perf report: Don't allow empty argument for '-t'.

Without this patch, perf report cause segfault if pass "" as '-t':

$ perf report -t ""

# To display the perf.data header info, please use --header/--header-only options.
#
# Samples: 37 of event 'syscalls:sys_enter_write'
# Event count (approx.): 37
#
# Children SelfCommand Shared Object Symbol
Segmentation fault

Since -t is used to add field-separator for generate table, -t "" is
actually meanless. This patch defines a new OPT_STRING_NOEMPTY() option
generator to ensure user never pass empty string to that option.

Signed-off-by: Wang Nan <[e-mail]>
Acked-by: Namhyung Kim <[e-mail]>
Cc: [e-mail]
Cc: Jiri Olsa <[e-mail]>
Cc: Zefan Li <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-report.c

···
 	OPT_STRING('w', "column-widths", &symbol_conf.col_width_list_str,
 		   "width[,width...]",
 		   "don't try to adjust column width, use these fixed values"),
-	OPT_STRING('t', "field-separator", &symbol_conf.field_sep, "separator",
+	OPT_STRING_NOEMPTY('t', "field-separator", &symbol_conf.field_sep, "separator",
 		   "separator for columns, no spaces will be added between "
 		   "columns '.' is reserved."),
 	OPT_BOOLEAN('U', "hide-unresolved", &report.hide_unresolved,

tools/perf/util/parse-options.c

···
 {
 	const char *s, *arg = NULL;
 	const int unset = flags & OPT_UNSET;
+	int err;
 
 	if (unset && p->opt)
 		return opterror(opt, "takes no value", flags);
···
 		return 0;
 
 	case OPTION_STRING:
+		err = 0;
 		if (unset)
 			*(const char **)opt->value = NULL;
 		else if (opt->flags & PARSE_OPT_OPTARG && !p->opt)
 			*(const char **)opt->value = (const char *)opt->defval;
 		else
-			return get_arg(p, opt, flags, (const char **)opt->value);
-		return 0;
+			err = get_arg(p, opt, flags, (const char **)opt->value);
+
+		/* PARSE_OPT_NOEMPTY: Allow NULL but disallow empty string. */
+		if (opt->flags & PARSE_OPT_NOEMPTY) {
+			const char *val = *(const char **)opt->value;
+
+			if (!val)
+				return err;
+
+			/* Similar to unset if we are given an empty string. */
+			if (val[0] == '\0') {
+				*(const char **)opt->value = NULL;
+				return 0;
+			}
+		}
+
+		return err;
 
 	case OPTION_CALLBACK:
 		if (unset)

tools/perf/util/parse-options.h

···
 	PARSE_OPT_LASTARG_DEFAULT = 16,
 	PARSE_OPT_DISABLED = 32,
 	PARSE_OPT_EXCLUSIVE = 64,
+	PARSE_OPT_NOEMPTY  = 128,
 };
 
 struct option;
···
 #define OPT_LONG(s, l, v, h)        { .type = OPTION_LONG, .short_name = (s), .long_name = (l), .value = check_vtype(v, long *), .help = (h) }
 #define OPT_U64(s, l, v, h)         { .type = OPTION_U64, .short_name = (s), .long_name = (l), .value = check_vtype(v, u64 *), .help = (h) }
 #define OPT_STRING(s, l, v, a, h)   { .type = OPTION_STRING,  .short_name = (s), .long_name = (l), .value = check_vtype(v, const char **), (a), .help = (h) }
+#define OPT_STRING_NOEMPTY(s, l, v, a, h)   { .type = OPTION_STRING,  .short_name = (s), .long_name = (l), .value = check_vtype(v, const char **), (a), .help = (h), .flags = PARSE_OPT_NOEMPTY}
 #define OPT_DATE(s, l, v, h) \
 	{ .type = OPTION_CALLBACK, .short_name = (s), .long_name = (l), .value = (v), .argh = "time", .help = (h), .callback = parse_opt_approxidate_cb }
 #define OPT_CALLBACK(s, l, v, a, h, f) \