[LTP] TST_GET_UNUSED_PORT returns ports < 1024

[Petr Vorel <pvorel@suse.cz>]

Hi Cyril,

> > when using the TST_GET_UNUSED_PORT macro you sometimes get ports lower
> > than 1024 which would require a testcase to have the
> > CAP_NET_BIND_SERVICE capability (or simply run as root).

> Looking at the code as far as I can tell the function returns the port
> in the network endianity, which is big endian. Intel CPUs are little
> endian, so if you want to print the port you actually have to use
> ntohs() function to convert it to the host endianity. And if you are
> passing that value in the sockaddr_in structure you must not use the
> htons() since the value is already in the correct byte order. And yes
> this is horribly confusing, but that's how it is.
Thanks for debugging the problem.

> I guess that we should write down this piece of information in the
> documentation, because it looks like the tst_get_unused_port shell
> helper does this incorrectly and prints the raw value instead of
> converting it with ntohs().
Correct, I've sent a patch fixing it for shell tests [1].
As I noted there, this problem was even on version for old API.

Not sure about docs as there is no docs for network API shell/C functions yet
But even the problem for shell was fixed by that patch it'd be worth to add note
about byte order to C code tst_get_unused_port() and/or header defining
TST_GET_UNUSED_PORT() developers using it in C.

Kind regards,
Petr

[1] https://patchwork.ozlabs.org/patch/1111167/