[genode] Label in l4_ipc_wait

[Stefan Kalkowski]

Hi Jaeyong,

On 01/25/2013 12:19 PM, jaeyong yoo wrote:
> Hello Genode,
> I have a question regarding Genode Label in Fiasco.OC kernel.
> 
> I noticed that Msgbuf_base::label() is the same as the return value of
> l4_ipc_wait (which is basically genode label):
> 
>         L4_INLINE l4_msgtag_t
>         l4_ipc_wait(l4_utcb_t *utcb, l4_umword_t *label, l4_timeout_t
> timeout) L4_NOTHROW;
> 
> 
> label is described as following:
> 
>          * \retval  label   Label assigned to the source object (IPC
> gate or IRQ).
> 
> 
> But, I can not see any corresponding label in ipc call as we can see below:
> 
>         L4_INLINE l4_msgtag_t
>         l4_ipc_call(l4_cap_idx_t object, l4_utcb_t *utcb, l4_msgtag_t
> tag, l4_timeout_t timeout) L4_NOTHROW;
> 
> 
> So, here goes the question:
> Since genode label is passed to the first data of Msgbuf_base::buf, does
> Fiasco.OC pickup this value and return when l4_ipc_wait function called?

In short no. In fact the first item send in each message is the
"local_name" of the Genode capability invoked. In general, this has
nothing to do with the "label" term of Fiasco.OC.

I guess you are confused by the overloaded term "label" in the Fiasco.OC
kernel. "Label" names two different things here. On the one hand there
is the "label" specifier in the message descriptor that classifies the
message, whether it is e.g.: normal-ipc, pagefault, exception,... . On
the other hand there is a "label" assigned to each IPC gate, or IRQ
kernel object. If you bind a thread to an IPC gate or IRQ object you can
define a label associated to that object. Whenever the gate or IRQ
object is invoked, the listening thread will receive that label in
addition to the actual message. With other words, when sending a message
through an IPC gate, the sender can't specify the label received on the
other side. The label is already determined by the IPC gate that was used.

In Genode we use this label mechanism of the Fiasco.OC kernel to encode
the Genode capability's "local_name" into it. For each new capability a
new IPC gate is created. All messages in Genode are send through such
IPC gates. When some client sends a message via a gate, it puts the
capability's "local_name" as the first item into the message. On the
other side, the server thread receives the message, and the associated
label of the gate, which the client invoked. The server will compare the
first item of the message - the "local_name"" - with the label received.
If they are equal, the request is valid. By this protocol, you can
detect fraud on the server side. Otherwise, some client could try to
guess the "local_name" of some other session, it doesn't possess.

I hope this clarifies your initial question.

Regards
Stefan

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

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