libc blocking functions/with_libc inside thread

Boris Mulder boris.mulder at ...434...
Mon Jun 26 14:46:08 CEST 2017

Hello Christian,

Actually the OpenVPN code hangs once it calls the libc socket()
function. Internally, this function calls a blocking write(), and this
write() is handled by Libc::Kernel.

So openVPN does not send or receive any packet yet as it is blocked at

Earlier, we have used lwip as a socket library. When we did that,
socket() (and connect() in TCP mode) did work, but it failed to send any
initial data to the server, likewise blocking on some function.

We are reaching the limit of our knowledge of genode libc and the
side-effects of the asynchronous entrypoint. At this point our debugging
went down into the libc kernel and there is a limit how deep we can go.
Help on this topic would be appreciated.

We uploaded the new 17.05 ready code of openVPN (including a run script
which can be run through make run/openvpn) onto .

If you have the time, could you try to run it and see if it is possible
to let it produce a single UDP packet? For this, it is not necessary to
setup a server. From there, we could pick it up again.

We appreciate it,


On 26-06-17 10:57, Christian Helmuth wrote:
> Hello Boris,
> On Fri, Jun 23, 2017 at 03:59:53PM +0200, Boris Mulder wrote:
>> The entrypoint creates the root component, spawns the thread and
>> returns. It will then handle RPC requests, as entrypoints do IIRC.
>> The program acts as a server (serving Nic sessions asynchronously) and
>> as a client to lxip vfs with libc. the code can be found in [1].
>> How can I have the entrypoint handle I/O signals in libc while also
>> being able to serve clients in Genode?
> This should happen automatically under the hood as libc processes
> signals in ordinary I/O signal handlers in the entrypoint.
> Are you able to run the scenario under linux and inspect the
> processing of both threads via GDB? I fear that I cannot help with
> specifics of OpenVPN, but may guide with more details about the
> blocking situation. It may be interesting to know if any network
> packets reach the OpenVPN code.
> Greets


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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