Page faults in managed dataspaces

[Denis Huber]

Dear Genode Community,

I want to implement a mechanism to monitor the access of a component to 
its address space.

My idea is to implement a monitoring component which provides managed 
dataspaces to a target component. Each managed dataspace has several 
designated dataspaces (allocated, but not attached, and with a fixed 
location in the managed dataspace). I want to use several dataspaces to 
control the access range of the target component.

Whenever the target component accesses an address in the managed 
dataspace, a page fault is triggered, because the managed dataspace has 
no dataspaces attached to it. The page fault is caught by a custom page 
fault handler. The page fault handler attaches the designated dataspace 
into the faulting managed dataspace and resolves the page fault.

To test my concept I implemented a prototypical system with a monitoring 
component (called "ckpt") [1] and a target component [2].

[1] 
https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/b502ffd962a87a5f9f790808b13554d6568f6d0b/src/test/concept_session_rm/server/main.cc
[2] 
https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/b502ffd962a87a5f9f790808b13554d6568f6d0b/src/test/concept_session_rm/client/main.cc

The monitoring component provides a service [3] to receive a Thread 
capability to pause the target component before detaching the dataspace 
and resume after detaching and to provide a managed dataspace to the client.

[3] 
https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/tree/b502ffd962a87a5f9f790808b13554d6568f6d0b/include/resource_session

The monitoring component runs a main loop which pauses the client's main 
thread and detaches all attached dataspaces from the managed dataspace.
The target component also runs a main loop which prints (reads) a number 
from the managed dataspace to the console and increments (writes) it in 
the managed dataspaces.

The run script is found here [4].

[4] 
https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/b502ffd962a87a5f9f790808b13554d6568f6d0b/run/concept_session_rm.run

The scenario works for the first 3 iterations of the monitoring 
component: Every 4 seconds it detaches the dataspaces from the managed 
dataspace and afterwards resolves the page faults by attaching the 
dataspaces back. After the 3. iteration, the target component accesses 
the theoretically empty managed dataspaces, but does not trigger a page 
fault. In fact, it reads and writes to the designated dataspaces as if 
it was attached.

By running the run script I get the following output:
[init -> target] Initialization started
[init -> target] Requesting session to Resource service
[init -> ckpt] Initialization started
[init -> ckpt] Creating page fault handler thread
[init -> ckpt] Announcing Resource service
[init -> target] Sending main thread cap
[init -> target] Requesting dataspace cap
[init -> target] Attaching dataspace cap
[init -> target] Initialization ended
[init -> target] Starting main loop
Genode::Pager_entrypoint::entry()::<lambda(Genode::Pager_object*)>:Could 
not resolve pf=6000 ip=10034bc
[init -> ckpt] Initialization ended
[init -> ckpt] Starting main loop
[init -> ckpt] Waiting for page faults
[init -> ckpt] Handling page fault: READ_FAULT pf_addr=0x00000000
[init -> ckpt]   attached sub_ds0 at address 0x00000000
[init -> ckpt] Waiting for page faults
[init -> target] 0
[init -> target] 1
[init -> target] 2
[init -> target] 3
[init -> ckpt] Iteration #0
[init -> ckpt]   valid thread
[init -> ckpt]   detaching sub_ds_cap0
[init -> ckpt]   sub_ds_cap1 already detached
Genode::Pager_entrypoint::entry()::<lambda(Genode::Pager_object*)>:Could 
not resolve pf=6000 ip=10034bc
[init -> ckpt] Handling page fault: READ_FAULT pf_addr=0x00000000
[init -> ckpt]   attached sub_ds0 at address 0x00000000
[init -> ckpt] Waiting for page faults
[init -> target] 4
[init -> target] 5
[init -> target] 6
[init -> target] 7
[init -> ckpt] Iteration #1
[init -> ckpt]   valid thread
[init -> ckpt]   detaching sub_ds_cap0
[init -> ckpt]   sub_ds_cap1 already detached
[init -> target] 8
[init -> target] 9
[init -> target] 10
[init -> target] 11
[init -> ckpt] Iteration #2
[init -> ckpt]   valid thread
[init -> ckpt]   sub_ds_cap0 already detached
[init -> ckpt]   sub_ds_cap1 already detached
[init -> target] 12
[init -> target] 13

As you can see: After "iteration #1" ended, no page fault was caused, 
although the target component printed and incremented the integer stored 
in the managed dataspace.

Could it be, that the detach method was not executed correctly?


Kind regards
Denis