Add selftest for BPF_CGROUP_INET4_CONNECT and BPF_CGROUP_INET6_CONNECT
attach types.

Try to connect(2) to specified IP:port and test that:
* remote IP:port pair is overridden;
* local end of connection is bound to specified IP.

All combinations of IPv4/IPv6 and TCP/UDP are tested.

Example:
  # tcpdump -pn -i lo -w connect.pcap 2>/dev/null &
  [1] 478
  # strace -qqf -e connect -o connect.trace ./test_sock_addr.sh
  Wait for testing IPv4/IPv6 to become available ... OK
  Load bind4 with invalid type (can pollute stderr) ... REJECTED
  Load bind4 with valid type ... OK
  Attach bind4 with invalid type ... REJECTED
  Attach bind4 with valid type ... OK
  Load connect4 with invalid type (can pollute stderr) libbpf: load bpf \
    program failed: Permission denied
  libbpf: -- BEGIN DUMP LOG ---
  libbpf:
  0: (b7) r2 = 23569
  1: (63) *(u32 *)(r1 +24) = r2
  2: (b7) r2 = 16777343
  3: (63) *(u32 *)(r1 +4) = r2
  invalid bpf_context access off=4 size=4
  [ 1518.404609] random: crng init done

  libbpf: -- END LOG --
  libbpf: failed to load program 'cgroup/connect4'
  libbpf: failed to load object './connect4_prog.o'
  ... REJECTED
  Load connect4 with valid type ... OK
  Attach connect4 with invalid type ... REJECTED
  Attach connect4 with valid type ... OK
  Test case #1 (IPv4/TCP):
          Requested: bind(192.168.1.254, 4040) ..
             Actual: bind(127.0.0.1, 4444)
          Requested: connect(192.168.1.254, 4040) from (*, *) ..
             Actual: connect(127.0.0.1, 4444) from (127.0.0.4, 56068)
  Test case #2 (IPv4/UDP):
          Requested: bind(192.168.1.254, 4040) ..
             Actual: bind(127.0.0.1, 4444)
          Requested: connect(192.168.1.254, 4040) from (*, *) ..
             Actual: connect(127.0.0.1, 4444) from (127.0.0.4, 56447)
  Load bind6 with invalid type (can pollute stderr) ... REJECTED
  Load bind6 with valid type ... OK
  Attach bind6 with invalid type ... REJECTED
  Attach bind6 with valid type ... OK
  Load connect6 with invalid type (can pollute stderr) libbpf: load bpf \
    program failed: Permission denied
  libbpf: -- BEGIN DUMP LOG ---
  libbpf:
  0: (b7) r6 = 0
  1: (63) *(u32 *)(r1 +12) = r6
  invalid bpf_context access off=12 size=4

  libbpf: -- END LOG --
  libbpf: failed to load program 'cgroup/connect6'
  libbpf: failed to load object './connect6_prog.o'
  ... REJECTED
  Load connect6 with valid type ... OK
  Attach connect6 with invalid type ... REJECTED
  Attach connect6 with valid type ... OK
  Test case #3 (IPv6/TCP):
          Requested: bind(face:b00c:1234:5678::abcd, 6060) ..
             Actual: bind(::1, 6666)
          Requested: connect(face:b00c:1234:5678::abcd, 6060) from (*, *)
             Actual: connect(::1, 6666) from (::6, 37458)
  Test case #4 (IPv6/UDP):
          Requested: bind(face:b00c:1234:5678::abcd, 6060) ..
             Actual: bind(::1, 6666)
          Requested: connect(face:b00c:1234:5678::abcd, 6060) from (*, *)
             Actual: connect(::1, 6666) from (::6, 39315)
  ### SUCCESS
  # egrep 'connect\(.*AF_INET' connect.trace | \
  > egrep -vw 'htons\(1025\)' | fold -b -s -w 72
  502   connect(7, {sa_family=AF_INET, sin_port=htons(4040),
  sin_addr=inet_addr("192.168.1.254")}, 128) = 0
  502   connect(8, {sa_family=AF_INET, sin_port=htons(4040),
  sin_addr=inet_addr("192.168.1.254")}, 128) = 0
  502   connect(9, {sa_family=AF_INET6, sin6_port=htons(6060),
  inet_pton(AF_INET6, "face:b00c:1234:5678::abcd", &sin6_addr),
  sin6_flowinfo=0, sin6_scope_id=0}, 128) = 0
  502   connect(10, {sa_family=AF_INET6, sin6_port=htons(6060),
  inet_pton(AF_INET6, "face:b00c:1234:5678::abcd", &sin6_addr),
  sin6_flowinfo=0, sin6_scope_id=0}, 128) = 0
  # fg
  tcpdump -pn -i lo -w connect.pcap 2> /dev/null
  # tcpdump -r connect.pcap -n tcp | cut -c 1-72
  reading from file connect.pcap, link-type EN10MB (Ethernet)
  17:57:40.383533 IP 127.0.0.4.56068 > 127.0.0.1.4444: Flags [S], seq 1333
  17:57:40.383566 IP 127.0.0.1.4444 > 127.0.0.4.56068: Flags [S.], seq 112
  17:57:40.383589 IP 127.0.0.4.56068 > 127.0.0.1.4444: Flags [.], ack 1, w
  17:57:40.384578 IP 127.0.0.1.4444 > 127.0.0.4.56068: Flags [R.], seq 1,
  17:57:40.403327 IP6 ::6.37458 > ::1.6666: Flags [S], seq 406513443, win
  17:57:40.403357 IP6 ::1.6666 > ::6.37458: Flags [S.], seq 2448389240, ac
  17:57:40.403376 IP6 ::6.37458 > ::1.6666: Flags [.], ack 1, win 342, opt
  17:57:40.404263 IP6 ::1.6666 > ::6.37458: Flags [R.], seq 1, ack 1, win
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>