tools lib traceevent: Fix get_field_str() for dynamic strings

If a field is a dynamic string, get_field_str() returned just the
offset/size value and not the string. Have it parse the offset/size
correctly to return the actual string. Otherwise filtering fails when
trying to filter fields that are dynamic strings.

Reported-by: Gopanapalli Pradeep <[e-mail]>
Signed-off-by: Steven Rostedt <[e-mail]>
Acked-by: Namhyung Kim <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/lib/traceevent/parse-filter.c

···
 	struct pevent *pevent;
 	unsigned long long addr;
 	const char *val = NULL;
+	unsigned int size;
 	char hex[64];
 
 	/* If the field is not a string convert it */
 	if (arg->str.field->flags & FIELD_IS_STRING) {
 		val = record->data + arg->str.field->offset;
+		size = arg->str.field->size;
+
+		if (arg->str.field->flags & FIELD_IS_DYNAMIC) {
+			addr = *(unsigned int *)val;
+			val = record->data + (addr & 0xffff);
+			size = addr >> 16;
+		}
 
 		/*
 		 * We need to copy the data since we can't be sure the field
 		 * is null terminated.
 		 */
-		if (*(val + arg->str.field->size - 1)) {
+		if (*(val + size - 1)) {
 			/* copy it */
 			memcpy(arg->str.buffer, val, arg->str.field->size);
 			/* the buffer is already NULL terminated */