[RFC PATCH bpf-next 2/3] bpf: add bpf_task_freeze_cgroup() to freeze the cgroup of a task

[Djalal Harouni]

This patch adds a new bpf helper bpf_task_freeze_cgroup() to freeze a
cgroup of a task and all its descendant cgroups. It requires the task
to be on the default cgroup v2 hierarchy.

For some cases we want to freeze the cgroup of a task based on some
signals, doing so from bpf is better than user space which could be
too late.

Planned users of this feature are: tetragon and systemd when freezing
a cgroup hierarchy that could be a K8s pod, container, system service
or a user session.

This helper will acquire the cgroup_mutex during its operation and
release it before it returns.

Signed-off-by: Djalal Harouni <tixxdz@xxxxxxxxx>
---
 kernel/bpf/helpers.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 9234174ccb21..8d510a1b265c 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2270,6 +2270,36 @@ bpf_task_get_cgroup1(struct task_struct *task, int hierarchy_id)
 		return NULL;
 	return cgrp;
 }
+
+/**
+ * bpf_task_freeze_cgroup - Freeze the task cgroup and all its descendant cgroups.
+ *
+ * @task: The target task
+ * @freeze: freeze state, passing value 1 causes the freezing of the cgroup
+ * and all descendant cgroups. Processes under this cgroup hierarchy will
+ * be stopped and will not run until the cgroup is explicitly unfrozen.
+ * Passing value 0 unthaws the task cgroup and its descendant cgroups.
+ *
+ * Return: %0 on success or a negative errno code on failure.
+ */
+__bpf_kfunc int bpf_task_freeze_cgroup(struct task_struct *task, int freeze)
+{
+	int ret;
+	struct cgroup *cgrp;
+
+	rcu_read_lock();
+	cgrp = task_dfl_cgroup(task);
+	if (!cgrp || !cgroup_tryget(cgrp)) {
+		rcu_read_unlock();
+		return -ENOENT;
+	}
+	rcu_read_unlock();
+
+	ret = cgroup_freeze_no_kn(cgrp, freeze);
+	cgroup_put(cgrp);
+
+	return ret;
+}
 #endif /* CONFIG_CGROUPS */
 
 /**
@@ -2577,6 +2607,7 @@ BTF_ID_FLAGS(func, bpf_cgroup_ancestor, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cgroup_from_id, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_task_under_cgroup, KF_RCU)
 BTF_ID_FLAGS(func, bpf_task_get_cgroup1, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_task_freeze_cgroup, KF_TRUSTED_ARGS | KF_SLEEPABLE)
 #endif
 BTF_ID_FLAGS(func, bpf_task_from_pid, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_throw)
-- 
2.34.1