149 lines
7.6 KiB
Markdown
149 lines
7.6 KiB
Markdown
---
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tags:
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- disks
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- operating-systems
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---
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# The boot process
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The primary or boot
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[partition](Disks.md#primary-extended-and-logical-partitions) of a harddisk
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contains a bootloader. It is the job of the bootloader to locate the
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[kernel](The_kernel.md) on the harddrive and inject it into memory so that they
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operating system can start. This is the boot process.
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## Boot loaders
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The boot loader loads the kernel into memory from the disk and then starts the
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kernel with a set of kernel parameters.
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Like the kernel itself, the boot loader requires a driver in order to access the
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disk but it can't use the same one as the kernel since at this point, the kernel
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is not yet loaded into memory. So it has its own special driver: this is either
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the BIOS or the UEFI firmware.
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A boot loader's core functionality includes the ability to do the following:
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- select from multiple kernels
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- switch between sets of kernel parameters
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- provide support for booting other operating systems
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### BIOS and UEFI
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BIOS and UEFI are both firmware that is installed directly on the motherboard of
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the computer. They are firmware because they are software that is permanent and
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programmed into read-only memory.
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In the context of disks, their most crucial role is locating the operating
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system on the harddisk and loading it into memory so that the bootstrapping
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process can begin. However they are also responsible for the computer clock and
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the management of peripherals.
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As we can see from the `fdisk` readout, the boot partition uses EFI, the storage
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partition associated with UEFI.
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```bash
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Device Start End Sectors Size Type
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/dev/nvme0n1p1 2048 1001471 999424 488M EFI System
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/dev/nvme0n1p2 1001472 59594751 58593280 27.9G Linux filesystem
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/dev/nvme0n1p3 59594752 1000214527 940619776 448.5G Linux filesystem
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```
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Whilst UEFI is installed on the hardware, most of its configuration is stored in
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the EFI partition on the disk, whereas with BIOS, everything is on the chip.
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This makes booting faster with UEFI.
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Even though most modern computers use UEFI, it may still be referred to as BIOS
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for user-continuity (for instance on WIndows). With Linux you have to explicitly
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create your boot process so the two are clearly distinguishable.
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### GRUB
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The de facto standard boot loader for Linux is GRUB: Grand Unified Boot Loader.
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You see the GRUB default menu when you first start a Linux machine. It will
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offer you various options for loading your installed OS or other OSs. GRUB is a
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[filesystem](./Filesystems.md) like the main disk. If you press `e` in this
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screen you can view and edit specific boot parameters. Pressing `c` gives you
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access to the GRUB command line interface. This allows you to interact with GRUB
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in the same way as you would with any other filesystem, allowing for advanced
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configuration.
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## The boot sequence
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1. The machine's BIOS or boot firmware (in the case of UEFI) loads and runs a
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boot loader. It finds the bootloader by looking for the part of the disk that
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contains either the MBR or GPT sector.
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2. The boot loader finds the kernel image on disk, loads it into memory and
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starts it.
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3. The kernel initializes the devices and its drivers.
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4. The kernel mounts the root filesystem.
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5. The kernel starts a program called **init**. It has a
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[process id](Processes.md#processes-ps) of 1. This is the point at which
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[user space](User_Space.md) starts.
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6. Init sets the rest of the system processes in motion.
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7. At the end of the boot process, init starts a process allowing you to log in.
