USB storage detachment / reattachment

Boris Mulder boris.mulder at ...434...
Tue Jan 31 13:13:57 CET 2017

All right, so far, the forwarding of sessions works. However, when
closing a session, there is an issue.

Whenever a client connection is closed, the client calls close() with a
session cap on the root. The root then has to look into its open
sessions, and compare the session caps of each of those open sessions
with the provided cap, and then further cleans up all data related to
that session.

For the service router example, it does the following on line 52

for (Forwarded_capability *cap = _caps.first(); cap; cap = cap->next()) {
	if (*cap == session)
		return cap;

it checks if these capabilities are equal using the '==' operator. In
Capability, this operator compares the internal pointers
Native_capability::Data *_data of each Capability object, which points
to an object containing metadata such as a Rpc destination and a key.

However, when this session capability is passed as argument to the
close() or upgrade() method of the root RPC interface, the unmarshaller
at the server side will always create a new Capability object with new
data using the Capability_space_tpl::import method (If I am not
mistaken), instead of using lookup(). This is done for instance on linux
and on nova in Therefore the cap pointers will never be equal
although they point to different duplicate cap data objects with the
same content. Is this the correct behaviour?

When testing it with print() by inserting the following line

log("testing... session = ", session, " cap = ", cap, " equal = ", session == cap);

it outputs the following:

session = cap<socket=27,key=474> cap = cap<socket=27,key=474> equal = 0

So the comparison will always fail, and the overloaded close() and
upgrade() methods of Root cannot close/upgrade the correct session.

Am I missing something here or is it not possible right now to locally
keep track of multiple forwarded session capabilities in this way?

Or is there a workaround?



On 30-01-17 11:09, Norman Feske wrote:
> Hi Boris,
>>> You just missed a tiny piece of the puzzle: The 'Slave::Connection' does
>>> not only provide the session interface of the slave's service but also
>>> the corresponding 'Session_capability' (it inherits
>>> 'CONNECTION::Client', so the 'Slave::Connection' _is_ a session
>>> capability). Instead of calling the 'File_system' methods, the media
>>> component would pass this 'Session_capability' to init as response to
>>> the 'File_system' session request that originated from init.
>> I assume here the session() method inherited from Genode::Root has to be
>> implemented such that it returns the capability that is the
>> Slave::Connection after that connection has been initiated?
> yes.
> Cheers
> Norman


Met vriendelijke groet / kind regards,

Boris Mulder

Cyber Security Labs B.V. | Gooimeer 6-31 | 1411 DD Naarden | The Netherlands
+31 35 631 3253 (office)

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