Merge branch 'net-bridge-add-stp_mode-attribute-for-stp-mode-selection'

Andy Roulin says:

====================
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 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

The user and kernel modes bypass call_usermodehelper() entirely,
addressing the security concerns discussed at [1]. Userspace is
responsible for ensuring an STP daemon manages the bridge, rather
than relying on the kernel to invoke /sbin/bridge-stp.

Patch 1 adds the kernel support. The mode can only be changed while
STP is disabled and is processed before IFLA_BR_STP_STATE in
br_changelink() so both can be set atomically in a single netlink
message.

Patch 2 adds documentation for the new attribute in the bridge docs.

Patch 3 adds a selftest with 9 test cases. The test requires iproute2
with IFLA_BR_STP_MODE support and can be run with virtme-ng:

  vng --run arch/x86/boot/bzImage --skip-modules \
      --overlay-rwdir /sbin --overlay-rwdir /tmp --overlay-rwdir /bin \
      --exec 'cp /path/to/iproute2-next/ip/ip /bin/ip && \
              cd tools/testing/selftests/net && \
              bash bridge_stp_mode.sh'

iproute2 support can be found here [2].

[1] https://lore.kernel.org/netdev/565B7F7D.80208@nod.at/
[2] https://github.com/aroulin/iproute2-next/tree/bridge-stp-mode
====================

Link: https://patch.msgid.link/20260405205224.3163000-1-aroulin@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>