[PATCH net-next 0/3] net: bridge: add stp_mode attribute for STP mode selection

[PATCH net-next 0/3] net: bridge: add stp_mode attribute for STP mode selection

[Andy Roulin]

The bridge-stp usermode helper is currently restricted to the initial
network namespace, preventing userspace STP daemons like mstpd from
operating on bridges in other namespaces. Since commit ff62198553e4
("bridge: Only call /sbin/bridge-stp for the initial network
namespace"), bridges in non-init namespaces silently fall back to
kernel STP with no way to request userspace STP.

This series adds a new IFLA_BR_STP_MODE bridge attribute that allows
explicit per-bridge control over STP mode selection. Three modes are
supported:

  - auto (default): existing behavior, try /sbin/bridge-stp in
    init_net, fall back to kernel STP otherwise
  - user: directly enable BR_USER_STP without invoking the helper,
    works in any network namespace
  - kernel: directly enable BR_KERNEL_STP without invoking the helper

The user and kernel modes bypass call_usermodehelper() entirely,
addressing the security concerns discussed at [1]. The caller is
responsible for managing the userspace STP daemon directly, rather
than relying on the kernel to invoke /sbin/bridge-stp.

Patch 1 adds the kernel support. The mode can only be changed while
STP is disabled and is processed before IFLA_BR_STP_STATE in
br_changelink() so both can be set atomically in a single netlink
message.

Patch 2 adds documentation for the new attribute in the bridge docs.

Patch 3 adds a selftest with 8 test cases. The test requires iproute2
with IFLA_BR_STP_MODE support and can be run with virtme-ng:

  vng --run arch/x86/boot/bzImage --skip-modules \
      --overlay-rwdir /sbin --overlay-rwdir /tmp --overlay-rwdir /bin \
      --exec 'cp /path/to/iproute2-next/ip/ip /bin/ip && \
              cd tools/testing/selftests/net && \
              bash bridge_stp_mode.sh'

iproute2 support can be found here [2].

[1] https://lore.kernel.org/netdev/565B7F7D.80208@nod.at/
[2] https://github.com/aroulin/iproute2-next/tree/bridge-stp-mode

Suggested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Andy Roulin <aroulin@nvidia.com>

Andy Roulin (3):
  net: bridge: add stp_mode attribute for STP mode selection
  docs: net: bridge: document stp_mode attribute
  selftests: net: add bridge STP mode selection test

 Documentation/networking/bridge.rst           |  22 ++
 include/uapi/linux/if_link.h                  |  40 +++
 net/bridge/br_device.c                        |   1 +
 net/bridge/br_netlink.c                       |  18 +-
 net/bridge/br_private.h                       |   1 +
 net/bridge/br_stp_if.c                        |  17 +-
 tools/testing/selftests/net/Makefile          |   1 +
 .../testing/selftests/net/bridge_stp_mode.sh  | 261 ++++++++++++++++++
 8 files changed, 353 insertions(+), 8 deletions(-)
 create mode 100755 tools/testing/selftests/net/bridge_stp_mode.sh

--
2.43.0

[PATCH net-next 1/3] net: bridge: add stp_mode attribute for STP mode selection

[Andy Roulin]

The bridge-stp usermode helper is currently restricted to the initial
network namespace, preventing userspace STP daemons (e.g. mstpd) from
operating on bridges in other network namespaces. Since commit
ff62198553e4 ("bridge: Only call /sbin/bridge-stp for the initial
network namespace"), bridges in non-init namespaces silently fall back
to kernel STP with no way to use userspace STP.

Add a new bridge attribute IFLA_BR_STP_MODE that allows explicit
per-bridge control over STP mode selection:

  BR_STP_MODE_AUTO (default) - Existing behavior: invoke the
    /sbin/bridge-stp helper in init_net only; fall back to kernel STP
    if it fails or in non-init namespaces.

  BR_STP_MODE_USER - Directly enable userspace STP (BR_USER_STP)
    without invoking the helper. Works in any network namespace. The
    caller is responsible for registering the bridge with the STP
    daemon after enabling STP.

