Just to avoid any confusion. Can you tell me if I got this right:
There are three possible ways to restict/allow communication between threads, using
* Thread IDs (current L4 v4 implementation) Any thread can communicate to any thread and send any message. Access checks need to be done by the server, that might need to verify some things using another 'security' server, requiring another IPC. The IPCs itself are very fast because no table lookups are necessary to verify if the communication is allows. However suity holes are much easier to created by buggy code that forgets to verify some accesses. Additionally the security checks are executed even slower than if they where done inside the kernel as in the other two methodes. These security checks can be done automatically using IPC redirection. This still requires an additional IPC, had some strange (unclean) quircks associated with it and I still doubt wether this methode is secure especially when used by buggy programs.
* Virtual Thread Objects (possible L4 v5 implementation) Each thread has a kernel table attached to it (1) which lists the threads it can communicate to. These virtual thread objects can be grranted, mapped and unmapped just like flexpages. So when a message is received the server can be sure it is send by a thread that is mapped it's thread ID to.
(1) Threads in the same address space might share the same table.
* Capabilities (current EROS implementation) Capabilities are similar to Virtual Thread Objects except that they have a server defined word attached to them.
I first want to make clear why the second and third methode are much better than the first:
* Methode 1 is insecured when used with buggy programs. The fact simply is that most, if not all, programs are buggy. I agree with shap that the cost of a program should be measured in total resources used (dollars or additional programming time for example) and not just computational resources. * In a system that requires security the 2nd and 3rd methodes are actually faster because only a single, cimple kernel check needs to be made agisnt several complex user checks for methode 1 (is this realy true? comments please). L4 is designed to be secure anyway and is therefore already to slow for systems that don't require security, so the penalty the insecure systems woudl suffer does not apply. * Additional security benefits are that the internal thread structure is not revealed to every program running on the system and some protection against DoS attacks is offered.
Now I want to make clear why capablities are much better than virtual thread objects:
* The extra word does not seem to decrease performance in any way (is this true?) so it a free feature, that can be used but doesn't have to. * The server defined word will probably be used to store a pointer to some client specific data structure containing important information that needs to be access often. You might say, yeah well mbut you can calculate this address from the sender ID, but this no longer works when clients start to grant and map server objects/capabilites to eachother, because the server doesn't know about these actions, unless some complex, slow protocol is used to update the serves information.
My conclusion is that Virtual Thread Objects/Capabilities are far supiror than just using Thread IDs and that the server defined word of Capabilities are very important to manage information of a server. The size of the capability should be equal to the pointer/word size of the machine.
Constructive criticism is very much appreciated,
Rudy Koot
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