net: bridge: add stp_mode attribute for STP mode selection

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>
Link: https://patch.msgid.link/20260405205224.3163000-2-aroulin@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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 */