User-level objects

Adam Lackorzynski adam at os.inf.tu-dresden.de
Wed Dec 10 00:17:26 CET 2014


On Tue Dec 09, 2014 at 10:32:09 -0500, teclis High Elf wrote:
> Hi, would someone be able to point me to example code for setting access
> control on a kernel object? Say I am brining up Fiasco on new hardware that
> has certain instructions that can only be executed when running in kernel
> privilege level but I want to control which tasks can use them. Could I
> create a new class of kernel object for my hardware instruction and then
> set access control on this class of kernel object so only my task can make
> the syscall. No other tasks i.e. Moe, Ned etc. would be able to instantiate
> my new kernel object and make the protected syscall. Or alternatively if
> some one could recommend a mechanism for controlling which tasks could make
> a new syscall in Fiasco L4Re that would be very helpful to.

Sounds like the easy and straightforward way is to have a singleton
kernel object. This one is then used to execute the instructions.
Filtering etc. can be implemented in user-space. Now there's no example
code on how to get the cap through to your program, it's nothing
standard to do usually. Further, giving it just to your task is
difficult as there's no way to identify a 'random' task in the kernel
that is dynamically created. What happens at boot time is that both
sigma0 and moe are equipped with their set of caps. This can be done
because the kernel creates both tasks so it can map something in there.
Other tasks are not known at this point as they do not exist yet.
Further, e.g. sigma0 and moe have control over your app's memory so they
could make your app do what they want. Thus I do not think that it is
reasonable to guard against those services. Now a plan could be roughly
like this: In the kernel, look at another singleton, e.g. Vlog, then
write your class, create a static object, use
initial_kobjects.register_obj(this, 10) in the constructor. With that it
will appear in moe under cap-idx 10. In moe, you add
root_name_space()->register_obj("xyz", Obj(Obj::F_rw, 10 << 12)); in
main.cc so it will be accessible in your ned script via L4.Env.xyz.
Then pass it on as usual to your program.


HTH,
Adam
-- 
Adam                 adam at os.inf.tu-dresden.de
  Lackorzynski         http://os.inf.tu-dresden.de/~adam/



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