[PATCH] zap_pid_ns_processes: don't send SIGKILL to sub-threads

Oleg Nesterov
2024-06-08T15:50:32Z

The comment above the idr_for_each_entry_continue() loop tries to explain
why we have to signal each thread in the namespace, but it is outdated.
This code no longer uses kill_proc_info(), we have a target task so we can
check thread_group_leader() and avoid the unnecessary group_send_sig_info.
Better yet, we can change pid_task() to use PIDTYPE_TGID rather than _PID,
this way it returns NULL if this pid is not a group-leader pid.

Also, change this code to check SIGNAL_GROUP_EXIT, the exiting process /
thread doesn't necessarily has a pending SIGKILL. Either way these checks
are racy without siglock, so the patch uses data_race() to shut up KCSAN.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/pid_namespace.c | 13 +++----------
1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 25f3cf679b35..0f9bd67c9e75 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -191,21 +191,14 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
* The last thread in the cgroup-init thread group is terminating.
* Find remaining pid_ts in the namespace, signal and wait for them
* to exit.
- *
- * Note: This signals each threads in the namespace - even those that
- * belong to the same thread group, To avoid this, we would have
- * to walk the entire tasklist looking a processes in this
- * namespace, but that could be unnecessarily expensive if the
- * pid namespace has just a few processes. Or we need to
- * maintain a tasklist for each pid namespace.
- *
*/
rcu_read_lock();
read_lock(&tasklist_lock);
nr = 2;
idr_for_each_entry_continue(&pid_ns->idr, pid, nr) {
- task = pid_task(pid, PIDTYPE_PID);
- if (task && !__fatal_signal_pending(task))
+ task = pid_task(pid, PIDTYPE_TGID);
+ /* reading signal->flags is racy without sighand->siglock */
+ if (task && !(data_race(task->signal->flags) & SIGNAL_GROUP_EXIT))
group_send_sig_info(SIGKILL, SEND_SIG_PRIV, task, PIDTYPE_MAX);
}
read_unlock(&tasklist_lock);
--
2.25.1.362.g51ebf55