  BR_STP_MODE_KERNEL - Directly enable kernel STP (BR_KERNEL_STP)
    without invoking the helper.

The mode can only be changed while STP is disabled (-EBUSY otherwise).
IFLA_BR_STP_MODE is processed before IFLA_BR_STP_STATE in
br_changelink(), so both can be set atomically in a single netlink
message.

This eliminates the need for call_usermodehelper() in user/kernel
modes, addressing the security concerns discussed in the thread at
https://lore.kernel.org/netdev/565B7F7D.80208@nod.at/ and providing
a cleaner alternative to extending the helper into namespaces.

Suggested-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Andy Roulin <aroulin@nvidia.com>
---
 include/uapi/linux/if_link.h | 40 ++++++++++++++++++++++++++++++++++++
 net/bridge/br_device.c       |  1 +
 net/bridge/br_netlink.c      | 18 +++++++++++++++-
 net/bridge/br_private.h      |  1 +
 net/bridge/br_stp_if.c       | 17 ++++++++-------
 5 files changed, 69 insertions(+), 8 deletions(-)

diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 83a96c56b8cad..87b2b671ec182 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -744,6 +744,11 @@ enum in6_addr_gen_mode {
  * @IFLA_BR_FDB_MAX_LEARNED
  *   Set the number of max dynamically learned FDB entries for the current
  *   bridge.
+ *
+ * @IFLA_BR_STP_MODE
+ *   Set the STP mode for the bridge, which controls how the bridge
+ *   selects between userspace and kernel STP. The valid values are
+ *   documented below in the ``BR_STP_MODE_*`` constants.
  */
 enum {
 	IFLA_BR_UNSPEC,
@@ -796,11 +801,46 @@ enum {
 	IFLA_BR_MCAST_QUERIER_STATE,
 	IFLA_BR_FDB_N_LEARNED,
 	IFLA_BR_FDB_MAX_LEARNED,
+	IFLA_BR_STP_MODE,
 	__IFLA_BR_MAX,
 };
 
 #define IFLA_BR_MAX	(__IFLA_BR_MAX - 1)
 
+/**
+ * DOC: Bridge STP mode values
+ *
+ * @BR_STP_MODE_AUTO
+ *   Default. The kernel invokes the ``/sbin/bridge-stp`` helper to hand
+ *   the bridge to a userspace STP daemon (e.g. mstpd). Only attempted in
+ *   the initial network namespace; in other namespaces this falls back to
+ *   kernel STP.
+ *
+ * @BR_STP_MODE_USER
+ *   Directly enable userspace STP (``BR_USER_STP``) without invoking the
+ *   ``/sbin/bridge-stp`` helper. Works in any network namespace. The
+ *   caller is responsible for registering the bridge with the userspace
+ *   STP daemon after enabling STP, and for deregistering it before
+ *   disabling STP.
+ *
+ * @BR_STP_MODE_KERNEL
+ *   Directly enable kernel STP (``BR_KERNEL_STP``) without invoking the
+ *   helper.
+ *
+ * The mode controls how the bridge selects between userspace and kernel
+ * STP when STP is enabled via ``IFLA_BR_STP_STATE``. It can only be
+ * changed while STP is disabled (``IFLA_BR_STP_STATE`` == 0), returns
+ * ``-EBUSY`` otherwise. The default value is ``BR_STP_MODE_AUTO``.
+ */
+enum {
+	BR_STP_MODE_AUTO,
+	BR_STP_MODE_USER,
+	BR_STP_MODE_KERNEL,
+	__BR_STP_MODE_MAX
+};
+
+#define BR_STP_MODE_MAX (__BR_STP_MODE_MAX - 1)
+
 struct ifla_bridge_id {
 	__u8	prio[2];
 	__u8	addr[6]; /* ETH_ALEN */
diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
index f7502e62dd357..a35ceae0a6f2c 100644
--- a/net/bridge/br_device.c
+++ b/net/bridge/br_device.c
@@ -518,6 +518,7 @@ void br_dev_setup(struct net_device *dev)
 	ether_addr_copy(br->group_addr, eth_stp_addr);
 
