Constant KERNEL_COMPANION_FILES 

pub(crate) const KERNEL_COMPANION_FILES: &[&str];

Files that distributions ship next to vmlinuz in /usr/lib/modules/<kver>/ which belong to that kernel binary, and so must travel with it.

• .vmlinuz.hmac: the kernel’s HMAC, which dracut’s fips module checks at boot in FIPS mode. Shipped by Fedora and RHEL derivatives’ kernel packages; ostree and Fedora’s grub2 kernel-install plugin install it next to the kernel as .vmlinuz-<kver>.hmac.

This is deliberately limited to companions of the kernel binary. Userspace metadata such as config, System.map or symvers.xz stays in the rootfs, where tools look for it. ostree also installs a devicetree file or dtb/ directory and an aboot.img from this directory, but those are separate boot inputs rather than companions of vmlinuz, so they are left alone.