bridge: MTU auto tuning ignores IFLA_MTU on NEWLINK

diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
index 10f0d33d8ccf..8aff7d077848 100644
--- a/net/bridge/br_netlink.c
+++ b/net/bridge/br_netlink.c
@@ -1559,6 +1559,9 @@  static int br_dev_newlink(struct net *src_net, struct net_device *dev,
 		spin_unlock_bh(&br->lock);
 	}
 
+	if (tb[IFLA_MTU])
+		br_opt_toggle(br, BROPT_MTU_SET_BY_USER, true);
+
 	err = br_changelink(dev, tb, data, extack);
 	if (err)
 		br_dev_delete(dev, NULL);
diff --git a/tools/testing/selftests/drivers/net/bridge/Makefile b/tools/testing/selftests/drivers/net/bridge/Makefile
new file mode 100644
index 000000000000..23e407c75a7f
--- /dev/null
+++ b/tools/testing/selftests/drivers/net/bridge/Makefile
@@ -0,0 +1,10 @@ 
+# SPDX-License-Identifier: GPL-2.0
+# Makefile for net selftests
+
+TEST_PROGS := \
+	bridge-user-mtu.sh
+
+TEST_FILES := \
+	net_forwarding_lib.sh
+
+include ../../../lib.mk
diff --git a/tools/testing/selftests/drivers/net/bridge/bridge-user-mtu.sh b/tools/testing/selftests/drivers/net/bridge/bridge-user-mtu.sh
new file mode 100755
index 000000000000..07e0ac972b00
--- /dev/null
+++ b/tools/testing/selftests/drivers/net/bridge/bridge-user-mtu.sh
@@ -0,0 +1,148 @@ 
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Ensure a bridge MTU does not automatically change when it has been specified
+# by the user.
+#
+# To run independently:
+# make TARGETS=drivers/net/bridge kselftest
+
+ALL_TESTS="
+	bridge_created_with_user_specified_mtu
+	bridge_created_without_user_specified_mtu
+	bridge_with_late_user_specified_mtu
+"
+
+REQUIRE_MZ=no
+NUM_NETIFS=0
+lib_dir=$(dirname "$0")
+source "${lib_dir}"/net_forwarding_lib.sh
+
+setup_prepare()
+{
+	for i in 1 3 5; do
+		ip link add "vtest${i}" mtu 9000 type veth peer name "vtest${i}b" mtu 9000
+	done
+}
+
+cleanup()
+{
+	for interface in vtest1 vtest3 vtest5 br-test0 br-test1 br-test2; do
+		if [[ -d "/sys/class/net/${interface}" ]]; then
+			ip link del "${interface}" &> /dev/null
+		fi
+	done
+}
+
+check_mtu()
+{
+	cur_mtu=$(<"/sys/class/net/$1/mtu")
+	[[ ${cur_mtu} -eq $2 ]]
+	exit_status=$?
+	return "${exit_status}"
+}
+
+check_bridge_user_specified_mtu()
+{
+	if [[ -z $1 ]]
+	then
+		exit 1
+	fi
+	mtu=$1
+
+	RET=0
+
+	ip link add dev br-test0 mtu "${mtu}" type bridge
+	ip link set br-test0 up
+	check_mtu br-test0 "${mtu}"
+	check_err $? "Bridge was not created with the user-specified MTU"
+
+	check_mtu vtest1 9000
+	check_err $? "vtest1 does not have MTU 9000"
+
+	ip link set dev vtest1 master br-test0
+	check_mtu br-test0 "${mtu}"
+	check_err $? "Bridge user-specified MTU incorrectly changed after adding an interface"
+
+	log_test "Bridge created with user-specified MTU (${mtu})"
+
+	ip link del br-test0
+}
+
+bridge_created_with_user_specified_mtu() {
+	# Check two user-specified MTU values
+	# - 1500: To ensure the default MTU (1500) is not special-cased, you
+	#         should be able to lock a bridge to the default MTU.
+	# - 2000: Ensure bridges are actually created with a user-specified MTU
+	check_bridge_user_specified_mtu 1500
+	check_bridge_user_specified_mtu 2000
+}
+
+bridge_created_without_user_specified_mtu()
+{
+	RET=0
+	ip link add dev br-test1 type bridge
+	ip link set br-test1 up
+	check_mtu br-test1 1500
+	check_err $? "Bridge was not created with the user-specified MTU"
+
+	ip link set dev vtest3 master br-test1
+	check_mtu br-test1 9000
+	check_err $? "Bridge without user-specified MTU did not change MTU"
+
+	log_test "Bridge created without user-specified MTU"
+
+	ip link del br-test1
+}
+
+check_bridge_late_user_specified_mtu()
+{
+	if [[ -z $1 ]]
+	then
+		exit 1
+	fi
+	mtu=$1
+
+	RET=0
+	ip link add dev br-test2 type bridge
+	ip link set br-test2 up
+	check_mtu br-test2 1500
+	check_err $? "Bridge was not created with default MTU (1500)"
+
+	ip link set br-test2 mtu "${mtu}"
+	check_mtu br-test2 "${mtu}"
+	check_err $? "User-specified MTU set after creation was not set"
+	check_mtu vtest5 9000
+	check_err $? "vtest5 does not have MTU 9000"
+
+	ip link set dev vtest5 master br-test2
+	check_mtu br-test2 "${mtu}"
+	check_err $? "Bridge late-specified MTU incorrectly changed after adding an interface"
+
+	log_test "Bridge created without user-specified MTU and changed after (${mtu})"
+
+	ip link del br-test2
+}
+
+bridge_with_late_user_specified_mtu()
+{
+	# Note: Unfortunately auto-tuning is not disabled when you set the MTU
+	# to it's current value, including the default of 1500. The reason is
+	# that dev_set_mtu_ext skips notifying any handlers if the MTU is set
+	# to the current value. Normally that makes sense, but is confusing
+	# since you might expect "ip link set br0 mtu 1500" to lock the MTU to
+	# 1500 but that will only happen if the MTU was not already 1500. So we
+	# only check a non-default value of 2000 here unlike the earlier
+	# bridge_created_with_user_specified_mtu test
+
+	# Check one user-specified MTU value
+	# - 2000: Ensure bridges actually change to a user-specified MTU
+	check_bridge_late_user_specified_mtu 2000
+}
+
+trap cleanup EXIT
+
+setup_prepare
+tests_run
+
+exit "${EXIT_STATUS}"
diff --git a/tools/testing/selftests/drivers/net/bridge/net_forwarding_lib.sh b/tools/testing/selftests/drivers/net/bridge/net_forwarding_lib.sh
new file mode 120000
index 000000000000..39c96828c5ef
--- /dev/null
+++ b/tools/testing/selftests/drivers/net/bridge/net_forwarding_lib.sh
@@ -0,0 +1 @@ 
+../../../net/forwarding/lib.sh
\ No newline at end of file