Trace Linux Boot on Meta Hosts

A production Linux server in Meta’s fleet is failing before it reaches the login prompt. As a DevOps engineer, you need to explain the Linux boot process clearly enough to diagnose where boot is breaking and what evidence to inspect at each stage.

Describe the boot sequence from power-on to the login prompt on a modern Linux system. Cover both the normal control flow and the security-relevant checks that happen along the way.

Requirements

1. Start at BIOS or UEFI firmware and explain what the firmware initializes and how it selects a boot device.
2. Explain the role of the bootloader (for example, GRUB), including how the kernel and initramfs are located and loaded.
3. Describe what the Linux kernel does immediately after control is transferred to it, including memory setup, hardware discovery, driver initialization, and mounting the initial root filesystem.
4. Explain the purpose of initramfs and when it is required.
5. Describe how the system transitions from the temporary root filesystem to the real root filesystem.
6. Explain how PID 1 starts (typically systemd), how services are brought up, and how the system reaches a console or graphical login prompt.
7. Call out at least three common failure points and how you would investigate them using concrete artifacts such as firmware settings, GRUB config, kernel command line, dmesg, emergency shell access, or journalctl.
8. Include security considerations such as Secure Boot, bootloader tampering risk, kernel parameter abuse, and protecting single-user or recovery paths.

Example framing

A strong answer should distinguish firmware, bootloader, kernel, initramfs, root filesystem handoff, and userspace initialization, while showing how to debug a host that hangs at any one of those stages.