Kernel booting process. Part 1. ================================================================================ From the bootloader to kernel -------------------------------------------------------------------------------- If you have read my previous [blog posts](http://0xax.blogspot.com/search/label/asm), you can see that sometime ago I started to get involved with low-level programming. I wrote some posts about x86_64 assembly programming for Linux. At the same time, I started to dive into the Linux source code. I have a great interest in understanding how low-level things work, how programs run on my computer, how they are located in memory, how the kernel manages processes and memory, how the network stack works on low-level and many many other things. So, I decided to write yet another series of posts about the Linux kernel for **x86_64**. Note that I'm not a professional kernel hacker and I don't write code for the kernel at work. It's just a hobby. I just like low-level stuff, and it is interesting for me to see how these things work. So if you notice anything confusing, or if you have any questions/remarks, ping me on twitter [0xAX](https://twitter.com/0xAX), drop me an [email](anotherworldofworld@gmail.com) or just create an [issue](https://github.com/0xAX/linux-insides/issues/new). I appreciate it. All posts will also be accessible at [linux-insides](https://github.com/0xAX/linux-insides) and if you find something wrong with my English or the post content, feel free to send a pull request. *Note that this isn't the official documentation, just learning and sharing knowledge.* **Required knowledge** * Understanding C code * Understanding assembly code (AT&T syntax) Anyway, if you just start to learn some tools, I will try to explain some parts during this and the following posts. Ok, simple introduction finishes and now we can start to dive into the kernel and low-level stuff. All code is actually for kernel - 3.18. If there are changes, I will update the posts accordingly. The Magic Power Button, What happens next? -------------------------------------------------------------------------------- Despite that this is a series of posts about the Linux kernel, we will not start from the kernel code (at least not in this paragraph). Ok, you press the magic power button on your laptop or desktop computer and it startes to work. After the motherboard sends a signal to the [power supply](https://en.wikipedia.org/wiki/Power_supply), the power supply provides the computer with the proper amount of electricity. Once the motherboard receives the [power good signal](https://en.wikipedia.org/wiki/Power_good_signal), it tries to start the CPU. The CPU resets all leftover data in its registers and sets up predefined values for each of them. [80386](https://en.wikipedia.org/wiki/Intel_80386) and later CPUs define the following predefined data in CPU registers after the computer resets: ``` IP 0xfff0 CS selector 0xf000 CS base 0xffff0000 ``` The processor starts working in [real mode](https://en.wikipedia.org/wiki/Real_mode). Let's back up a little to try and understand memory segmentation in this mode. Real mode is supported on all x86-compatible processors, from the [8086](https://en.wikipedia.org/wiki/Intel_8086) all the way to the modern Intel 64-bit CPUs. The 8086 processor has a 20-bit address bus, which means that it could work with 0-2^20 bytes address space (1 megabyte). But it only has 16-bit registers, and with 16-bit registers the maximum address is 2^16 or 0xffff (64 kilobytes). [Memory segmentation](http://en.wikipedia.org/wiki/Memory_segmentation) is used to make use of all the address space available. All memory is divided into small, fixed-size segments of 65535 bytes, or 64 KB. Since we cannot address memory above 64 KB with 16 bit registers, an alternate method is devised. An address consists of two parts: the beginning address of the segment and an offset from this address. To get a physical address in memory, we need to multiply the segment part by 16 and add the offset part: ``` PhysicalAddress = Segment * 16 + Offset ``` For example if `CS:IP` is `0x2000:0x0010`, the corresponding