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## Boot diagnostics
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Linux keeps a log of the boot process as part of the [journal](./journald.md)
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log of everything that happens on disk. We can use the command `journalct -k` to
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view the diagnostics generated during the boot. Here is a selection:
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```
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Jul 07 07:12:58 archbish kernel: Linux version 5.18.5-arch1-1 (linux@archlinux) (gcc (GCC) 12.1.0, GNU ld (GNU Binutils) 2.38) #1 SMP PREEMPT_DYNAMIC Thu, 16 Jun 2022 20:40:45 +0000
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Jul 07 07:12:58 archbish kernel: Command line: BOOT_IMAGE=/boot/vmlinuz-linux root=UUID=2ee6b834-0857-49dc-b8ba-a24d46d228ae rw loglevel=3 quiet
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Jul 07 07:12:58 archbish kernel: x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
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Jul 07 07:12:58 archbish kernel: x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
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Jul 07 07:12:58 archbish kernel: x86/fpu: Supporting XSAVE feature 0x020: 'AVX-512 opmask'
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Jul 07 07:12:58 archbish kernel: x86/fpu: Supporting XSAVE feature 0x040: 'AVX-512 Hi256'
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Jul 07 07:12:58 archbish kernel: x86/fpu: xstate_offset[2]: 576, xstate_sizes[2]: 256
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Jul 07 07:12:58 archbish kernel: x86/fpu: xstate_offset[5]: 832, xstate_sizes[5]: 64
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Jul 07 07:12:58 archbish kernel: x86/fpu: xstate_offset[6]: 896, xstate_sizes[6]: 512
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Jul 07 07:12:58 archbish kernel: x86/fpu: xstate_offset[7]: 1408, xstate_sizes[7]: 1024
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Jul 07 07:12:58 archbish kernel: x86/fpu: xstate_offset[9]: 2432, xstate_sizes[9]: 8
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Jul 07 07:12:58 archbish kernel: x86/fpu: Enabled xstate features 0x2e7, context size is 2440 bytes, using 'compacted' format.
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Jul 07 07:12:58 archbish kernel: signal: max sigframe size: 3632
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Jul 07 07:12:58 archbish kernel: BIOS-provided physical RAM map:
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Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x0000000000000000-0x000000000009efff] usable
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Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x000000000009f000-0x00000000000fffff] reserved
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Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x0000000000100000-0x000000008ef6efff] usable
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Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x000000008ef6f000-0x000000008f03efff] type 20
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Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x000000008f03f000-0x0000000093f2efff] reserved
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Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x0000000093f2f000-0x0000000094b2efff] ACPI NVS
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Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x0000000094b2f000-0x0000000094bfefff] ACPI data
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Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x0000000094bff000-0x0000000094bfffff] usable
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Jul 07 07:12:58 archbish kernel: NX (Execute Disable) protection: active
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Jul 07 07:12:58 archbish kernel: efi: EFI v2.70 by Lenovo
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Jul 07 07:12:58 archbish kernel: efi: ACPI=0x94bfe000 ACPI 2.0=0x94bfe014 SMBIOS=0x90cbd000 SMBIOS 3.0=0x90cb0000 TPMFinalLog=0x9498f000 MEMATTR=0x8aa30018 ESRT=0x8ab9c000
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Jul 07 07:12:58 archbish kernel: SMBIOS 3.2.0 present.
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Jul 07 07:12:58 archbish kernel: DMI: LENOVO 20W4000NUK/20W4000NUK, BIOS N34ET40W (1.40 ) 06/25/2021
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Jul 07 07:12:58 archbish kernel: tsc: Detected 2400.000 MHz processor
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Jul 07 07:12:58 archbish kernel: tsc: Detected 2419.200 MHz TSC
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Jul 07 07:12:58 archbish kernel: e820: update [mem 0x00000000-0x00000fff] usable ==> reserved
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Jul 07 07:12:58 archbish kernel: e820: remove [mem 0x000a0000-0x000fffff] usable
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Jul 07 07:12:58 archbish kernel: last_pfn = 0x45f800 max_arch_pfn = 0x400000000
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Jul 07 07:12:58 archbish kernel: x86/PAT: Configuration [0-7]: WB WC UC- UC WB WP UC- WT
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Jul 07 07:12:58 archbish kernel: last_pfn = 0x94c00 max_arch_pfn = 0x400000000
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Jul 07 07:12:58 archbish kernel: esrt: Reserving ESRT space from 0x000000008ab9c000 to 0x000000008ab9c150.
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Jul 07 07:12:58 archbish kernel: e820: update [mem 0x8ab9c000-0x8ab9cfff] usable ==> reserved
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```
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We can see that the first thing that is registered is the OS image that contains
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the kernel. From this point on, the kernel is active and doing its work.
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## Initial kernel actions
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The diagnostics above are a running commentary provided by the kernel itself
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about its actions. The details will be specific to the machine in question but
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the general sequence is as follows for all operating systems:
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1. CPU inspection
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2. Memory inspection
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3. Device bus discovery
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4. Device discovery
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5. Auxiliary kernel subsystem setup (networks etc)
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6. Mounting of the root filesystem
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7. Starting of user space
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