tools lib traceevent: Check string is really printable

When libtraceevent parses format fields, it assumes that array of 1 byte
is string but it's not always true. The kvm_emulate_insn contains 15 u8
array of insn that contains (binary) instructions. Thus when it's
printed, it'll have broken output like below:

kvm_emulate_insn: [FAILED TO PARSE] rip=3238197797 csbase=0 len=2 \
insn=<89>P^]<B4>& flags=5 failed=0

With this patch:

kvm_emulate_insn: [FAILED TO PARSE] rip=3238197797 csbase=0 len=2 \
insn=ARRAY[89, 10, 5d, c3, 8d, b4, 26, 00, 00, 00, 00, 55, 89, e5, 3e] flags=5 failed=0

Suggested-by: Steven Rostedt <[e-mail]>
Signed-off-by: Namhyung Kim <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/lib/traceevent/event-parse.c

···
 	trace_seq_printf(s, fmt, buf[0], buf[1], buf[2], buf[3], buf[4], buf[5]);
 }
 
+static int is_printable_array(char *p, unsigned int len)
+{
+	unsigned int i;
+
+	for (i = 0; i < len && p[i]; i++)
+		if (!isprint(p[i]))
+		    return 0;
+	return 1;
+}
+
 static void print_event_fields(struct trace_seq *s, void *data, int size,
 			       struct event_format *event)
 {
···
 				len = offset >> 16;
 				offset &= 0xffff;
 			}
-			if (field->flags & FIELD_IS_STRING) {
+			if (field->flags & FIELD_IS_STRING &&
+			    is_printable_array(data + offset, len)) {
 				trace_seq_printf(s, "%s", (char *)data + offset);
 			} else {
 				trace_seq_puts(s, "ARRAY[");
···
 							 *((unsigned char *)data + offset + i));
 				}
 				trace_seq_putc(s, ']');
+				field->flags &= ~FIELD_IS_STRING;
 			}
 		} else {
 			val = pevent_read_number(event->pevent, data + field->offset,