Re: [PATCH net-next 1/3] net: bridge: add stp_mode attribute for STP mode selection

[Nikolay Aleksandrov <razor@blackwall.org>]

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/ 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 <aroulin@nvidia.com>
> ---
>   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]
>   #ifdef CONFIG_BRIDGE_VLAN_FILTERING
> diff --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 */