We learnt previously that an initramfs in Linux is a “CPIO archive”, so I set out to write something that can read and write them, in order to learn more about the format. What I found was an interesting discrepancy between how GNU cpio and the Linux Kernel parse CPIO files. It seems pretty innocuous, but I thought I’d document it for posterity. The CPIO header CPIO is an exceedingly simple format.