opensnoop to trace openat

tools/opensnoop.bt

@@ -21,15 +21,22 @@ BEGIN
 	printf("%-6s %-16s %4s %3s %s\n", "PID", "COMM", "FD", "ERR", "PATH");
 }
 
+// workaround for #132: the next two code blocks can't be combined yet
 tracepoint:syscalls:sys_enter_open
 {
 	@filename[tid] = args->filename;
 }
 
-tracepoint:syscalls:sys_exit_open
+tracepoint:syscalls:sys_enter_openat
+{
+	@filename[tid] = args->filename;
+}
+
+tracepoint:syscalls:sys_exit_open,
+tracepoint:syscalls:sys_exit_openat
 /@filename[tid]/
 {
-	$ret = args->ret;
+	$ret = *(ctx + 16);		// workaround for #132
 	$fd = $ret > 0 ? $ret : -1;
 	$errno = $ret > 0 ? 0 : - $ret;