[V2,bpf-next,1/2] bpf: add bpf_task_get_cgroup kfunc

Commit Message

Jose Fernandez March 16, 2024, 4:22 p.m. UTC

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 <[e-mail]>
Reviewed-by: Tycho Andersen <[e-mail]>
---
V1 -> V2: Return a pointer to the cgroup instead of the cgroup ID

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


base-commit: 4c8644f86c854c214aaabbcc24a27fa4c7e6a951

Comments

Yonghong Song March 18, 2024, 3:54 p.m. UTC | #1

On 3/16/24 9:22 AM, Jose Fernandez wrote:
> 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 <[e-mail]>
> Reviewed-by: Tycho Andersen <[e-mail]>

Acked-by: Yonghong Song <[e-mail]>

Stanislav Fomichev March 18, 2024, 4:07 p.m. UTC | #2

On 03/16, Jose Fernandez wrote:
> 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 <[e-mail]>
> Reviewed-by: Tycho Andersen <[e-mail]>

Acked-by: Stanislav Fomichev <[e-mail]>

Patch

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)