Re: [PATCH net-next 1/3] net: bridge: add stp_mode attribute for STP mode selection
From: Nikolay Aleksandrov <razor@blackwall.org>
Date: 2026-03-25 07:46:36
Also in:
bridge, linux-doc, linux-kselftest, lkml
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/ (local) 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 <redacted>
---
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]
quoted hunk ↗ jump to hunk
#ifdef CONFIG_BRIDGE_VLAN_FILTERINGdiff --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 */