[PATCH net-next v4 1/3] net: bridge: add stp_mode attribute for STP mode selection

Andy Roulin
2026-04-05T20:52:31Z

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.
Userspace is responsible for ensuring an STP daemon manages the
bridge.

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, or set to the
same value (-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. The mode can also be changed in the
same message that disables STP.

The stp_mode struct field is u8 since all possible values fit, while
NLA_U32 is used for the netlink attribute since it occupies the same
space in the netlink message as NLA_U8.

A new stp_helper_active boolean tracks whether the /sbin/bridge-stp
helper was invoked during br_stp_start(), so that br_stp_stop() only
calls the helper for stop when it was called for start. This avoids
calling the helper asymmetrically when stp_mode changes between
start and stop.

Suggested-by: Ido Schimmel <idosch@nvidia.com>
Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Andy Roulin <aroulin@nvidia.com>
---

Notes:
v2:
* Add rt-link.yaml netlink spec update.
* Allow idempotent stp_mode set while STP is active.
* Move stp_mode next to root_port to fill a struct hole.
* Rephrase BR_STP_MODE_USER doc.
v3:
* Name enum br_stp_mode for YNL codegen.
* Add enum-name to rt-link.yaml spec.
v4:
* Use u8 for stp_mode struct field.
* Add stp_helper_active bool to track whether the
usermode helper was invoked during start.
* Allow mode change when STP is being disabled in the
same netlink message.

Documentation/netlink/specs/rt-link.yaml | 12 ++++++++
include/uapi/linux/if_link.h | 39 ++++++++++++++++++++++++
net/bridge/br_device.c | 1 +
net/bridge/br_netlink.c | 24 ++++++++++++++-
net/bridge/br_private.h | 2 ++
net/bridge/br_stp_if.c | 19 +++++++-----
6 files changed, 89 insertions(+), 8 deletions(-)

diff --git a/Documentation/netlink/specs/rt-link.yaml b/Documentation/netlink/specs/rt-link.yaml
index df4b56beb8187..495836a569e8f 100644
--- a/Documentation/netlink/specs/rt-link.yaml
+++ b/Documentation/netlink/specs/rt-link.yaml
@@ -833,6 +833,14 @@ definitions:
entries:
- p2p
- mp
+ -
+ name: br-stp-mode
+ type: enum
+ enum-name: br-stp-mode
+ entries:
+ - auto
+ - user
+ - kernel

attribute-sets:
-
@@ -1543,6 +1551,10 @@ attribute-sets:
-
name: fdb-max-learned
type: u32
+ -
+ name: stp-mode
+ type: u32
+ enum: br-stp-mode
-
name: linkinfo-brport-attrs
name-prefix: ifla-brport-
diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 83a96c56b8cad..f159be39a6e78 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 +801,45 @@ 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.
+ * Userspace is responsible for ensuring an STP daemon manages the
+ * bridge.
+ *
+ * @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 {
+ 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..6fd5386a1d646 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 +1309,23 @@ 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 (mode != br->stp_mode) {
+ bool stp_off = br->stp_enabled == BR_NO_STP ||
+ (data[IFLA_BR_STP_STATE] &&
+ !nla_get_u32(data[IFLA_BR_STP_STATE]));
+
+ if (!stp_off) {
+ 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 +1654,7 @@ 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 +1707,8 @@ 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..361a9b84451ec 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -523,6 +523,8 @@ struct net_bridge {
unsigned char topology_change;
unsigned char topology_change_detected;
u16 root_port;
+ u8 stp_mode;
+ bool stp_helper_active;
unsigned long max_age;
unsigned long hello_time;
unsigned long forward_delay;
diff --git a/net/bridge/br_stp_if.c b/net/bridge/br_stp_if.c
index cc4b27ff1b088..28c1d3f7e22f6 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)
@@ -162,8 +164,9 @@ 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->stp_helper_active = !err;
br_debug(br, "userspace STP started\n");
} else {
br->stp_enabled = BR_KERNEL_STP;
@@ -180,12 +183,14 @@ 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_helper_active) {
+ int err = br_stp_call_user(br, "stop");
+
+ if (err)
+ br_err(br, "failed to stop userspace STP (%d)\n", err);
+ br->stp_helper_active = false;
+ }

/* To start timers on any ports left in blocking */
spin_lock_bh(&br->lock);
--
2.43.0