tests: cover more Diag-related code

[matttbe]

There are some improvements that can be done on the tests side to cover more code in some areas (code coverage), e.g. the Diag part:

• subflow_get_info_size() from diag.c is not covered
• mptcp_diag_dump_one() from mptcp_diag.c is not covered

[geliangtang]

Hi Matt, I assigned this issue to myself since my colleague Gang Yan in my group is looking at it.

[matttbe]

Thanks! A good task to get familiar with the code and the test suite.

Please note that subflow_get_info_size() might move out of net/mptcp with this patch, but if it is easy, it still makes sense to have a test covering this part.

[Dwyane-Yan]

Hi Matt, recently I've been trying to accept and parse messages from kernel, but I've encountered quite a few issues. For instance, when I call recvmsg, I get an 'NLMSG_ERROR' which is 'No such file or directory'. When I change '.nlmsg_flags' to 'NLM_F_REQUEST | NLM_F_DUMP', the error doesn't occur, but this results in the kernel not calling the '.dump_one' interface. Do you have any suggestions? Or do you have any experience with overriding the dump_one method in other modules?

[matttbe]

@Dwyane-Yan no sorry, I'm not sure what is needed here.

My understanding is that dump_one could be used to display info about a specific MPTCP connection by giving the token. Maybe pm_nl_ctl and/org ss can be extended to display info about a specific connection?

Also, because the kernel code is not covered by our test suite, it is possible it doesn't work as expected, and also need to be modified.