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

[Jose Fernandez <josef@netflix.com>]

This patch enhances the BPF helpers by adding a kfunc to retrieve the
cgroup v2 ID of a specific 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 previous and next tasks.

The bpf_task_get_cgroup_id kfunc returns a task's cgroup ID, correctly
implementing RCU read locking and unlocking for safe data access, and
leverages existing cgroup.h helpers to fetch the cgroup and its ID.

Signed-off-by: Jose Fernandez <josef@netflix.com>
Reviewed-by: Tycho Andersen <tycho@tycho.pizza>
---
 kernel/bpf/helpers.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index a89587859571..8038b2bd3488 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2266,6 +2266,27 @@ bpf_task_get_cgroup1(struct task_struct *task, int hierarchy_id)
 		return NULL;
 	return cgrp;
 }
+
+/**
+ * bpf_task_get_cgroup_id - Get the cgroup ID of a task.
+ * @task: The target task
+ *
+ * This function returns the ID of 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.
+ */
+__bpf_kfunc u64 bpf_task_get_cgroup_id(struct task_struct *task)
+{
+	struct cgroup *cgrp;
+	u64 cgrp_id;
+
+	rcu_read_lock();
+	cgrp = task_dfl_cgroup(task);
+	cgrp_id = cgroup_id(cgrp);
+	rcu_read_unlock();
+	return cgrp_id;
+}
 #endif /* CONFIG_CGROUPS */
 
 /**
@@ -2573,6 +2594,7 @@ 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_id, KF_RCU)
 #endif
 BTF_ID_FLAGS(func, bpf_task_from_pid, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_throw)
-- 
2.40.1