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 --- 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: 4c8644f86c854c214aaabbcc24a27fa4c7e6a951