 	br->stp_enabled = BR_NO_STP;
+	br->stp_mode = BR_STP_MODE_AUTO;
 	br->group_fwd_mask = BR_GROUPFWD_DEFAULT;
 	br->group_fwd_mask_required = BR_GROUPFWD_DEFAULT;
 
diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
index 0264730938f4b..4c607d5d17a49 100644
--- a/net/bridge/br_netlink.c
+++ b/net/bridge/br_netlink.c
@@ -1270,6 +1270,9 @@ static const struct nla_policy br_policy[IFLA_BR_MAX + 1] = {
 		NLA_POLICY_EXACT_LEN(sizeof(struct br_boolopt_multi)),
 	[IFLA_BR_FDB_N_LEARNED] = { .type = NLA_REJECT },
 	[IFLA_BR_FDB_MAX_LEARNED] = { .type = NLA_U32 },
+	[IFLA_BR_STP_MODE] = NLA_POLICY_RANGE(NLA_U32,
+					      BR_STP_MODE_AUTO,
+					      BR_STP_MODE_MAX),
 };
 
 static int br_changelink(struct net_device *brdev, struct nlattr *tb[],
@@ -1306,6 +1309,17 @@ static int br_changelink(struct net_device *brdev, struct nlattr *tb[],
 			return err;
 	}
 
+	if (data[IFLA_BR_STP_MODE]) {
+		u32 mode = nla_get_u32(data[IFLA_BR_STP_MODE]);
+
+		if (br->stp_enabled != BR_NO_STP) {
+			NL_SET_ERR_MSG_MOD(extack,
+					   "Can't change STP mode while STP is enabled");
+			return -EBUSY;
+		}
+		br->stp_mode = mode;
+	}
+
 	if (data[IFLA_BR_STP_STATE]) {
 		u32 stp_enabled = nla_get_u32(data[IFLA_BR_STP_STATE]);
 
@@ -1634,6 +1648,7 @@ static size_t br_get_size(const struct net_device *brdev)
 	       nla_total_size(sizeof(u8)) +     /* IFLA_BR_NF_CALL_ARPTABLES */
 #endif
 	       nla_total_size(sizeof(struct br_boolopt_multi)) + /* IFLA_BR_MULTI_BOOLOPT */
+	       nla_total_size(sizeof(u32)) +    /* IFLA_BR_STP_MODE */
 	       0;
 }
 
@@ -1686,7 +1701,8 @@ static int br_fill_info(struct sk_buff *skb, const struct net_device *brdev)
 	    nla_put(skb, IFLA_BR_MULTI_BOOLOPT, sizeof(bm), &bm) ||
 	    nla_put_u32(skb, IFLA_BR_FDB_N_LEARNED,
 			atomic_read(&br->fdb_n_learned)) ||
-	    nla_put_u32(skb, IFLA_BR_FDB_MAX_LEARNED, br->fdb_max_learned))
+	    nla_put_u32(skb, IFLA_BR_FDB_MAX_LEARNED, br->fdb_max_learned) ||
+	    nla_put_u32(skb, IFLA_BR_STP_MODE, br->stp_mode))
 		return -EMSGSIZE;
 
 #ifdef CONFIG_BRIDGE_VLAN_FILTERING
diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index 6dbca845e625d..e4bb9c3f28726 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -540,6 +540,7 @@ struct net_bridge {
 		BR_KERNEL_STP,		/* old STP in kernel */
 		BR_USER_STP,		/* new RSTP in userspace */
 	} stp_enabled;
+	u32				stp_mode;
 
 	struct net_bridge_mcast		multicast_ctx;
 
diff --git a/net/bridge/br_stp_if.c b/net/bridge/br_stp_if.c
index cc4b27ff1b088..fa2271c5d84fe 100644
--- a/net/bridge/br_stp_if.c
+++ b/net/bridge/br_stp_if.c
@@ -149,7 +149,9 @@ static void br_stp_start(struct net_bridge *br)
 {
 	int err = -ENOENT;
 
