[PATCH net-next 3/3] selftests: net: add bridge STP mode selection test

[Andy Roulin <aroulin@nvidia.com>]

Add a selftest for the IFLA_BR_STP_MODE bridge attribute that verifies:

1. stp_mode defaults to auto on new bridges
2. stp_mode can be toggled between user, kernel, and auto
3. Changing stp_mode while STP is active is rejected with -EBUSY
4. stp_mode user in a network namespace yields userspace STP (stp_state=2)
5. stp_mode kernel forces kernel STP (stp_state=1)
6. stp_mode auto in a netns preserves traditional fallback to kernel STP
7. stp_mode and stp_state can be set atomically in a single message
8. stp_mode persists across STP disable/enable cycles

Test 4 is the key use case: it demonstrates that userspace STP can now
be enabled in non-init network namespaces by setting stp_mode to user
before enabling STP.

Test 7 verifies the atomic usage pattern where both attributes are set
in a single netlink message, which is supported because br_changelink()
processes IFLA_BR_STP_MODE before IFLA_BR_STP_STATE.

The test gracefully skips if the installed iproute2 does not support
the stp_mode attribute.

Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Andy Roulin <aroulin@nvidia.com>
---
 tools/testing/selftests/net/Makefile          |   1 +
 .../testing/selftests/net/bridge_stp_mode.sh  | 261 ++++++++++++++++++
 2 files changed, 262 insertions(+)
 create mode 100755 tools/testing/selftests/net/bridge_stp_mode.sh

