Re: [PATCH mptcp-net v3 2/2] selftests: mptcp: join: re-add the initial subflow endp with fullmesh

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- [Medium] The new subtest "delete and re-add, fullmesh, after reset"
  (endpoint_tests() in tools/testing/selftests/net/mptcp/mptcp_join.sh)…
- [Low] In "delete and re-add, fullmesh, after reset" (endpoint_tests(),
  mptcp_join.sh lines 4493-4507), the reset, the endpoint delete/re-add,…

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selftests: mptcp: join: re-add the initial subflow endp with fullmesh

This commit adds two subtests to endpoint_tests() in mptcp_join.sh. Both
delete and re-add the endpoint used by the initial subflow with the
fullmesh flag set. The second one first resets the initial subflow, then
removes its endpoint.

> diff --git a/tools/testing/selftests/net/mptcp/mptcp_join.sh b/tools/testing/selftests/net/mptcp/mptcp_join.sh
> index 18ce7136a2b0..d2d105cef4a9 100755
> --- a/tools/testing/selftests/net/mptcp/mptcp_join.sh
> +++ b/tools/testing/selftests/net/mptcp/mptcp_join.sh
> @@ -4445,6 +4445,68 @@ endpoint_tests()

[ ... ]

> +	# same, but the initial subflow is reset before removing its endpoint
> +	if reset_with_events "delete and re-add, fullmesh, after reset" &&
> +	   continue_if mptcp_lib_kallsyms_has "subflow_rebuild_header$"; then
> +		pm_nl_set_limits $ns1 0 2
> +		pm_nl_set_limits $ns2 0 2
> +		pm_nl_add_endpoint $ns2 10.0.1.2 id 1 dev ns2eth1 flags subflow,fullmesh
> +		pm_nl_add_endpoint $ns2 10.0.2.2 id 2 dev ns2eth2 flags subflow,fullmesh
> +		{ timeout_test=120 test_linkfail=128 speed=5 \
> +			run_tests $ns1 $ns2 10.0.1.1 & } 2>/dev/null
> +		local tests_pid=$!
> +
> +		wait_event ns2 MPTCP_LIB_EVENT_ESTABLISHED 1
> +		wait_mpj 1
> +		chk_subflow_nr "before reset" 2
> +
> +		if ip netns exec "${ns1}" ${iptables} -A INPUT -s "10.0.1.2" \
> +		      -p tcp --tcp-option 30 -j REJECT --reject-with tcp-reset; then
> +			wait_event ns2 MPTCP_LIB_EVENT_SUB_CLOSED 1
> +			ip netns exec "${ns1}" ${iptables} -F INPUT
> +
> +			pm_nl_del_endpoint $ns2 1 10.0.1.2
> +			pm_nl_add_endpoint $ns2 10.0.1.2 id 1 dev ns2eth1 flags subflow,fullmesh
> +			wait_mpj 2
> +			chk_mptcp_info subflows 2 subflows 2

[Severity: Medium]
Can this subtest pass even if the initial subflow was never reset?

wait_event() gives up after 100 polls and reports nothing to its caller,
so a timeout looks the same as success:

tools/testing/selftests/net/mptcp/mptcp_join.sh:wait_event() {
    ...
	for _ in $(seq 100); do
		count=$(grep -cw "type:${evt}" "${evts}")
		[ "${count}" -ge "${exp}" ] && break
		sleep 0.1
	done
}

The REJECT rule only causes a reset if ns2 sends an option 30 packet
from 10.0.1.2 while the rule is in place. Nothing forces traffic onto
the initial subflow during that window.

If no reset happens, the rule is flushed and the endpoint of a subflow
that is still established gets deleted and re-added. That is the same
scenario the previous "delete and re-add, fullmesh" subtest already
covers.

The later checks pass in that case as well. "chk_mptcp_info subflows 2
subflows 2" holds because closing the initial subflow does not decrement
extra_subflows, and the re-add adds one join. "chk_join_nr 2 2 2" holds
too.

So the TCP_CLOSE path in __mptcp_subflow_disconnect() in
net/mptcp/protocol.c may never be reached:

	if (((1 << ssk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)) ||
	    fastclosing) {
		WARN_ON_ONCE(tcp_disconnect(ssk, 0));
		mptcp_subflow_ctx_reset(subflow);

The commit message says this subtest covers that case ("also after a
reset of the initial subflow, before the removal of its endpoint").

"userspace pm no dup close events after error" follows its wait_event()
with "chk_subflows_total 1 1" to confirm the reset happened. Could this
subtest add a similar check after the SUB_CLOSED wait, for example
chk_evt_nr ns2 MPTCP_LIB_EVENT_SUB_CLOSED 1 or chk_rst_nr?

> +		fi
> +
> +		mptcp_lib_kill_group_wait $tests_pid
> +		kill_events_pids
> +
> +		chk_join_nr 2 2 2

[Severity: Low]
Should this chk_join_nr be inside the iptables branch above?

The if around the iptables command exists to tolerate a rule that cannot
be installed. In that case endpoint id 1 is never re-added.

Then only one MP_JOIN happens, from the id 2 fullmesh endpoint. The
check here sits outside the branch, so it reports "got 1 JOIN[s] syn rx
expected 2" and the subtest fails instead of being skipped.

"userspace pm no dup close events after error" in userspace_tests() uses
the same iptables guard but keeps every dependent check inside the
branch. Also, reset_with_tcp_filter() calls mark_as_skipped when it
cannot set a REJECT rule.

> +	fi
> +
>  	# remove and re-add
>  	if reset_with_events "delete re-add signal" &&
>  	   continue_if mptcp_lib_kallsyms_has "subflow_rebuild_header$"; then

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Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261019185512.458313-1-w453y.me%40gmail.com