[PATCH V3 bpf-next 1/2] bpf: add bpf_task_get_cgroup kfunc

[Jose Fernandez <josef@netflix.com>]

This patch enhances the BPF helpers by adding a kfunc to retrieve the
cgroup v2 of a task, addressing a previous limitation where only
bpf_task_get_cgroup1 was available for cgroup v1. The new kfunc is
particularly useful for scenarios where obtaining the cgroup ID of a
task other than the "current" one is necessary, which the existing
bpf_get_current_cgroup_id helper cannot accommodate. A specific use
case at Netflix involved the sched_switch tracepoint, where we had to
get the cgroup IDs of both the prev and next tasks.

The bpf_task_get_cgroup kfunc acquires and returns a reference to a
task's default cgroup, ensuring thread-safe access by correctly
implementing RCU read locking and unlocking. It leverages the existing
cgroup.h helper, and cgroup_tryget to safely acquire a reference to it.

Signed-off-by: Jose Fernandez <josef@netflix.com>
Reviewed-by: Tycho Andersen <tycho@tycho.pizza>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Stanislav Fomichev <sdf@google.com>
---
V2 -> V3: No changes
V1 -> V2: Return a pointer to the cgroup instead of the cgroup ID

 kernel/bpf/helpers.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index a89587859571..bbd19d5eedb6 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2266,6 +2266,31 @@ bpf_task_get_cgroup1(struct task_struct *task, int hierarchy_id)
 		return NULL;
 	return cgrp;
 }
+
+/**
+ * bpf_task_get_cgroup - Acquire a reference to the default cgroup of a task.
+ * @task: The target task
+ *
+ * This function returns the task's default cgroup, primarily
+ * designed for use with cgroup v2. In cgroup v1, the concept of default
+ * cgroup varies by subsystem, and while this function will work with
+ * cgroup v1, it's recommended to use bpf_task_get_cgroup1 instead.
+ * A cgroup returned by this kfunc which is not subsequently stored in a
+ * map, must be released by calling bpf_cgroup_release().
+ *
+ * Return: On success, the cgroup is returned. On failure, NULL is returned.
+ */
+__bpf_kfunc struct cgroup *bpf_task_get_cgroup(struct task_struct *task)
+{
+	struct cgroup *cgrp;
+
+	rcu_read_lock();
+	cgrp = task_dfl_cgroup(task);
+	if (!cgroup_tryget(cgrp))
+		cgrp = NULL;
+	rcu_read_unlock();
+	return cgrp;
+}
 #endif /* CONFIG_CGROUPS */
 
 /**
@@ -2573,6 +2598,7 @@ 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_get_cgroup, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 #endif
 BTF_ID_FLAGS(func, bpf_task_from_pid, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_throw)

base-commit: c733239f8f530872a1f80d8c45dcafbaff368737
-- 
2.40.1