This patch makes a few changes to the network_helpers.c

1) Enforce SO_RCVTIMEO and SO_SNDTIMEO
   This patch enforces timeout to the network fds through setsockopt
   SO_RCVTIMEO and SO_SNDTIMEO.

   It will remove the need for SOCK_NONBLOCK that requires a more demanding
   timeout logic with epoll/select, e.g. epoll_create, epoll_ctrl, and
   then epoll_wait for timeout.

   That removes the need for connect_wait() from the
   cgroup_skb_sk_lookup.c. The needed change is made in
   cgroup_skb_sk_lookup.c.

2) start_server():
   Add optional addr_str and port to start_server().
   That removes the need of the start_server_with_port().  The caller
   can pass addr_str==NULL and/or port==0.

   I have a future tcp-hdr-opt test that will pass a non-NULL addr_str
   and it is in general useful for other future tests.

   "int timeout_ms" is also added to control the timeout
   on the "accept(listen_fd)".

3) connect_to_fd(): Fully use the server_fd.
   The server sock address has already been obtained from
   getsockname(server_fd).  The sockaddr includes the family,
   so the "int family" arg is redundant.

   Since the server address is obtained from server_fd,  there
   is little reason not to get the server's socket type from the
   server_fd also.  getsockopt(server_fd) can be used to do that,
   so "int type" arg is also removed.

   "int timeout_ms" is added.

4) connect_fd_to_fd():
   "int timeout_ms" is added.
   Some code is also refactored to connect_fd_to_addr() which is
   shared with connect_to_fd().

5) Preserve errno:
   Some callers need to check errno, e.g. cgroup_skb_sk_lookup.c.
   Make changes to do it more consistently in save_errno_close()
   and log_err().
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200702004852.2103003-1-kafai@fb.com