BTF-playground: Add btf_module_read.c example

This is based on kernel selftests/bpf:

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

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

BTF-playground/btf_module_read.c

@@ -0,0 +1,83 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+#include <bpf/libbpf.h>
+#include <stdlib.h>
+#include <errno.h>
+
+#include <stdio.h>
+
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <getopt.h>
+
+#include <bpf/btf.h> /* Notice libbpf BTF include */
+
+#include <linux/err.h>
+
+static const char *module_name = "tun";
+static const char *symbol_name = "tun_struct";
+
+static const struct option long_options[] = {
+	{ "debug",	no_argument,	NULL,	'd' },
+	{ 0, 0, NULL, 0 }
+};
+
+int print_all_levels(enum libbpf_print_level level,
+		     const char *format, va_list args)
+{
+	return vfprintf(stderr, format, args);
+}
+
+#define pr_err(fmt, ...) \
+	fprintf(stderr, "%s:%d - " fmt, __FILE__, __LINE__, ##__VA_ARGS__)
+
+
+int main(int argc, char **argv)
+{
+	struct btf *vmlinux_btf, *module_btf;
+	int opt, longindex = 0;
+        __s32 type_id;
+	int err = 0;
+
+	/* Parse commands line args */
+	while ((opt = getopt_long(argc, argv, "d",
+				  long_options, &longindex)) != -1) {
+		switch (opt) {
+		case 'd':
+			libbpf_set_print(print_all_levels);
+			// verifier_logs = true;
+			break;
+		default:
+			pr_err("Unrecognized option '%s'\n", argv[optind - 1]);
+			return EXIT_FAILURE;
+		}
+	}
+	argc -= optind;
+	argv += optind;
+
+	vmlinux_btf = btf__load_vmlinux_btf();
+	err = libbpf_get_error(vmlinux_btf);
+	if (err) {
+		pr_err("ERROR(%d): btf__load_vmlinux_btf()\n", err);
+		goto out;
+	}
+
+	module_btf = btf__load_module_btf(module_name, vmlinux_btf);
+	err = libbpf_get_error(module_btf);
+	if (err) {
+		pr_err("ERROR(%d): btf__load_module_btf() module_name:%s\n",
+		       err, module_name);
+		goto out;
+	}
+
+        type_id = btf__find_by_name(module_btf, symbol_name);
+
+	printf("Module:%s Symbol:%s have BTF id:%d\n",
+	       module_name, symbol_name, type_id);
+
+out:
+	if (err)
+		return EXIT_FAILURE;
+	return EXIT_SUCCESS;
+}