[PATCH net v3 0/3] vsock: add write-once semantics to child_ns_mode

[Bobby Eshleman <bobbyeshleman@gmail.com>]

Two administrator processes may race when setting child_ns_mode: one
sets it to "local" and creates a namespace, but another changes it to
"global" in between. The first process ends up with a namespace in the
wrong mode. Make child_ns_mode write-once so that a namespace manager
can set it once, check the value, and be guaranteed it won't change
before creating its namespaces. Writing a different value after the
first write returns -EBUSY.

One patch for the implementation, one for docs, and one for tests.

---
Changes in v3:
- Simplify code be reverting approach of using only a single variable to
  represent both lock state and child_ns_mode state. Instead, use
  child_ns_mode and child_ns_mode_locked.
- Update documentation to clarify the value-dependent behavior of
  child_ns_mode writes (that is, same value is ok, different value gets
  -EBUSY).
- fixed some line length > 80 checkpatch issues in vmtest.sh
- Link to v2: https://lore.kernel.org/r/20260218-vsock-ns-write-once-v2-0-19e4c50d509a@meta.com

Changes in v2:
- break docs, tests, and implementation into separate patches
- clarify commit message
- only use child_ns_mode, do not add additional child_ns_mode_locked
  variable
- add documentation to Documentation/
- Link to v1: https://lore.kernel.org/r/20260217-vsock-ns-write-once-v1-1-a1fb30f289a9@meta.com

---
Bobby Eshleman (3):
      selftests/vsock: change tests to respect write-once child ns mode
      vsock: lock down child_ns_mode as write-once
      vsock: document write-once behavior of the child_ns_mode sysctl

 Documentation/admin-guide/sysctl/net.rst |  3 +++
 include/net/af_vsock.h                   | 13 ++++++++--
 include/net/netns/vsock.h                |  3 +++
 net/vmw_vsock/af_vsock.c                 | 15 ++++++++----
 tools/testing/selftests/vsock/vmtest.sh  | 41 ++++++++++++++++----------------
 5 files changed, 48 insertions(+), 27 deletions(-)
---
base-commit: ccd8e87748ad083047d6c8544c5809b7f96cc8df
change-id: 20260217-vsock-ns-write-once-8834d684e0a2

Best regards,
-- 
Bobby Eshleman <bobbyeshleman@meta.com>