extern int read_cow_header(int (*reader)(__u64, char *, int, void *),
void *arg, __u32 *version_out,
char **backing_file_out, time_t *mtime_out,
extern int read_cow_header(int (*reader)(__u64, char *, int, void *),
void *arg, __u32 *version_out,
char **backing_file_out, time_t *mtime_out,