ktrace-CO-RE/ktrace01: Add --debug option to userspace loader

Use this --debug to better understand what libbpf does of
CO-RE relocation tricks and lookup of BTF-IDs.

Example output:

 [...]
 libbpf: loading kernel BTF '/sys/kernel/btf/vmlinux': 0
 libbpf: map 'ktrace01.data': created successfully, fd=4
 libbpf: sec 'kprobe/udp_send_skb': found 4 CO-RE relocations
 libbpf: prog 'udp_send_skb': relo #0: kind <byte_off> (0), spec is [2] struct pt_regs.di (0:14 @ offset 112)
 libbpf: CO-RE relocating [0] struct pt_regs: found target candidate [176] struct pt_regs in [vmlinux]
 libbpf: prog 'udp_send_skb': relo #0: matching candidate #0 [176] struct pt_regs.di (0:14 @ offset 112)
 libbpf: prog 'udp_send_skb': relo #0: patched insn #0 (LDX/ST/STX) off 112 -> 112
 libbpf: prog 'udp_send_skb': relo #1: kind <byte_off> (0), spec is [7] struct sk_buff___local.hash (0:1 @ offset 4)
 libbpf: CO-RE relocating [0] struct sk_buff___local: found target candidate [2965] struct sk_buff in [vmlinux]
 [...]

Notice how the BTF-ID of 'struct sk_buff' is resolved to be 2965.

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

ktrace-CO-RE/ktrace01.c

@@ -10,6 +10,7 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <getopt.h>
 
 #define pr_err(fmt, ...) \
 	fprintf(stderr, "%s:%d - " fmt, __FILE__, __LINE__, ##__VA_ARGS__)
@@ -36,21 +37,45 @@ void read_trace_pipe(void)
 	}
 }
 
+int print_all_levels(enum libbpf_print_level level,
+		     const char *format, va_list args)
+{
+	return vfprintf(stderr, format, args);
+}
+
+static const struct option long_options[] = {
+	{ "debug",	no_argument,	NULL,	'd' },
+	{ 0, 0, NULL, 0 }
+};
 
 int main(int argc, char **argv)
 {
 	struct bpf_object *obj = NULL;
 	struct bpf_link *link = NULL;
 	struct bpf_program *prog;
+	int opt, longindex = 0;
 	char filename[256];
-	//char pin_file[256];
-	// char *pin_file;
 	char buf[100];
 	int err;
-//	int c;
 
 	snprintf(filename, sizeof(filename), "%s_kern.o", argv[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;
+
 	obj = bpf_object__open_file(filename, NULL);
 	err = libbpf_get_error(obj);
 	if (err) {