physical address will be: ```python >>> hex((0x2000 << 4) + 0x0010) '0x20010' ``` But if we take the largest segment part and offset: `0xffff:0xffff`, it will be: ```python >>> hex((0xffff << 4) + 0xffff) '0x10ffef' ``` which is 65519 bytes over first megabyte. Since only one megabyte is accessible in real mode, `0x10ffef` becomes `0x00ffef` with disabled [A20](https://en.wikipedia.org/wiki/A20_line). Ok, now we know about real mode and memory addressing. Let's get back to discuss about register values after reset: `CS` register consists of two parts: the visible segment selector and hidden base address. We know predefined `CS` base and `IP` value, so the logical address will be: ``` 0xffff0000:0xfff0 ``` The starting address is formed by adding the base address to the value in the EIP register: ```python >>> 0xffff0000 + 0xfff0 '0xfffffff0' ``` We get `0xfffffff0` which is 4GB - 16 bytes. This point is called the [Reset vector](http://en.wikipedia.org/wiki/Reset_vector). This is the memory location at which the CPU expects to find the first instruction to execute after reset. It contains a [jump](http://en.wikipedia.org/wiki/JMP_%28x86_instruction%29) instruction which usually points to the BIOS entry point. For example, if we look in the [coreboot](http://www.coreboot.org/) source code, we see: ```assembly .section ".reset" .code16 .globl reset_vector reset_vector: .byte 0xe9 .int _start - ( . + 2 ) ... ``` Here we can see the jmp instruction [opcode](http://ref.x86asm.net/coder32.html#xE9) - 0xe9 and its destination address - `_start - ( . + 2)`, and we can see that the `reset` section is 16 bytes and starts at `0xfffffff0`: ``` SECTIONS { _ROMTOP = 0xfffffff0; . = _ROMTOP; .reset . : { *(.reset) . = 15 ; BYTE(0x00); } } ``` Now the BIOS starts: after initializing and checking the hardware, it needs to find a bootable device. A boot order is stored in the BIOS configuration, controlling which devices the kernel attempts to boot from. When attempting to boot from a hard drive, the BIOS tries to find a boot sector. On hard drives partitioned with an MBR partition layout, the boot sector is stored in the first 446 bytes of the first sector (which is 512 bytes). The final two bytes of the first sector are `0x55` and `0xaa`, which signals the BIOS that this device is bootable. For example: ```assembly ; ; Note: this example is written in Intel Assembly syntax ; [BITS 16] [ORG 0x7c00] boot: mov al, '!' mov ah, 0x0e mov bh, 0x00 mov bl, 0x07 int 0x10 jmp $ times 510-($-$$) db 0 db 0x55 db 0xaa ``` Build and run it with: ``` nasm -f bin boot.nasm && qemu-system-x86_64 boot ``` This will instruct [QEMU](http://qemu.org) to use the `boot` binary we just built as a disk image. Since the binary generated by the assembly code above fulfills the requirements of the boot sector (the origin is set to `0x7c00`, and we end with the magic sequence), QEMU will treat the binary as the master boot record(MBR) of a disk image. You will see: ![Simple bootloader which prints only `!`](http://oi60.tinypic.com/2qbwup0.jpg) In this example we can see that the code will be executed in 16 bit real mode and will start at 0x7c00 in memory. After starting it calls the [0x10](http://www.ctyme.com/intr/rb-0106.htm) interrupt which just prints the `!` symbol. It fills the rest of the 510 bytes with zeros and finishes with the two magic bytes `0xaa` and `0x55`. You can see a binary dump of this with the `objdump` util: ``` nasm -f bin boot.nasm objdump -D -b binary -mi386 -Maddr16,data16,intel boot ``` A real-world boot sector has code to continue the boot process and the partition table instead of a bunch of 0's and an exclamation mark :) From this point onwards, BIOS hands over control to the bootloader. **NOTE**: As you can read above the CPU is in real mode. In real mode, calculating the physical address in memory is done as following: ``` PhysicalAddress = Segment * 16 + Offset ``` The same as mentioned before. We have only 16 bit general purpose registers, the maximum value of a 16 bit register is `0xffff`, so if we take the largest values, the result will be: ```python >>> hex((0xffff * 16) + 0xffff) '0x10ffef' ``` Where `0x10ffef` is equal to `1MB + 64KB - 16b`. But a [8086](https://en.wikipedia.org/wiki/Intel_8086) processor, which is the first processor with real mode, has a 20 bit address line and `2^20 = 1048576` is 1MB. This means the actual memory available is 1MB. General real mode's memory map is: ``` 0x00000000 - 0x000003FF - Real Mode Interrupt Vector Table 0x00000400 - 0x000004FF - BIOS Data Area 0x00000500 - 0x00007BFF - Unused 0x00007C00 - 0x00007DFF - Our Bootloader 0x00007E00 - 0x0009FFFF - Unused 0x000A0000 - 0x000BFFFF - Video RAM (VRAM) Memory 0x000B0000 - 0x000B7777 - Monochrome Video Memory 0x000B8000 - 0x000BFFFF - Color Video Memory 0x000C0000 - 0x000C7FFF - Video ROM BIOS 0x000C8000 - 0x000EFFFF - BIOS Shadow Area 0x000F0000 - 0x000FFFFF - System BIOS ``` In the beginning of this post I wrote that the first instruction executed by the CPU is located at address `0xFFFFFFF0`, which is much larger than `0xFFFFF` (1MB). How can the CPU access this in real mode? This is in the [coreboot](http://www.coreboot.org/Developer_Manual/Memory_map) documentation: ``` 0xFFFE_0000 - 0xFFFF_FFFF: 128 kilobyte ROM mapped into address space ``` At the start of execution, the BIOS is not in RAM, but in ROM. Bootloader -------------------------------------------------------------------------------- There are a number of bootloaders that can boot Linux, such as [GRUB 2](https://www.gnu.org/software/grub/) and [syslinux](http://www.syslinux.org/wiki/index.php/The_Syslinux_Project). The Linux kernel has a [Boot protocol](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt) which specifies the requirements for bootloaders to implement Linux support. This example will describe GRUB 2. Now that the BIOS has chosen a boot device and transferred control to the boot sector code, execution starts from [boot.img](http://git.savannah.gnu.org/gitweb/?p=grub.git;a=blob;f=grub-core/boot/i386/pc/boot.S;hb=HEAD). This code is very simple due to the limited amount of space available, and contains a pointer which is used to jump to the location of GRUB 2's core image. The core image begins with [diskboot.img](http://git.savannah.gnu.org/gitweb/?p=grub.git;a=blob;f=grub-core/boot/i386/pc/diskboot.S;hb=HEAD), which is usually stored immediately after the first sector in the unused space before the first partition. The above code loads the rest of the core image into memory, which contains GRUB 2's kernel and drivers for handling filesystems. After loading the rest of the core image, it executes [grub_main](http://git.savannah.gnu.org/gitweb/?p=grub.git;a=blob;f=grub-core/kern/main.c). `grub_main` initializes the console, gets the base address for modules, sets the root device, loads/parses the grub configuration file, loads modules etc. At the end of execution, `grub_main` moves grub to normal mode. `grub_normal_execute` (from `grub-core/normal/main.c`) completes the last preparation and shows a menu to select an operating system. When we select one of the grub menu entries, `grub_menu_execute_entry` runs, which executes the grub `boot` command, booting the selected operating system. As we can read in the kernel boot protocol, the bootloader must read and fill some fields of the kernel setup header, which starts at `0x01f1` offset from the kernel setup code. The kernel header [arch/x86/boot/header.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S) starts from: ```assembly .globl hdr hdr: setup_sects: .byte 0 root_flags: .word ROOT_RDONLY syssize: .long 0 ram_size: .word 0 vid_mode: .word SVGA_MODE root_dev: .word 0 boot_flag: .word 0xAA55 ``` The bootloader must fill this and the rest of the headers (only marked as `write` in the Linux boot protocol, for example [this](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt#L354)) with values which it either got from command line or calculated. We will not see a description and explanation of all fields of the kernel setup header, we will get back to that when the kernel uses them. You can find a description of all fields in the [boot protocol](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt#L156). As we can see in the kernel boot protocol, the memory map will be the