2024-10-19 11:00:03 +01:00
|
|
|
---
|
|
|
|
|
tags:
|
|
|
|
|
- disks
|
|
|
|
|
- operating-systems
|
|
|
|
|
---
|
|
|
|
|
|
2024-11-04 14:26:42 +00:00
|
|
|
# Boot process
|
2024-10-19 11:00:03 +01:00
|
|
|
|
|
|
|
|
The primary or boot
|
|
|
|
|
[partition](Disks.md#primary-extended-and-logical-partitions) of a harddisk
|
|
|
|
|
contains a bootloader. It is the job of the bootloader to locate the
|
|
|
|
|
[kernel](The_kernel.md) on the harddrive and inject it into memory so that they
|
|
|
|
|
operating system can start. This is the boot process.
|
|
|
|
|
|
|
|
|
|
## Boot loaders
|
|
|
|
|
|
|
|
|
|
The boot loader loads the kernel into memory from the disk and then starts the
|
|
|
|
|
kernel with a set of kernel parameters.
|
|
|
|
|
|
|
|
|
|
Like the kernel itself, the boot loader requires a driver in order to access the
|
|
|
|
|
disk but it can't use the same one as the kernel since at this point, the kernel
|
|
|
|
|
is not yet loaded into memory. So it has its own special driver: this is either
|
|
|
|
|
the BIOS or the UEFI firmware.
|
|
|
|
|
|
|
|
|
|
A boot loader's core functionality includes the ability to do the following:
|
|
|
|
|
|
|
|
|
|
- select from multiple kernels
|
|
|
|
|
- switch between sets of kernel parameters
|
|
|
|
|
- provide support for booting other operating systems
|
|
|
|
|
|
|
|
|
|
### BIOS and UEFI
|
|
|
|
|
|
|
|
|
|
BIOS and UEFI are both firmware that is installed directly on the motherboard of
|
|
|
|
|
the computer. They are firmware because they are software that is permanent and
|
|
|
|
|
programmed into read-only memory.
|
|
|
|
|
|
|
|
|
|
In the context of disks, their most crucial role is locating the operating
|
|
|
|
|
system on the harddisk and loading it into memory so that the bootstrapping
|
|
|
|
|
process can begin. However they are also responsible for the computer clock and
|
|
|
|
|
the management of peripherals.
|
|
|
|
|
|
|
|
|
|
As we can see from the `fdisk` readout, the boot partition uses EFI, the storage
|
|
|
|
|
partition associated with UEFI.
|
|
|
|
|
|
|
|
|
|
```bash
|
|
|
|
|
Device Start End Sectors Size Type
|
|
|
|
|
/dev/nvme0n1p1 2048 1001471 999424 488M EFI System
|
|
|
|
|
/dev/nvme0n1p2 1001472 59594751 58593280 27.9G Linux filesystem
|
|
|
|
|
/dev/nvme0n1p3 59594752 1000214527 940619776 448.5G Linux filesystem
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
Whilst UEFI is installed on the hardware, most of its configuration is stored in
|
|
|
|
|
the EFI partition on the disk, whereas with BIOS, everything is on the chip.
|
|
|
|
|
This makes booting faster with UEFI.
|
|
|
|
|
|
|
|
|
|
Even though most modern computers use UEFI, it may still be referred to as BIOS
|
|
|
|
|
for user-continuity (for instance on WIndows). With Linux you have to explicitly
|
|
|
|
|
create your boot process so the two are clearly distinguishable.
|
|
|
|
|
|
|
|
|
|
### GRUB
|
|
|
|
|
|
|
|
|
|
The de facto standard boot loader for Linux is GRUB: Grand Unified Boot Loader.
|
|
|
|
|
|
2024-10-20 19:50:20 +01:00
|
|
|
|
2024-10-19 11:00:03 +01:00
|
|
|
|
|
|
|
|
You see the GRUB default menu when you first start a Linux machine. It will
|
|
|
|
|
offer you various options for loading your installed OS or other OSs. GRUB is a
|
|
|
|
|
[filesystem](./Filesystems.md) like the main disk. If you press `e` in this
|
|
|
|
|
screen you can view and edit specific boot parameters. Pressing `c` gives you
|
|
|
|
|
access to the GRUB command line interface. This allows you to interact with GRUB
|
|
|
|
|
in the same way as you would with any other filesystem, allowing for advanced
|
|
|
|
|
configuration.
|
|
|
|
|
|
|
|
|
|
## The boot sequence
|
|
|
|
|
|
|
|
|
|
1. The machine's BIOS or boot firmware (in the case of UEFI) loads and runs a
|
|
|
|
|
boot loader. It finds the bootloader by looking for the part of the disk that
|
|
|
|
|
contains either the MBR or GPT sector.
