This patch adds a selftest for the `bpf_task_get_cgroup` kfunc. The test focuses on the use case of obtaining the cgroup ID of the previous task in a `sched_switch` tracepoint.

The selftest involves creating a test cgroup, attaching a BPF program that utilizes the `bpf_task_get_cgroup` during a `sched_switch` tracepoint, and validating that the obtained cgroup ID for the previous task matches the expected cgroup ID.

Signed-off-by: Jose Fernandez josef@netflix.com Reviewed-by: Tycho Andersen tycho@tycho.pizza --- V2 -> v3: Use _open_and_load(), usleep helper, and drop map usage V1 -> V2: Refactor test to work with a cgroup pointer instead of the ID

.../bpf/prog_tests/task_get_cgroup.c | 43 +++++++++++++++++++ .../bpf/progs/test_task_get_cgroup.c | 32 ++++++++++++++ 2 files changed, 75 insertions(+) create mode 100644 tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c create mode 100644 tools/testing/selftests/bpf/progs/test_task_get_cgroup.c

diff --git a/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c b/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c new file mode 100644 index 000000000000..031623067e7e --- /dev/null +++ b/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c @@ -0,0 +1,43 @@ +// SPDX-License-Identifier: GPL-2.0 +// Copyright 2024 Netflix, Inc. + +#include <test_progs.h> +#include <cgroup_helpers.h> +#include "test_task_get_cgroup.skel.h" +#include <unistd.h> + +#define TEST_CGROUP "/test-task-get-cgroup/" + +void test_task_get_cgroup(void) +{ + struct test_task_get_cgroup *skel; + int err, fd; + __u64 expected_cgroup_id; + + fd = test__join_cgroup(TEST_CGROUP); + if (!ASSERT_OK(fd < 0, "test_join_cgroup_TEST_CGROUP")) + return; + + skel = test_task_get_cgroup__open_and_load(); + if (!ASSERT_OK_PTR(skel, "test_task_get_cgroup__open_and_load")) + goto cleanup; + + err = test_task_get_cgroup__attach(skel); + if (!ASSERT_OK(err, "test_task_get_cgroup__attach")) + goto cleanup; + + skel->bss->pid = getpid(); + expected_cgroup_id = get_cgroup_id(TEST_CGROUP); + if (!ASSERT_GT(expected_cgroup_id, 0, "get_cgroup_id")) + goto cleanup; + + /* Trigger nanosleep to enter the sched_switch tracepoint */ + /* The previous task should be this process */ + usleep(100); + + ASSERT_EQ(skel->bss->cgroup_id, expected_cgroup_id, "cgroup_id"); + +cleanup: + test_task_get_cgroup__destroy(skel); + close(fd); +} diff --git a/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c new file mode 100644 index 000000000000..30d4499c6bc5 --- /dev/null +++ b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c @@ -0,0 +1,32 @@ +// SPDX-License-Identifier: GPL-2.0 +// Copyright 2024 Netflix, Inc. + +#include "vmlinux.h" +#include <bpf/bpf_helpers.h> +#include <bpf/bpf_tracing.h> + +struct cgroup *bpf_task_get_cgroup(struct task_struct *task) __ksym; +void bpf_cgroup_release(struct cgroup *cgrp) __ksym; + +int pid = 0; +u64 cgroup_id = 0; + +SEC("tp_btf/sched_switch") +int BPF_PROG(sched_switch, bool preempt, struct task_struct *prev, + struct task_struct *next) +{ + struct cgroup *cgrp; + + if (prev->pid != pid) + return 0; + + cgrp = bpf_task_get_cgroup(prev); + if (cgrp == NULL) + return 0; + cgroup_id = cgrp->kn->id; + + bpf_cgroup_release(cgrp); + return 0; +} + +char _license[] SEC("license") = "GPL";

On Mon, Mar 18, 2024 at 10:04 PM Jose Fernandez josef@netflix.com 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 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;
  

+}

Tejun,

Does this patch look good to you?