Intercepting ram session of child

[Norman Feske]

Hi Denis,
	
> This means, the Rpc_objects can communicate, if they are managed by the 
> same Entrypoint, but not if they are in different Entrypoints. Just out 
> of curiosity: Can I delegate a capability from one Entrypoint to another?

I am not sure what you mean by "communicate" exactly here. The important
point is that core's RAM session tried to look up another RAM session by
using a the capability it received (as argument to the transfer-quota
call) as key. The operation to look up an RPC object by a given
capability is provided by the entrypoint (via the 'apply' method).
However, the entrypoint knows only those RPC objects that it manages. So
when presented with a capability that refers to an RPC object managed by
some other entrypoint (in your case, the entrypoint living in your
"random" program), it finds no matching RPC object among its managed
objects.

> I think, the child constructor created an Rpc_object<Parent>, which
> is managed by the child's Entrypoint. Then the Rpc_object<Parent>
> called an RPC method from my Rpc_object<Ram_session>, which also is
> managed by this Entrypoint. Thus, the Entrypoint blocked on the
> remote procedure call to Rpc_objcet<Ram_session> waiting for itself
> to finish the call from Rpc_object<Ram_session>. The
> Rpc_object<Ram_session> needs to use this Entrypoint, but it was
> blocked. Thus, it went waiting for the Entrypoint.

Exactly!

> Now Rpc_object<Parent> is waiting for Rpc_object<Ram_session> and
> vice vera => deadlock.
> 
> Please correct me, if I am wrong :)

Your analysis is flawless. :-)

For this exact reason, the Genode::Child constructor takes the RAM
session (to be used locally to initialize the child) and RAM-session
capability as two distinct arguments instead of internally creating a
Ram_session_client object with the capability.

Cheers
Norman

-- 
Dr.-Ing. Norman Feske
Genode Labs

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