sys/net/gnrc/tcp: fix gnrc_tcp_open() netif handling

[maribu]

Contribution description

gnrc_tcp_open() previously would eventually fail with a timeout without sending any data when no netif was specified and a link-local target address was used. This fixes the behavior:

• If there is only one netif, we just pick that
• If there are multiple netifs, fail directly with -EINVAL rather than sending out nothing and waiting for a timeout.

Testing procedure

In one terminal run:

$ make BOARD=native -C tests/net/gnrc_sock_tcp -j && tests/net/gnrc_sock_tcp/bin/native/*.elf -w tap1
[...]
RIOT native interrupts/signals initialized.
RIOT native board initialized.
RIOT native hardware initialization complete.

main(): This is RIOT! (Version: 2025.01-devel-50-ge419c)
RIOT GNRC_TCP test application
> ifconfig
Iface  5  HWaddr: 26:DE:D0:85:BD:B2 
          L2-PDU:1500  MTU:1500  HL:64  Source address length: 6
          Link type: wireless
          inet6 addr: fe80::24de:d0ff:fe85:bdb2  scope: link  VAL
          inet6 group: ff02::1
          inet6 group: ff02::1:ff85:bdb2

> sock_tcp_listen [fe80::24de:d0ff:fe85:bdb2]:1234
sock_tcp_listen: argc=2, argv[0] = sock_tcp_listen, argv[1] = [fe80::24de:d0ff:fe85:bdb2]:1234
sock_tcp_listen: returns 0

In a second (while keeping the TCP server from the first terminal running) run:

$ make BOARD=native -C tests/net/gnrc_sock_tcp -j && tests/net/gnrc_sock_tcp/bin/native/*.elf -w tap0
[...]
RIOT native interrupts/signals initialized.
RIOT native board initialized.
RIOT native hardware initialization complete.

main(): This is RIOT! (Version: 2025.01-devel-50-ge419c)
RIOT GNRC_TCP test application
> sock_tcp_connect [fe80::24de:d0ff:fe85:bdb2]:1234 0
sock_tcp_connect: argc=3, argv[0] = sock_tcp_connect, argv[1] = [fe80::24de:d0ff:fe85:bdb2]:1234, argv[2] = 0
sock_tcp_connect: returns 0

Issues/PRs references

None

[riot-ci]

Murdock results

✔️ PASSED

e714707 sys/net/gnrc/tcp: fix gnrc_tcp_open() netif handling

Success	Failures	Total	Runtime
10251	0	10251	15m:37s

Artifacts

• Documentation preview

[benpicco]

Suggested change

	if (tcb->ll_iface == SOCK_ADDR_ANY_NETIF) {
	if (ipv6_addr_is_link_local(tcb->peer_addr) && tcb->ll_iface == SOCK_ADDR_ANY_NETIF) {

[maribu]

I think for link local, we don't even need to check if only one netif is available: Any interface will be equally valid then, so we could always just take the first netif for link local addresses and unspecified netif.

[benpicco]

No, always chosing the first interface when no interface is specified is way too surprising/confusing.

Just return an error if you'd need to make a choice, like gnrc_udp does.

[maribu]

I cannot find where UDP does this. Do you know where the code lives? If that is implemented already, I could split this out info a common function and reuse this. That way, we can ensure that TCP and UDP will behave consistently in this regard.

[maribu]

OK, I just free coded this for now.

[maribu]

Thx ❤️