-	if (net_eq(dev_net(br->dev), &init_net))
+	/* AUTO mode: try bridge-stp helper in init_net only */
+	if (br->stp_mode == BR_STP_MODE_AUTO &&
+	    net_eq(dev_net(br->dev), &init_net))
 		err = br_stp_call_user(br, "start");
 
 	if (err && err != -ENOENT)
@@ -162,7 +164,7 @@ static void br_stp_start(struct net_bridge *br)
 	else if (br->bridge_forward_delay > BR_MAX_FORWARD_DELAY)
 		__br_set_forward_delay(br, BR_MAX_FORWARD_DELAY);
 
-	if (!err) {
+	if (br->stp_mode == BR_STP_MODE_USER || !err) {
 		br->stp_enabled = BR_USER_STP;
 		br_debug(br, "userspace STP started\n");
 	} else {
@@ -180,12 +182,13 @@ static void br_stp_start(struct net_bridge *br)
 
 static void br_stp_stop(struct net_bridge *br)
 {
-	int err;
-
 	if (br->stp_enabled == BR_USER_STP) {
-		err = br_stp_call_user(br, "stop");
-		if (err)
-			br_err(br, "failed to stop userspace STP (%d)\n", err);
+		if (br->stp_mode == BR_STP_MODE_AUTO) {
+			int err = br_stp_call_user(br, "stop");
+
+			if (err)
+				br_err(br, "failed to stop userspace STP (%d)\n", err);
+		}
 
 		/* To start timers on any ports left in blocking */
 		spin_lock_bh(&br->lock);
-- 
2.43.0

Re: [PATCH net-next 1/3] net: bridge: add stp_mode attribute for STP mode selection

[Ido Schimmel]

On Tue, Mar 24, 2026 at 11:49:40AM -0700, Andy Roulin wrote:
>  include/uapi/linux/if_link.h | 40 ++++++++++++++++++++++++++++++++++++

I forgot that this requires a spec update. See:

Documentation/netlink/specs/rt-link.yaml

But wait at least 24h before posting v2.

>  net/bridge/br_device.c       |  1 +
>  net/bridge/br_netlink.c      | 18 +++++++++++++++-
>  net/bridge/br_private.h      |  1 +
>  net/bridge/br_stp_if.c       | 17 ++++++++-------
>  5 files changed, 69 insertions(+), 8 deletions(-)

Re: [PATCH net-next 1/3] net: bridge: add stp_mode attribute for STP mode selection

[Nikolay Aleksandrov]

