When servers set the C-flag in their MP_CAPABLE to tell clients not to
create subflows to the initial address and port, clients will likely not
use their other endpoints. That's because the in-kernel path-manager
uses the 'subflow' endpoints to create subflows only to the initial
address and port.

The first patch fixes that, and the second one validates it.

More details: https://github.com/multipath-tcp/mptcp_net-next/issues/536

Please see the comment in the first patch, and don't hesitate to reply :)

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Changes in v2:
- patch 1: move cond to new helper (Geliang) + rename var, squash comm.
- patch 2: move accidental .c modif to the parent commit (Geliang)
- Link to v1: https://lore.kernel.org/r/20250915-pm-c-flag-client-default-v1-0-6bb241e4d991@kernel.org

---
Matthieu Baerts (NGI0) (2):
      mptcp: pm: in-kernel: usable client side with C-flag
      selftests: mptcp: join: validate C-flag + def limit

 net/mptcp/pm.c                                  |  7 +++--
 net/mptcp/pm_kernel.c                           | 37 +++++++++++++++++++++++++
 net/mptcp/protocol.h                            |  7 +++++
 tools/testing/selftests/net/mptcp/mptcp_join.sh | 11 ++++++++
 4 files changed, 60 insertions(+), 2 deletions(-)
---
base-commit: 01d3ba86f43b6fe8e91cfd00ba178d6997095a04
change-id: 20250912-pm-c-flag-client-default-f676d8acfdc7

Best regards,
-- 
Matthieu Baerts (NGI0) <matttbe@kernel.org>

Hi Matthieu,

Thank you for your modifications, that's great!

Our CI did some validations and here is its report:

- KVM Validation: normal: Success! ✅
- KVM Validation: debug: Unstable: 1 failed test(s): packetdrill_mp_capable 🔴
- KVM Validation: btf-normal (only bpftest_all): Success! ✅
- KVM Validation: btf-debug (only bpftest_all): Success! ✅
- Task: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/17764130273

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/cbd9718b7233
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1002880


If there are some issues, you can reproduce them using the same environment as
the one used by the CI thanks to a docker image, e.g.:

    $ cd [kernel source code]
    $ docker run -v "${PWD}:${PWD}:rw" -w "${PWD}" --privileged --rm -it \
        --pull always mptcp/mptcp-upstream-virtme-docker:latest \
        auto-normal

For more details:

    https://github.com/multipath-tcp/mptcp-upstream-virtme-docker


Please note that despite all the efforts that have been already done to have a
stable tests suite when executed on a public CI like here, it is possible some
reported issues are not due to your modifications. Still, do not hesitate to
help us improve that ;-)

Cheers,
MPTCP GH Action bot
Bot operated by Matthieu Baerts (NGI0 Core)