Vmlinuz

File Format
Name	vmlinuz
Ontology	Electronic File Formats Executables vmlinuz
Extension(s)	(none)
Compression	lossless

vmlinuz is a file comprising of Executable and Linkable Format (ELF) and Common Object File Format (COFF), that contains a pre-compiled, (typically) compressed Linux kernel image, and is required to boot into a Linux environment. While the filename may vary across Linux distributions, e.g. vmlinuz-6.15.9-201.fc42.x86_64 or vmlinuz-linux the filename generally starts with vmlinuz.

This file is also commonly paired with initramfs, residing in /boot directory, enables loading of extra kernel objects (.ko) along with any other added data that is necessary to;

• mount the filesystem, for example provide OpenZFS on deployments that uses OpenZFS filesystem as its rootfs,
• Early load files, such as AMD/Intel CPU microcodes,
• Provide a rescue environment if in case there is an issue while trying to initialize pivot_root.

Without vmlinuz, the system may fail booting at the bootloader stage, typically instructing the user the stated file is not found.

Discussion

There are no agreed amount of vmlinuz and initramfs to have inside any Linux installation. Fedora Linux retains a maximum of three, any older versions are replaced with newer versions when the system updates includes newer kernel in distro repository. Arch Linux, on the other hand defaults to one, which will automatically replace the contents as soon as linux package is located during system upgrade.

"vmlinux" as opposed to "vmlinuz, are considered to be uncompressed and unbootable variant of vmlinuz. While stream compression typically used are either:

• gunzip or gzip,
• bzip2,
• LZ4,
• lzop,
• LZMA,
• XZ,
• Zstandard

The claim that vmlinux being "unbootable" is disputed, however, it is possible to boot uncompressed kernel image. These are possibly isolated to specific environments where vmlinuz is explicitly not used.

Identification

A x86 vmlinuz contains 48 64 72 53 at offset 0x202, which in ASCII format is the string HdrS.

Software

• Generation:
  • make bzImage
• Other:
  • file - Used to identify the type of file by its magic bytes
  • extract-vmlinux - From Linus Torvald's Linux kernel repository. Used to extract vmlinuz
  • GNU Binutils → objdump - For examining the file
  • GNU Binutils → readelf - For examining the ELF portion of the file

Links

• vmlinux page on Wikipedia
• How to extract and disassemble vmlinuz file - Trend Oceans