On 24/03/2026 20:49, Andy Roulin wrote:
> The bridge-stp usermode helper is currently restricted to the initial
> network namespace, preventing userspace STP daemons (e.g. mstpd) from
> operating on bridges in other network namespaces. Since commit
> ff62198553e4 ("bridge: Only call /sbin/bridge-stp for the initial
> network namespace"), bridges in non-init namespaces silently fall back
> to kernel STP with no way to use userspace STP.
> 
> Add a new bridge attribute IFLA_BR_STP_MODE that allows explicit
> per-bridge control over STP mode selection:
> 
>    BR_STP_MODE_AUTO (default) - Existing behavior: invoke the
>      /sbin/bridge-stp helper in init_net only; fall back to kernel STP
>      if it fails or in non-init namespaces.
> 
>    BR_STP_MODE_USER - Directly enable userspace STP (BR_USER_STP)
>      without invoking the helper. Works in any network namespace. The
>      caller is responsible for registering the bridge with the STP
>      daemon after enabling STP.
> 
>    BR_STP_MODE_KERNEL - Directly enable kernel STP (BR_KERNEL_STP)
>      without invoking the helper.
> 
> The mode can only be changed while STP is disabled (-EBUSY otherwise).
> IFLA_BR_STP_MODE is processed before IFLA_BR_STP_STATE in
> br_changelink(), so both can be set atomically in a single netlink
> message.
> 
> This eliminates the need for call_usermodehelper() in user/kernel
> modes, addressing the security concerns discussed in the thread at
> https://lore.kernel.org/netdev/565B7F7D.80208@nod.at/ and providing
> a cleaner alternative to extending the helper into namespaces.
> 
> Suggested-by: Ido Schimmel <idosch@nvidia.com>
> Reviewed-by: Ido Schimmel <idosch@nvidia.com>
> Assisted-by: Claude:claude-opus-4-6
> Signed-off-by: Andy Roulin <aroulin@nvidia.com>
> ---
>   include/uapi/linux/if_link.h | 40 ++++++++++++++++++++++++++++++++++++
>   net/bridge/br_device.c       |  1 +
>   net/bridge/br_netlink.c      | 18 +++++++++++++++-
>   net/bridge/br_private.h      |  1 +
>   net/bridge/br_stp_if.c       | 17 ++++++++-------
>   5 files changed, 69 insertions(+), 8 deletions(-)
> 
[snip]
>   #ifdef CONFIG_BRIDGE_VLAN_FILTERING
> diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
> index 6dbca845e625d..e4bb9c3f28726 100644
> --- a/net/bridge/br_private.h
> +++ b/net/bridge/br_private.h
> @@ -540,6 +540,7 @@ struct net_bridge {
>   		BR_KERNEL_STP,		/* old STP in kernel */
>   		BR_USER_STP,		/* new RSTP in userspace */
>   	} stp_enabled;
> +	u32				stp_mode;
>   
>   	struct net_bridge_mcast		multicast_ctx;
>   
[snip]

Not critical but there's a 4 byte hole in the same cache line betwen root_port
and max_age, if you move stp_mode there we get:

/* size: 1728, cachelines: 27, members: 53 */
/* sum members: 1722, holes: 2, sum holes: 6 */

vs

/* size: 1736, cachelines: 28, members: 53 */
/* sum members: 1722, holes: 4, sum holes: 14 */

[PATCH net-next 2/3] docs: net: bridge: document stp_mode attribute

[Andy Roulin]

Add documentation for the IFLA_BR_STP_MODE bridge attribute in the
"User space STP helper" section of the bridge documentation. Reference
the BR_STP_MODE_* values via kernel-doc and describe the use case for
network namespace environments.

Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Andy Roulin <aroulin@nvidia.com>
---
 Documentation/networking/bridge.rst | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/Documentation/networking/bridge.rst b/Documentation/networking/bridge.rst
index ef8b73e157b26..c1e6ea52c9e59 100644
--- a/Documentation/networking/bridge.rst
+++ b/Documentation/networking/bridge.rst
@@ -148,6 +148,28 @@ called by the kernel when STP is enabled/disabled on a bridge
 stp_state <0|1>``).  The kernel enables user_stp mode if that command returns
 0, or enables kernel_stp mode if that command returns any other value.
 
+STP mode selection
+------------------
+
+The ``IFLA_BR_STP_MODE`` bridge attribute allows explicit control over how
+STP operates when enabled, bypassing the ``/sbin/bridge-stp`` helper
+entirely for the ``user`` and ``kernel`` modes.
+
+.. kernel-doc:: include/uapi/linux/if_link.h
+   :doc: Bridge STP mode values
+
+The default mode is ``BR_STP_MODE_AUTO``, which preserves the traditional
+behavior of invoking the ``/sbin/bridge-stp`` helper. The ``user`` and
+``kernel`` modes are particularly useful in network namespace environments
+where the helper mechanism is not available, as ``call_usermodehelper()``
+is restricted to the initial network namespace.
+
+Example::
+
+  ip link set dev br0 type bridge stp_mode user stp_state 1
+
+The mode can only be changed while STP is disabled.
+
 VLAN
 ====
 
-- 
2.43.0

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

[Andy Roulin]

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

Re: [PATCH net-next 0/3] net: bridge: add stp_mode attribute for STP mode selection

[Jonas Gorski]

