[genode] Label in l4_ipc_wait

[jaeyong yoo]

It indeed clarifies my question!

Thanks a lot.
Jaeyong

On Wed, Jan 30, 2013 at 7:01 PM, Stefan Kalkowski <
stefan.kalkowski at ...1...> wrote:

> 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/
>
>
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