[PATCH V2 bpf-next 2/2] selftests/bpf: add selftest for bpf_task_get_cgroup

[Jose Fernandez <josef@netflix.com>]

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>
---
V1 -> V2: Refactor test to work with a cgroup pointer instead of the ID

 .../bpf/prog_tests/task_get_cgroup.c          | 58 +++++++++++++++++++
 .../bpf/progs/test_task_get_cgroup.c          | 37 ++++++++++++
 2 files changed, 95 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..67ed65d0c461
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c
@@ -0,0 +1,58 @@
+// 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;
+	pid_t pid;
+	__u64 cgroup_id, expected_cgroup_id;
+	const struct timespec req = {
+		.tv_sec = 1,
+		.tv_nsec = 0,
+	};
+
+	fd = test__join_cgroup(TEST_CGROUP);
+	if (!ASSERT_OK(fd < 0, "test_join_cgroup_TEST_CGROUP"))
+		return;
+
+	skel = test_task_get_cgroup__open();
+	if (!ASSERT_OK_PTR(skel, "test_task_get_cgroup__open"))
+		goto cleanup;
+
+	err = test_task_get_cgroup__load(skel);
+	if (!ASSERT_OK(err, "test_task_get_cgroup__load"))
+		goto cleanup;
+
+	err = test_task_get_cgroup__attach(skel);
+	if (!ASSERT_OK(err, "test_task_get_cgroup__attach"))
+		goto cleanup;
+
+	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 */
+	syscall(__NR_nanosleep, &req, NULL);
+
+	err = bpf_map_lookup_elem(bpf_map__fd(skel->maps.pid_to_cgid_map), &pid,
+				  &cgroup_id);
+
+	if (!ASSERT_OK(err, "bpf_map_lookup_elem"))
+		goto cleanup;
+
+	ASSERT_EQ(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..580f8f0657d5
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c
@@ -0,0 +1,37 @@
+// 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;
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 4096);
+	__type(key, __u32);
+	__type(value, __u64);
+} pid_to_cgid_map SEC(".maps");
+
+SEC("tp_btf/sched_switch")
+int BPF_PROG(sched_switch, bool preempt, struct task_struct *prev,
+	     struct task_struct *next)
+{
+	struct cgroup *cgrp;
+	u64 cgroup_id;
+	u32 pid;
+
+	cgrp = bpf_task_get_cgroup(prev);
+	if (cgrp == NULL)
+		return 0;
+	cgroup_id = cgrp->kn->id;
+	pid = prev->pid;
+	bpf_map_update_elem(&pid_to_cgid_map, &pid, &cgroup_id, BPF_ANY);
+
+	bpf_cgroup_release(cgrp);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.40.1