selftests/bpf: Add selftests for nested_trust

Add selftests for nested_strust to check whehter PTR_UNTRUSTED is cleared
as expected, the result as follows:

 #141/1   nested_trust/test_read_cpumask:OK
 #141/2   nested_trust/test_skb_field:OK                    <<<<
 #141/3   nested_trust/test_invalid_nested_user_cpus:OK
 #141/4   nested_trust/test_invalid_nested_offset:OK
 #141/5   nested_trust/test_invalid_skb_field:OK            <<<<
 #141     nested_trust:OK

The #141/2 and #141/5 are newly added.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230713025642.27477-3-laoar.shao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -10,6 +10,13 @@
 
 char _license[] SEC("license") = "GPL";
 
+struct {
+	__uint(type, BPF_MAP_TYPE_SK_STORAGE);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+	__type(key, int);
+	__type(value, u64);
+} sk_storage_map SEC(".maps");
+
 /* Prototype for all of the program trace events below:
  *
  * TRACE_EVENT(task_newtask,
@@ -31,3 +38,12 @@ int BPF_PROG(test_invalid_nested_offset, struct task_struct *task, u64 clone_fla
 	bpf_cpumask_first_zero(&task->cpus_mask);
 	return 0;
 }
+
+/* Although R2 is of type sk_buff but sock_common is expected, we will hit untrusted ptr first. */
+SEC("tp_btf/tcp_probe")
+__failure __msg("R2 type=untrusted_ptr_ expected=ptr_, trusted_ptr_, rcu_ptr_")
+int BPF_PROG(test_invalid_skb_field, struct sock *sk, struct sk_buff *skb)
+{
+	bpf_sk_storage_get(&sk_storage_map, skb->next, 0, 0);
+	return 0;
+}

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

@@ -10,10 +10,25 @@
 
 char _license[] SEC("license") = "GPL";
 
+struct {
+	__uint(type, BPF_MAP_TYPE_SK_STORAGE);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+	__type(key, int);
+	__type(value, u64);
+} sk_storage_map SEC(".maps");
+
 SEC("tp_btf/task_newtask")
 __success
 int BPF_PROG(test_read_cpumask, struct task_struct *task, u64 clone_flags)
 {
 	bpf_cpumask_test_cpu(0, task->cpus_ptr);
 	return 0;
 }
+
+SEC("tp_btf/tcp_probe")
+__success
+int BPF_PROG(test_skb_field, struct sock *sk, struct sk_buff *skb)
+{
+	bpf_sk_storage_get(&sk_storage_map, skb->sk, 0, 0);
+	return 0;
+}