tools: bpftool: introduce --json and --pretty options

These two options can be used to ask for a JSON output (--j or -json),
and to make this JSON human-readable (-p or --pretty).

A json_writer object is created when JSON is required, and will be used
in follow-up commits to produce JSON output.

Note that --pretty implies --json.

Update for the manual pages and interactive help messages comes in a
later patch of the series.

Signed-off-by: Quentin Monnet <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/bpf/bpftool/main.c

···
 static int last_argc;
 static char **last_argv;
 static int (*last_do_help)(int argc, char **argv);
+json_writer_t *json_wtr;
+bool pretty_output;
+bool json_output;
 
 void usage(void)
 {
···
 int main(int argc, char **argv)
 {
 	static const struct option options[] = {
+		{ "json",	no_argument,	NULL,	'j' },
 		{ "help",	no_argument,	NULL,	'h' },
+		{ "pretty",	no_argument,	NULL,	'p' },
 		{ "version",	no_argument,	NULL,	'V' },
 		{ 0 }
 	};
-	int opt;
+	int opt, ret;
 
 	last_do_help = do_help;
+	pretty_output = false;
+	json_output = false;
 	bin_name = argv[0];
 
-	while ((opt = getopt_long(argc, argv, "Vh",
+	while ((opt = getopt_long(argc, argv, "Vhpj",
 				  options, NULL)) >= 0) {
 		switch (opt) {
 		case 'V':
 			return do_version(argc, argv);
 		case 'h':
 			return do_help(argc, argv);
+		case 'p':
+			pretty_output = true;
+			/* fall through */
+		case 'j':
+			json_output = true;
+			break;
 		default:
 			usage();
 		}
···
 	if (argc < 0)
 		usage();
 
+	if (json_output) {
+		json_wtr = jsonw_new(stdout);
+		if (!json_wtr) {
+			err("failed to create JSON writer\n");
+			return -1;
+		}
+		jsonw_pretty(json_wtr, pretty_output);
+	}
+
 	bfd_init();
 
-	return cmd_select(cmds, argc, argv, do_help);
+	ret = cmd_select(cmds, argc, argv, do_help);
+
+	if (json_output)
+		jsonw_destroy(&json_wtr);
+
+	return ret;
 }

tools/bpf/bpftool/main.h

···
 #include <linux/bpf.h>
 #include <linux/kernel.h>
 
+#include "json_writer.h"
+
 #define err(msg...)	fprintf(stderr, "Error: " msg)
 #define warn(msg...)	fprintf(stderr, "Warning: " msg)
 #define info(msg...)	fprintf(stderr, msg)
···
 };
 
 extern const char *bin_name;
+
+extern json_writer_t *json_wtr;
+extern bool json_output;
 
 bool is_prefix(const char *pfx, const char *str);
 void fprint_hex(FILE *f, void *arg, unsigned int n, const char *sep);