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Matthieu Baerts (NGI0)
f84281bb11
commit2293c57484upstream. During the connection establishment, a peer can tell the other one that it cannot establish new subflows to the initial IP address and port by setting the 'C' flag [1]. Doing so makes sense when the sender is behind a strict NAT, operating behind a legacy Layer 4 load balancer, or using anycast IP address for example. When this 'C' flag is set, the path-managers must then not try to establish new subflows to the other peer's initial IP address and port. The in-kernel PM has access to this info, but the userspace PM didn't. The RFC8684 [1] is strict about that: (...) therefore the receiver MUST NOT try to open any additional subflows toward this address and port. So it is important to tell the userspace about that as it is responsible for the respect of this flag. When a new connection is created and established, the Netlink events now contain the existing but not currently used 'flags' attribute. When MPTCP_PM_EV_FLAG_DENY_JOIN_ID0 is set, it means no other subflows to the initial IP address and port -- info that are also part of the event -- can be established. Link: https://datatracker.ietf.org/doc/html/rfc8684#section-3.1-20.6 [1] Fixes:702c2f646d("mptcp: netlink: allow userspace-driven subflow establishment") Reported-by: Marek Majkowski <marek@cloudflare.com> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/532 Reviewed-by: Mat Martineau <martineau@kernel.org> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org> Link: https://patch.msgid.link/20250912-net-mptcp-pm-uspace-deny_join_id0-v1-2-40171884ade8@kernel.org Signed-off-by: Jakub Kicinski <kuba@kernel.org> [ Conflicts in mptcp_pm.yaml, and mptcp_pm.h, because these files have been added later by commitbc8aeb2045("Documentation: netlink: add a YAML spec for mptcp"), and commit9d1ed17f93("uapi: mptcp: use header file generated from YAML spec"), which are not in this version. Applying the same modifications, but only in mptcp.h. Conflict in pm_netlink.c, because of a difference in the context, introduced by commitb9f4554356("mptcp: annotate lockless access for token"), which is not in this version. ] Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>