[PATCH v5 0/4] pid_namespace: make init creation more flexible

Pavel Tikhomirov
2026-03-13T11:10:38Z

The first patch properly annotates accesses to ->child_reaper with
_ONCE macroses, to protect unlocked accesses from possible cpu/compiler
optimization problems.

The second patch makes sure that the init is always a first process in
the pid namespace, previously this was only checked for set_tid case.

The third patch allows to join pid namespace before pid namespace init
is created, that allows to create pid namespace by one process and then
create pid namespace init from another process after setns(). Please see
the detailed description in the patch commit message. It depends on the
second patch.

The forth and the final patch is a comprehansive test, that tests both
basic usecase of creating pid namespace and init separately, and a more
specific usecase which shows how we can improve clone3(set_tid)
usability after this change.

This change is generally useful as it makes clone3(set_tid) more
universal, and let's it work in all the cases evenly. Also it is highly
useful to CRIU to handle nested containers.

v2: Use *_ONCE for ->child_reaper accesses atomicity, and avoid taking
task_list lock for reading it. Rebase to master.
v3: Separate *_ONCE change and "init is first" checks into separate
commits.
v4: Update second patch commit message. Include Oleg's review tags.
v5: Handle one more READ_ONCE case. Include Andrei's review tags. Base
on top of mm tree.

This series is also available here:
https://github.com/Snorch/linux/commits/allow-creating-pid-namespace-init-after-setns-v5-mm-tree/

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

Pavel Tikhomirov (4):
pid_namespace: avoid optimization of accesses to ->child_reaper
pid: check init is created first after idr alloc
pid_namespace: allow opening pid_for_children before init was created
selftests: Add tests for creating pidns init via setns

kernel/exit.c | 3 +-
kernel/fork.c | 5 +-
kernel/pid.c | 19 +-
kernel/pid_namespace.c | 9 -
.../selftests/pid_namespace/.gitignore | 1 +
.../testing/selftests/pid_namespace/Makefile | 2 +-
.../pid_namespace/pidns_init_via_setns.c | 238 ++++++++++++++++++
7 files changed, 257 insertions(+), 20 deletions(-)
create mode 100644 tools/testing/selftests/pid_namespace/pidns_init_via_setns.c

base-commit: d1b4433308592363d133a788f0443d5d63872bdd
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2.53.0