socket: Export the FIB number of sockets

[markj]

Summary

Sponsored by: Klara, Inc.
Sponsored by: Stormshield

[zlei]

Looks good to me.

sys/sys/socketvar.h

618–619

struct xsocket is used as KPI. I wonder if we should prefer fixed sized int, aka int32_t for exported so_fibnum. Thought on either 32bit or 64bit architectures sizeof int == sizeof int32_t.

[zlei]

sys/kern/uipc_socket.c

5036

The fib of socket is readonly once set. So I think no need to read it under lock.

[markj]

sys/kern/uipc_socket.c

5036

Somewhat surprisingly, this is not true: see the SO_SETFIB setsockopt handler.

Right now, that option handler does not use any locking. I have a patch which adds it in order to synchronize a new check. In particular, I want to disallow changing the FIB of a listening socket, so we have:

SOCK_LOCK(so);
if (SOLISTENING(so)) {
    error = EINVAL;
    ...

since SOLISTENING() requires some synchronization.

So, I prefer to keep the locking here, even though it is formally unnecessary for now.

[zlei]

sys/kern/uipc_socket.c

5036

Yes, you are right. I missed SO_SETFIB setsockopt handler while doing the quick review.