selftests/net: Repair RST passive reset selftest

Currently, the test is racy and seems to not pass anymore.

In order to rectify it, aim on TCP_TW_RST.
Doesn't seem way too good with this sleep() part, but it seems as
a reasonable compromise for the test. There is a plan in-line comment on
how-to improve it, going to do it on the top, at this moment I want it
to run on netdev/patchwork selftests dashboard.

It also slightly changes tcp_ao-lib in order to get SO_ERROR propagated
to test_client_verify() return value.

Fixes: c6df7b2361d7 ("selftests/net: Add TCP-AO RST test")
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20240130-tcp-ao-test-key-mgmt-v2-3-d190430a6c60@arista.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

selftests/net: Add TCP-AO RST test

Check that both active and passive reset works and correctly sign
segments with TCP-AO or don't send RSTs if not possible to sign.
A listening socket with backlog = 0 gets one connection in accept
queue, another in syn queue. Once the server/listener socket is
forcibly closed, client sockets aren't connected to anything.
In regular situation they would receive RST on any segment, but
with TCP-AO as there's no listener, no AO-key and unknown ISNs,
no RST should be sent.

And "passive" reset, where RST is sent on reply for some segment
(tcp_v{4,6}_send_reset()) - there use TCP_REPAIR to corrupt SEQ numbers,
which later results in TCP-AO signed RST, which will be verified and
client socket will get EPIPE.

No TCPAORequired/TCPAOBad segments are expected during these tests.

Sample of the output:
> # ./rst_ipv4
> 1..15
> # 1462[lib/setup.c:254] rand seed 1686611171
> TAP version 13
> ok 1 servered 1000 bytes
> ok 2 Verified established tcp connection
> ok 3 sk[0] = 7, connection was reset
> ok 4 sk[1] = 8, connection was reset
> ok 5 sk[2] = 9
> ok 6 MKT counters are good on server
> ok 7 Verified established tcp connection
> ok 8 client connection broken post-seq-adjust
> ok 9 client connection was reset
> ok 10 No segments without AO sign (server)
> ok 11 Signed AO segments (server): 0 => 30
> ok 12 No segments with bad AO sign (server)
> ok 13 No segments without AO sign (client)
> ok 14 Signed AO segments (client): 0 => 30
> ok 15 No segments with bad AO sign (client)
> # Totals: pass:15 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

selftests/net: Add TCP-AO library

Provide functions to create selftests dedicated to TCP-AO.
They can run in parallel, as they use temporary net namespaces.
They can be very specific to the feature being tested.
This will allow to create a lot of TCP-AO tests, without complicating
one binary with many --options and to create scenarios, that are
hard to put in bash script that uses one binary.

Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>