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.