|
|
|
|
|
2. The boot loader finds the kernel image on disk, loads it into memory and
|
|
|
|
|
starts it.
|
|
|
|
|
3. The kernel initializes the devices and its drivers.
|
|
|
|
|
4. The kernel mounts the root filesystem.
|
|
|
|
|
5. The kernel starts a program called **init**. It has a
|
|
|
|
|
[process id](Processes.md#processes-ps) of 1. This is the point at which
|
|
|
|
|
[user space](User_Space.md) starts.
|
|
|
|
|
6. Init sets the rest of the system processes in motion.
|
|
|
|
|
7. At the end of the boot process, init starts a process allowing you to log in.
|
|
|
|
|
|
|
|
|
|
## Boot diagnostics
|
|
|
|
|
|
|
|
|
|
Linux keeps a log of the boot process as part of the [journal](./journald.md)
|
|
|
|
|
log of everything that happens on disk. We can use the command `journalct -k` to
|
|
|
|
|
view the diagnostics generated during the boot. Here is a selection:
|
|
|
|
|
|
|
|
|
|
```
|
|
|
|
|
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
|
|
|
|
|
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
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: x86/fpu: Supporting XSAVE feature 0x020: 'AVX-512 opmask'
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: x86/fpu: Supporting XSAVE feature 0x040: 'AVX-512 Hi256'
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: x86/fpu: xstate_offset[2]: 576, xstate_sizes[2]: 256
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: x86/fpu: xstate_offset[5]: 832, xstate_sizes[5]: 64
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: x86/fpu: xstate_offset[6]: 896, xstate_sizes[6]: 512
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: x86/fpu: xstate_offset[7]: 1408, xstate_sizes[7]: 1024
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: x86/fpu: xstate_offset[9]: 2432, xstate_sizes[9]: 8
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: x86/fpu: Enabled xstate features 0x2e7, context size is 2440 bytes, using 'compacted' format.
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: signal: max sigframe size: 3632
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: BIOS-provided physical RAM map:
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x0000000000000000-0x000000000009efff] usable
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x000000000009f000-0x00000000000fffff] reserved
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x0000000000100000-0x000000008ef6efff] usable
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x000000008ef6f000-0x000000008f03efff] type 20
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x000000008f03f000-0x0000000093f2efff] reserved
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x0000000093f2f000-0x0000000094b2efff] ACPI NVS
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x0000000094b2f000-0x0000000094bfefff] ACPI data
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: BIOS-e820: [mem 0x0000000094bff000-0x0000000094bfffff] usable
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: NX (Execute Disable) protection: active
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: efi: EFI v2.70 by Lenovo
|
|
|
|
|
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
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: SMBIOS 3.2.0 present.
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: DMI: LENOVO 20W4000NUK/20W4000NUK, BIOS N34ET40W (1.40 ) 06/25/2021
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: tsc: Detected 2400.000 MHz processor
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: tsc: Detected 2419.200 MHz TSC
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: e820: update [mem 0x00000000-0x00000fff] usable ==> reserved
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: e820: remove [mem 0x000a0000-0x000fffff] usable
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: last_pfn = 0x45f800 max_arch_pfn = 0x400000000
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: x86/PAT: Configuration [0-7]: WB WC UC- UC WB WP UC- WT
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: last_pfn = 0x94c00 max_arch_pfn = 0x400000000
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: esrt: Reserving ESRT space from 0x000000008ab9c000 to 0x000000008ab9c150.
|
|
|
|
|
Jul 07 07:12:58 archbish kernel: e820: update [mem 0x8ab9c000-0x8ab9cfff] usable ==> reserved
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
We can see that the first thing that is registered is the OS image that contains
|
|
|
|
|
the kernel. From this point on, the kernel is active and doing its work.
|
|
|
|
|
|
|
|
|
|
## Initial kernel actions
|
|
|
|
|
|
|
|
|
|
The diagnostics above are a running commentary provided by the kernel itself
|
|
|
|
|
about its actions. The details will be specific to the machine in question but
|
|
|
|
|
the general sequence is as follows for all operating systems:
|
|
|
|
|
|
|
|
|
|
1. CPU inspection
|
|
|
|
|
2. Memory inspection
|
|
|
|
|
3. Device bus discovery
|
|
|
|
|
4. Device discovery
|
|
|
|
|
5. Auxiliary kernel subsystem setup (networks etc)
|
|
|
|
|
6. Mounting of the root filesystem
|
|
|
|
|
7. Starting of user space
|
