Config update timing

[Norman Feske]

Hi Ben,

On 27.02.2018 17:04, Nobody III wrote:
> What guarantees are there regarding config update timing? For example,
> if a process requires a policy change to get access to a required
> service, is the server guaranteed to get the config update before the
> service request?

there are no guarantees of the timing of signals, not to speak of the
response times to those signals by a server. Reconfiguring a server's
policy and adding a client (matching the policy) at the same time cannot
work reliably.

There are two approaches to overcome this problem.

In the case where you want to restrict access to the file system, you
may plug the chroot component between the client and the server. This is
the approach that I am using the depot-download subsystem (described in
the release notes of the upcoming release). This side-steps the problem
by not changing the server's policy dynamically.

Another principle idea is to defer the creation of the client until the
server has somehow acknowledged the new policy. This approach is at work
in the USB driver. The USB driver has a config option for reporting its
active configuration. These reports give the feedback needed to decide
when the policy for a new client is actually effective.

Cheers
Norman

-- 
Dr.-Ing. Norman Feske
Genode Labs

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