set the 
> property are choosing a valid value".
> 
> Since its set by org.freedesktop.DBus.Properties.Set, the ideal way 
> would be to register a callback that would validate the user input based 
> on custom algo and then only would let the .Set continue.
> 
> Without an interceptor, the user settings may be as junk as "abcdef" and 
> the *settings damon* can only react to that change and can put the 
> previous value back into it post verification.
> 
> I did a lot of search on ways to register an interceptor and did not see 
> one. I then asked in #dbus and they told me that there is no way to 
> intercept that.

The way to do it entirely depends on the DBus binding in use.  I'm not
sure why people in #dbus would tell you it cannot be done.

The systemd/sd-bus vtable entry for a "writable property" allows a
property to be managed directly by systemd (pass property by pointer) or
via indirect get/set methods:
https://github.com/systemd/systemd/blob/master/src/systemd/sd-bus-vtable.h#L123

If you look through the systemd repository you will find some
SD_BUS_WRITABLE_PROPERTY calls where the get/set methods are used and
those can clearly cause a Dbus.Error instead of returning success.

phosphor-settingsd is currently using a pyphosphor wrapper around the
python dbus bindings.  Here we implement the
org.freedesktop.Dbus.Properties.Set[All]:

https://github.com/openbmc/pyphosphor/blob/master/obmc/dbuslib/bindings.py#L76
https://github.com/openbmc/pyphosphor/blob/master/obmc/dbuslib/bindings.py#L95

I think you could enhance this to call a set/get function, if present, for
specific properties.  Those can throw an exception which will turn into
a Dbus.Error.

> So the solution what I have at the moment is to
> 
> 1) Disable the property set via freedesktop
> 2) Introduce a custom Setter "org.openbmc.Settings.Host.Set" which can 
> do the job of validating the inputs also.
> 
> Please let me know if there are better approaches.
> 
> Thanks.
> 
> Vishwanatha Subbanna.

> _______________________________________________
> openbmc mailing list
> openbmc at lists.ozlabs.org
> https://lists.ozlabs.org/listinfo/openbmc


-- 
Patrick Williams
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