Undefined behaviour of connect(fd, NULL, 0);

[Neil Brown <neilb@suse.de>]

Hi Netdev.

We have a customer who was reporting strangely unpredictable behaviour of an
in-house application that used networking.

It called connect on a non-blocking socket and subsequently called
   connect(fd, NULL, 0)

to check if the connection had succeeded.
This would sometime "work" and sometimes close the connection.

Looking at the code (sys_connect, move_addr_to_kernel, inet_stream_connect),
it seems that in this case an uninitialised on-stack address is passed
to inet_stream_connect and it makes a decision based on ->sa_family (which is
uninitialised).

It seems clear that connect(fd, NULL, 0) is the wrong thing to do in this
circumstance, but I think it would be good if it failed consistently rather
than unpredictably.

Would it be appropriate for move_addr_to_kernel to zero out the remainder of
the address?
   memset(kaddr+ulen, 0, MAX_SOCK_ADDR-ulen);
??

Then connect(fd, NULL, 0) would always break the connection.

Thanks,
NeilBrown