On 24/03/2026 19:49, Andy Roulin wrote:
> The bridge-stp usermode helper is currently restricted to the initial
> network namespace, preventing userspace STP daemons like mstpd from
> operating on bridges in other namespaces. Since commit ff62198553e4
> ("bridge: Only call /sbin/bridge-stp for the initial network
> namespace"), bridges in non-init namespaces silently fall back to
> kernel STP with no way to request userspace STP.
> 
> This series adds a new IFLA_BR_STP_MODE bridge attribute that allows
> explicit per-bridge control over STP mode selection. Three modes are
> supported:
> 
>   - auto (default): existing behavior, try /sbin/bridge-stp in
>     init_net, fall back to kernel STP otherwise
>   - user: directly enable BR_USER_STP without invoking the helper,
>     works in any network namespace
>   - kernel: directly enable BR_KERNEL_STP without invoking the helper

I like that very much! This will also allow selftests for
switchdev/dsa drivers for correct (mst) STP state (change) handling.

> The user and kernel modes bypass call_usermodehelper() entirely,
> addressing the security concerns discussed at [1]. The caller is
> responsible for managing the userspace STP daemon directly, rather
> than relying on the kernel to invoke /sbin/bridge-stp.

Should the caller directly manage the STP daemon, or could the STP
daemon also just automatically manage bridges with
IFLA_BR_STP_STATE=BR_STP_MODE_KERNEL (and IFLA_BR_STP_STATE != 0)?

The latter would require less changes for network managers, as they
wouldn't need to be aware of (individual) STP daemon
implementations.

But I guess either is fine, as long as the latter behavior
configurable.

Best regards,
Jonas

Re: [PATCH net-next 0/3] net: bridge: add stp_mode attribute for STP mode selection

[Andy Roulin]

On 3/25/26 00:28, Jonas Gorski wrote:
> On 24/03/2026 19:49, Andy Roulin wrote:
>> The bridge-stp usermode helper is currently restricted to the initial
>> network namespace, preventing userspace STP daemons like mstpd from
>> operating on bridges in other namespaces. Since commit ff62198553e4
>> ("bridge: Only call /sbin/bridge-stp for the initial network
>> namespace"), bridges in non-init namespaces silently fall back to
>> kernel STP with no way to request userspace STP.
>>
>> This series adds a new IFLA_BR_STP_MODE bridge attribute that allows
>> explicit per-bridge control over STP mode selection. Three modes are
>> supported:
>>
>>    - auto (default): existing behavior, try /sbin/bridge-stp in
>>      init_net, fall back to kernel STP otherwise
>>    - user: directly enable BR_USER_STP without invoking the helper,
>>      works in any network namespace
>>    - kernel: directly enable BR_KERNEL_STP without invoking the helper
> 
> I like that very much! This will also allow selftests for
> switchdev/dsa drivers for correct (mst) STP state (change) handling.
> 
>> The user and kernel modes bypass call_usermodehelper() entirely,
>> addressing the security concerns discussed at [1]. The caller is
>> responsible for managing the userspace STP daemon directly, rather
>> than relying on the kernel to invoke /sbin/bridge-stp.
> 
> Should the caller directly manage the STP daemon, or could the STP
> daemon also just automatically manage bridges with
> IFLA_BR_STP_STATE=BR_STP_MODE_KERNEL (and IFLA_BR_STP_STATE != 0)?
> 
> The latter would require less changes for network managers, as they
> wouldn't need to be aware of (individual) STP daemon
> implementations.
> 
> But I guess either is fine, as long as the latter behavior
> configurable.

STP daemons can listen to netlink and automatically discover the bridges
with IFLA_BR_STP_STATE=BR_STP_MODE_USER (or stp_state=BR_STP_USER).

Will change uapi doc in v2 from

  The caller is responsible for registering the bridge with the 
  

   userspace STP daemon after enabling STP, and for deregistering it 
  

   before disabling STP. 
  


to

   No /sbin/bridge-stp helper is invoked; userspace is responsible for 
  

   ensuring an STP daemon manages the bridge.