Compatibility between D-Bus and kdbus

Lennart Poettering mzqohf at
Tue Nov 25 08:25:54 PST 2014

On Tue, 25.11.14 11:08, Simon McVittie (simon.mcvittie at wrote:

> > This might be a problem. Right now, QtDBus assumes that any match rule it adds 
> > will be handled successfully. If the resource limit is low enough that an 
> > application could hit it, we'll need to start handling the failure case. 
> Yes, and that's hard; GDBus doesn't handle this either. On the other
> hand, at least kdbus adds match rules via a synchronous ioctl rather
> than an async message.

Are you sure? I thought it was async in gdbus... sd-bus currently adds
them all in sync mode, and I actually have a todo list item to make
this async, taking inspiration from gdbus. Syncing on AddMatch is
actually really slow, given that there are so many involved.

> Somewhere else (in this thread? on the Google Code project?) I suggested
> adding an ioctl for "atomically replace this bloom filter with this
> bloom filter", which would let a binding consolidate two similar match
> rules A and B into a broader rule, say A|B|C, without needing to worry
> about exceeding the limit again. Pseudocode:
>     try:
>         add match rule D
>     catch resource limit exceeded:
>         find two sufficiently similar match rules A and B
>         atomically replace A with A|B|C
>         remove B
>         retry adding D

If figure you mean:

It appears to be implemented.

> > org.freedesktop.DBus.GetId
> Possible gap?

It's communicated in the HELLO ioctl struct when you connect. A client
should cache it.


Lennart Poettering, Red Hat

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