[Genode] Possibility of genode capability-selector conflict while thread creation in Fiasco.OC

[Stefan Kalkowski]

Hi Jaeyong,

On 01/31/2013 09:06 AM, jaeyong yoo wrote:
> Hello, genode!
> 
> In Thread_base::start() function, there is a capability inserting part:
> 
>         Cap_index *i = cap_map()->insert(state.id, state.cap);
>         l4_utcb_tcr_u(state.utcb)->user[UTCB_TCR_BADGE]      = (unsigned
> long) i;
>         l4_utcb_tcr_u(state.utcb)->user[UTCB_TCR_THREAD_OBJ] = (addr_t)this;
> 
> in the above, /state.kcap/ is the cap-selector in the Genode CORE (where
> Cpu_root is there).
> And, Thread_base::start() function tries to create the capability with
> the same value of state.kcap (it looks rather than delegating, uses
> direct value).
> My question here is that, "could it be possible that the state.kcap
> value is already taken by some other capability in the Genode application?"
> 

Your suspicion is comprehensible, but fortunately there is no such
conflict. The capability space of each task respectively protection
domain is split up. The first part is under control of core, the other
one is for free use by the application. This is just a convention
defined by Genode's port to Fiasco.OC. It's not determined by the kernel.

The area controlled by core contains all capabilities that are
asynchronously mapped to the protection domain by core. These are: the
task capability, and 3 private capabilities for each thread of the
domain. One of these capabilities, which each thread owns, is a IPC gate
capability that is associated with that thread. This capability lies
within the state.kcap capability index of the task. It's not the
capability index in core. Core only propagates this information via the
cpu session.

The second area of the capability space, which is not under core's
control, contains all capabilities, which are mapped synchronously via IPC.

I hope this completes your picture of Genode/Fiasco.OC' capability
handling a bit.

Best regards
Stefan

> Best regards,
> Jaeyong
> 
> 
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-- 
Stefan Kalkowski
Genode Labs

http://www.genode-labs.com/ · http://genode.org/