selftests/bpf: Add selftest for bpf_task_under_cgroup() in sleepable prog

The result is as follows:

  $ tools/testing/selftests/bpf/test_progs --name=task_under_cgroup
  #237     task_under_cgroup:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Without the previous patch, there will be RCU warnings in dmesg when
CONFIG_PROVE_RCU is enabled. While with the previous patch, there will
be no warnings.
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20231007135945.4306-2-laoar.shao@gmail.com

tools/testing/selftests/bpf/prog_tests/task_under_cgroup.c

@@ -30,8 +30,15 @@ void test_task_under_cgroup(void)
 	if (!ASSERT_OK(ret, "test_task_under_cgroup__load"))
 		goto cleanup;
 
-	ret = test_task_under_cgroup__attach(skel);
-	if (!ASSERT_OK(ret, "test_task_under_cgroup__attach"))
+	/* First, attach the LSM program, and then it will be triggered when the
+	 * TP_BTF program is attached.
+	 */
+	skel->links.lsm_run = bpf_program__attach_lsm(skel->progs.lsm_run);
+	if (!ASSERT_OK_PTR(skel->links.lsm_run, "attach_lsm"))
+		goto cleanup;
+
+	skel->links.tp_btf_run = bpf_program__attach_trace(skel->progs.tp_btf_run);
+	if (!ASSERT_OK_PTR(skel->links.tp_btf_run, "attach_tp_btf"))
 		goto cleanup;
 
 	pid = fork();

tools/testing/selftests/bpf/progs/test_task_under_cgroup.c

@@ -18,7 +18,7 @@ const volatile __u64 cgid;
 int remote_pid;
 
 SEC("tp_btf/task_newtask")
-int BPF_PROG(handle__task_newtask, struct task_struct *task, u64 clone_flags)
+int BPF_PROG(tp_btf_run, struct task_struct *task, u64 clone_flags)
 {
 	struct cgroup *cgrp = NULL;
 	struct task_struct *acquired;
@@ -48,4 +48,30 @@ int BPF_PROG(handle__task_newtask, struct task_struct *task, u64 clone_flags)
 	return 0;
 }
 
+SEC("lsm.s/bpf")
+int BPF_PROG(lsm_run, int cmd, union bpf_attr *attr, unsigned int size)
+{
+	struct cgroup *cgrp = NULL;
+	struct task_struct *task;
+	int ret = 0;
+
+	task = bpf_get_current_task_btf();
+	if (local_pid != task->pid)
+		return 0;
+
+	if (cmd != BPF_LINK_CREATE)
+		return 0;
+
+	/* 1 is the root cgroup */
+	cgrp = bpf_cgroup_from_id(1);
+	if (!cgrp)
+		goto out;
+	if (!bpf_task_under_cgroup(task, cgrp))
+		ret = -1;
+	bpf_cgroup_release(cgrp);
+
+out:
+	return ret;
+}
+
 char _license[] SEC("license") = "GPL";