libnl issue with NETLINK_CONNECTOR and PROC_EVENTS on OpenWrt

[Thomas Haller]

On Tue, 2016-11-15 at 17:45 +0100, Hrvoje Varga wrote:
> Hi all,
> 
> I am experiencing an issue with an application that uses libnl to
> subscribe and later on receive PROC_EVENTS messages from the kernel.
> The application is built to work on OpenWrt platform but the initial
> development was done using the Arch Linux on my own local machine.
> 
> The problem is that I am not receiving any events from Linux in a
> case
> when the application is built for OpenWrt which is running on one of
> our devices. If this application is built and ran on Arch Linux, the
> application is behaving as expected; the PROC_EVENTS messages are
> received and successfully processed.
> 
> Note that I have built the OpenWrt with enabled CONNECTOR and
> PROC_EVENTS kernel configuration parameters.
> 
> Next is a non-compilable example code which I use to allocate a
> socket, connect and subscribe to receive events about processes.
> 
>     struct nl_sock *nls = nl_socket_alloc();
>     if (nls == NULL) {
>         return -1;
>     }
> 
>     int rc = nl_connect(nls, NETLINK_CONNECTOR);
>     if (rc < 0) {
>           return -1;
>     }
> 
>     nl_socket_disable_seq_check(nls);
>     nl_socket_add_memberships(nls, CN_IDX_PROC, 0);
>     nl_socket_modify_cb(nls, NL_CB_MSG_IN, NL_CB_CUSTOM,
> connector_proc_msg_recv_cb, NULL);
>     nl_socket_set_nonblocking(nls);
> 
> Note here that I am calling nl_recvmsgs_default() function behind the
> scenes. The connector_proc_msg_recv_cb() callback function is not
> called in case of OpenWrt.
> 
> Now, the reason that I consider that libnl has an issue is that I
> tried to refactor my application to use the raw Linux NETLINK API
> directly instead of using libnl. In this case, the application is
> working as expected even on the OpenWrt.
> 
> I'm hoping that someone here can tell me what's wrong and how I can
> fix this. I'm willing to provide more info when necessary.
> 
> Below is the environment that I use to run the application:
> 
> Device:
> - OpenWrt 15.05 (Chaos Calmer)
> - Linux 3.18.36
> - libnl 3.2.21 (downloaded from http://www.infradead.org/~tgr/libnl/f
> iles/)
> 
> Local Machine:
> - Arch Linux
> - Linux 4.8.7
> - libnl 3.2.28 (downloaded from https://github.com/thom311/libnl/rele
> ases)
> 

Hi,

what you describe doesn't look wrong, although not seeing the entire
code. Also does your issue not ring a bell.

Note that NL_CB_MSG_IN is the first hook and you should get events
there.

What happens if you enable debug logging via `export NLDBG=100`?
Does strace help?

As you are using different libnl versions, I would first check whether
it's due to that... rebuilding libnl3 should be simple enough.
Maybe try how 3.2.21 works on Arch, or how 3.2.28 works on OpenWrt.
If that shows a difference, I would bisect libnl.


Sorry for the late reply.

Best,
Thomas
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