selftests/bpf: Add selftest for PTR_UNTRUSTED

Add a new selftest to check the PTR_UNTRUSTED condition. Below is the
result,

 #160     ptr_untrusted:OK
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230713025642.27477-5-laoar.shao@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2023 Yafang Shao <laoar.shao@gmail.com> */
+
+#include <string.h>
+#include <linux/bpf.h>
+#include <test_progs.h>
+#include "test_ptr_untrusted.skel.h"
+
+#define TP_NAME "sched_switch"
+
+void serial_test_ptr_untrusted(void)
+{
+	struct test_ptr_untrusted *skel;
+	int err;
+
+	skel = test_ptr_untrusted__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open"))
+		goto cleanup;
+
+	/* First, attach lsm prog */
+	skel->links.lsm_run = bpf_program__attach_lsm(skel->progs.lsm_run);
+	if (!ASSERT_OK_PTR(skel->links.lsm_run, "lsm_attach"))
+		goto cleanup;
+
+	/* Second, attach raw_tp prog. The lsm prog will be triggered. */
+	skel->links.raw_tp_run = bpf_program__attach_raw_tracepoint(skel->progs.raw_tp_run,
+								    TP_NAME);
+	if (!ASSERT_OK_PTR(skel->links.raw_tp_run, "raw_tp_attach"))
+		goto cleanup;
+
+	err = strncmp(skel->bss->tp_name, TP_NAME, strlen(TP_NAME));
+	ASSERT_EQ(err, 0, "cmp_tp_name");
+
+cleanup:
+	test_ptr_untrusted__destroy(skel);
+}

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2023 Yafang Shao <laoar.shao@gmail.com> */
+
+#include "vmlinux.h"
+#include <bpf/bpf_tracing.h>
+
+char tp_name[128];
+
+SEC("lsm/bpf")
+int BPF_PROG(lsm_run, int cmd, union bpf_attr *attr, unsigned int size)
+{
+	switch (cmd) {
+	case BPF_RAW_TRACEPOINT_OPEN:
+		bpf_probe_read_user_str(tp_name, sizeof(tp_name) - 1,
+					(void *)attr->raw_tracepoint.name);
+		break;
+	default:
+		break;
+	}
+	return 0;
+}
+
+SEC("raw_tracepoint")
+int BPF_PROG(raw_tp_run)
+{
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";