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

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