Trying to understand how capabilities are implemented

[Martin Stein]

Hi Albin!

On 19.08.23 03:35, Albin Otterhäll wrote:
> 1. The RPC object A is created inside the protection domain 1, which result in the object identity A being created by the kernel.
> 
> 2. A has a name, which is a natural number.
>     Q1: Is the RPC object's name the value of a class variable of type int?

In short: Yes, you can consider it a simple integer that is known to 
both the kernel and the affected user-land program. The name references 
this object only within this one program (respectively protection domain).

Just in case you're interested in some code background:

In header [1] you can find the classes Rpc_object and Rpc_entrypoint - a 
thread that is used for handling RPCs on RPC objects. An RPC object 
initially comes without a capability (the objects name), but a 
capability is created and returned when linking the object via 
Rpc_entrypoint::manage to an entrypoint. At this point the object 
becomes ready to be called from outside the program.

As you can see in header [2], Capability has no members but inherits 
from Untyped_capability, AKA Native_capability, which contains a pointer 
_data. Now, this is where it becomes platform-specific. However, a very 
simple case is the "base-hw" kernel. There, the _data pointer itself (as 
long value) is used as local name of the object [4]. This long value is 
the name that is registered in the domain's cap space inside the kernel 
for that particular RPC object.

> 3. Protection domain 1 has a capability space insied the kernel.
>     Q2: Is the protection domain seperate from the capability space in the kernel, and they're "connected" with some map somewhere?

In general, this is very kernel-specific. However, I know the internals 
only for the "base-hw" kernel. In this kernel, the cap space is not 
separate from its protection domain. Each domain object [5] contains a 
tree _cap_tree of all RPC object names known to this domain (see [6] for 
more details).

>     Q3: Is the capability space a map between natural numbers and some form of address to the object identity? I.e. the map work similar to a set of OO references, there the keys are variable names; and the values are the memory addresses of object identities.

Again very kernel-specific and, again, my knowledge about "base-hw": In 
this kernel your depiction is very accurate. Each RPC object name is an 
Object_identity_reference in the kernel and contains a pointer 
Object_identity *_identity.

>     Q4: If the answer to Q3 is yes, does that mean that delegagion is some form of mechanism to share the adress of the object identity with other protection domains?

If I understand you correctly, yes and no. To the user it looks like 
sharing an address in OO but you might better think of it as sharing the 
_ability_ to access the object. In contrast to address sharing, 
delegation creates a new local name/reference for the object identity in 
the target domain/cap space. In my domain the object might be referenced 
by the name "3" while in a domain I have delegated to, the object might 
be referenced by the name "987". However, even if delegation would 
result in the same name "3", the link between this name and the object 
is unique to the target domain as is that between my "3" and the object. 
This is well illustrated by § 3.1 Figure 3 in the book.

I hope that was helpful!?

[1] base/include/base/rpc_server.h
[2] base/include/base/capability.h
[3] base/include/base/native_capability.h
[4] base-hw/src/lib/base/capability.cc:59
[5] base-hw/src/core/kernel/pd.h
[6] base-hw/src/core/kernel/object.h
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