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index 6bced3ed798b0..053c7b83c76dd 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -15,6 +15,7 @@ TEST_PROGS := \
 	big_tcp.sh \
 	bind_bhash.sh \
 	bpf_offload.py \
+	bridge_stp_mode.sh \
 	bridge_vlan_dump.sh \
 	broadcast_ether_dst.sh \
 	broadcast_pmtu.sh \
diff --git a/tools/testing/selftests/net/bridge_stp_mode.sh b/tools/testing/selftests/net/bridge_stp_mode.sh
new file mode 100755
index 0000000000000..9c99d5b6fd667
--- /dev/null
+++ b/tools/testing/selftests/net/bridge_stp_mode.sh
@@ -0,0 +1,261 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# shellcheck disable=SC2034,SC2154,SC2317
+#
+# Test for bridge STP mode selection (IFLA_BR_STP_MODE).
+#
+# Verifies that:
+# - stp_mode defaults to auto on new bridges
+# - stp_mode can be toggled between user, kernel, and auto
+# - stp_mode change is rejected while STP is active (-EBUSY)
+# - stp_mode user in a netns yields userspace STP (stp_state=2)
+# - stp_mode kernel forces kernel STP (stp_state=1)
+# - stp_mode auto preserves traditional fallback to kernel STP
+# - stp_mode and stp_state can be set atomically in one message
+# - stp_mode persists across STP disable/enable cycles
+
+source lib.sh
+
+require_command jq
+
+ALL_TESTS="
+	test_default_auto
+	test_set_modes
+	test_reject_change_while_stp_active
+	test_user_mode_in_netns
+	test_kernel_mode
+	test_auto_mode
+	test_atomic_mode_and_state
+	test_mode_persistence
+"
+
+bridge_info_get()
+{
+	ip -n "$NS1" -d -j link show "$1" | \
+		jq -r ".[0].linkinfo.info_data.$2"
+}
+
+check_stp_mode()
+{
+	local br=$1; shift
+	local expected=$1; shift
+	local msg=$1; shift
+	local val
+
+	val=$(bridge_info_get "$br" stp_mode)
+	[ "$val" = "$expected" ]
+	check_err $? "$msg: expected $expected, got $val"
+}
+
+check_stp_state()
+{
+	local br=$1; shift
+	local expected=$1; shift
+	local msg=$1; shift
+	local val
+
+	val=$(bridge_info_get "$br" stp_state)
+	[ "$val" = "$expected" ]
+	check_err $? "$msg: expected $expected, got $val"
+}
+
+# Create a bridge in NS1, bring it up, and defer its deletion.
+bridge_create()
+{
+	ip -n "$NS1" link add "$1" type bridge
+	ip -n "$NS1" link set "$1" up
+	defer ip -n "$NS1" link del "$1"
+}
+
+setup_prepare()
+{
+	setup_ns NS1
+}
+
+cleanup()
+{
+	defer_scopes_cleanup
+	cleanup_all_ns
+}
+
+# Check that stp_mode defaults to auto when creating a bridge.
+test_default_auto()
+{
+	RET=0
+
+	ip -n "$NS1" link add br-test type bridge
+	defer ip -n "$NS1" link del br-test
+
+	check_stp_mode br-test auto "stp_mode default"
+
+	log_test "stp_mode defaults to auto"
+}
+
+# Test setting stp_mode to user, kernel, and back to auto.
+test_set_modes()
+{
+	RET=0
+
+	ip -n "$NS1" link add br-test type bridge
+	defer ip -n "$NS1" link del br-test
+
+	ip -n "$NS1" link set dev br-test type bridge stp_mode user
+	check_err $? "Failed to set stp_mode to user"
+	check_stp_mode br-test user "after set user"
+
+	ip -n "$NS1" link set dev br-test type bridge stp_mode kernel
+	check_err $? "Failed to set stp_mode to kernel"
+	check_stp_mode br-test kernel "after set kernel"
+
+	ip -n "$NS1" link set dev br-test type bridge stp_mode auto
+	check_err $? "Failed to set stp_mode to auto"
+	check_stp_mode br-test auto "after set auto"
+
+	log_test "stp_mode set user/kernel/auto"
+}
+
+# Verify that stp_mode cannot be changed while STP is active.
+test_reject_change_while_stp_active()
+{
+	RET=0
+
+	bridge_create br-test
+
+	ip -n "$NS1" link set dev br-test type bridge stp_mode kernel
+	check_err $? "Failed to set stp_mode to kernel"
+
+	ip -n "$NS1" link set dev br-test type bridge stp_state 1
+	check_err $? "Failed to enable STP"
+
+	# Changing stp_mode while STP is active should fail.
+	ip -n "$NS1" link set dev br-test type bridge stp_mode auto 2>/dev/null
+	check_fail $? "Changing stp_mode should fail while STP is active"
+
+	check_stp_mode br-test kernel "mode unchanged after rejected change"
+
+	# Disable STP, then change should succeed.
+	ip -n "$NS1" link set dev br-test type bridge stp_state 0
+	check_err $? "Failed to disable STP"
+
+	ip -n "$NS1" link set dev br-test type bridge stp_mode auto
+	check_err $? "Changing stp_mode should succeed after STP is disabled"
+
+	log_test "reject stp_mode change while STP is active"
+}
+
+# Test that stp_mode user in a non-init netns yields userspace STP
+# (stp_state == 2). This is the key use case: userspace STP without
+# needing /sbin/bridge-stp or being in init_net.
+test_user_mode_in_netns()
+{
+	RET=0
+
+	bridge_create br-test
+
+	ip -n "$NS1" link set dev br-test type bridge stp_mode user
+	check_err $? "Failed to set stp_mode to user"
+
+	ip -n "$NS1" link set dev br-test type bridge stp_state 1
+	check_err $? "Failed to enable STP"
+
+	check_stp_state br-test 2 "stp_state with user mode"
+
+	log_test "stp_mode user in netns yields userspace STP"
+}
+
+# Test that stp_mode kernel forces kernel STP (stp_state == 1)
+# regardless of whether /sbin/bridge-stp exists.
+test_kernel_mode()
+{
+	RET=0
+
+	bridge_create br-test
+
+	ip -n "$NS1" link set dev br-test type bridge stp_mode kernel
+	check_err $? "Failed to set stp_mode to kernel"
+
+	ip -n "$NS1" link set dev br-test type bridge stp_state 1
+	check_err $? "Failed to enable STP"
+
+	check_stp_state br-test 1 "stp_state with kernel mode"
+
+	log_test "stp_mode kernel forces kernel STP"
+}
+
+# Test that stp_mode auto preserves traditional behavior: in a netns
+# (non-init_net), bridge-stp is not called and STP falls back to
+# kernel mode (stp_state == 1).
+test_auto_mode()
+{
+	RET=0
+
+	bridge_create br-test
+
+	# Auto mode is the default; enable STP in a netns.
+	ip -n "$NS1" link set dev br-test type bridge stp_state 1
+	check_err $? "Failed to enable STP"
+
+	# In a netns with auto mode, bridge-stp is skipped (init_net only),
+	# so STP should fall back to kernel mode (stp_state == 1).
+	check_stp_state br-test 1 "stp_state with auto mode in netns"
+
+	log_test "stp_mode auto preserves traditional behavior"
+}
+
+# Test that stp_mode and stp_state can be set in a single netlink
+# message. This is the intended atomic usage pattern.
+test_atomic_mode_and_state()
+{
+	RET=0
+
+	bridge_create br-test
+
+	# Set both stp_mode and stp_state in one command.
+	ip -n "$NS1" link set dev br-test type bridge stp_mode user stp_state 1
+	check_err $? "Failed to set stp_mode user and stp_state 1 atomically"
+
+	check_stp_state br-test 2 "stp_state after atomic set"
+
+	log_test "atomic stp_mode user + stp_state 1 in single message"
+}
+
+# Test that stp_mode persists across STP disable/enable cycles.
+test_mode_persistence()
+{
+	RET=0
+
+	bridge_create br-test
+
+	# Set user mode and enable STP.
+	ip -n "$NS1" link set dev br-test type bridge stp_mode user
+	ip -n "$NS1" link set dev br-test type bridge stp_state 1
+	check_err $? "Failed to enable STP with user mode"
+
+	# Disable STP.
+	ip -n "$NS1" link set dev br-test type bridge stp_state 0
+	check_err $? "Failed to disable STP"
+
+	# Verify mode is still user.
+	check_stp_mode br-test user "stp_mode after STP disable"
+
+	# Re-enable STP -- should use user mode again.
+	ip -n "$NS1" link set dev br-test type bridge stp_state 1
+	check_err $? "Failed to re-enable STP"
+
+	check_stp_state br-test 2 "stp_state after re-enable"
+
+	log_test "stp_mode persists across STP disable/enable cycles"
+}
+
+# Check iproute2 support before setting up resources.
+if ! ip link add type bridge help 2>&1 | grep -q "stp_mode"; then
+	echo "SKIP: iproute2 too old, missing stp_mode support"
+	exit "$ksft_skip"
+fi
+
+trap cleanup EXIT
+
+setup_prepare
+tests_run
+
+exit "$EXIT_STATUS"
-- 
2.43.0