following after loading the kernel: ```shell | Protected-mode kernel | 100000 +------------------------+ | I/O memory hole | 0A0000 +------------------------+ | Reserved for BIOS | Leave as much as possible unused ~ ~ | Command line | (Can also be below the X+10000 mark) X+10000 +------------------------+ | Stack/heap | For use by the kernel real-mode code. X+08000 +------------------------+ | Kernel setup | The kernel real-mode code. | Kernel boot sector | The kernel legacy boot sector. X +------------------------+ | Boot loader | <- Boot sector entry point 0x7C00 001000 +------------------------+ | Reserved for MBR/BIOS | 000800 +------------------------+ | Typically used by MBR | 000600 +------------------------+ | BIOS use only | 000000 +------------------------+ ``` So when the bootloader transfers control to the kernel, it starts at: ``` 0x1000 + X + sizeof(KernelBootSector) + 1 ``` where `X` is the address of the kernel bootsector loaded. In my case `X` is `0x10000`, as we can see in a memory dump: ![kernel first address](http://oi57.tinypic.com/16bkco2.jpg) The bootloader has now loaded the Linux kernel into memory, filled the header fields and jumped to it. Now we can move directly to the kernel setup code. Start of Kernel Setup -------------------------------------------------------------------------------- Finally we are in the kernel. Technically the kernel hasn't run yet, we need to set up the kernel, memory manager, process manager etc first. Kernel setup execution starts from [arch/x86/boot/header.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S) at [_start](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L293). It is a little strange at first sight, as there are several instructions before it. A Long time ago the Linux kernel had its own bootloader, but now if you run for example: ``` qemu-system-x86_64 vmlinuz-3.18-generic ``` You will see: ![Try vmlinuz in qemu](http://oi60.tinypic.com/r02xkz.jpg) Actually `header.S` starts from [MZ](https://en.wikipedia.org/wiki/DOS_MZ_executable) (see image above), error message printing and following [PE](https://en.wikipedia.org/wiki/Portable_Executable) header: ```assembly #ifdef CONFIG_EFI_STUB # "MZ", MS-DOS header .byte 0x4d .byte 0x5a #endif ... ... ... pe_header: .ascii "PE" .word 0 ``` It needs this to load an operating system with [UEFI](https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface). We won't see how this works right now, we'll see this in one of the next chapters. So the actual kernel setup entry point is: ``` // header.S line 292 .globl _start _start: ``` The bootloader (grub2 and others) knows about this point (`0x200` offset from `MZ`) and makes a jump directly to this point, despite the fact that `header.S` starts from `.bstext` section which prints an error message: ``` // // arch/x86/boot/setup.ld // . = 0; // current position .bstext : { *(.bstext) } // put .bstext section to position 0 .bsdata : { *(.bsdata) } ``` So the kernel setup entry point is: ```assembly .globl _start _start: .byte 0xeb .byte start_of_setup-1f 1: // // rest of the header // ``` Here we can see a `jmp` instruction opcode - `0xeb` to the `start_of_setup-1f` point. `Nf` notation means `2f` refers to the next local `2:` label. In our case it is label `1` which goes right after jump. It contains the rest of the setup [header](https://github.com/torvalds/linux/blob/master/Documentation/x86/boot.txt#L156). Right after the setup header we see the `.entrytext` section which starts at the `start_of_setup` label. Actually this is the first code that runs (aside from the previous jump instruction of course). After the kernel setup got the control from the bootloader, the first `jmp` instruction is located at `0x200` (first 512 bytes) offset from the start of the kernel real mode. This we can read in the Linux kernel boot protocol and also see in the grub2 source code: ```C state.gs = state.fs = state.es = state.ds = state.ss = segment; state.cs = segment + 0x20; ``` It means that segment registers will have following values after kernel setup starts: ``` gs = fs = es = ds = ss = 0x1000 cs = 0x1020 ``` in my case when the kernel is loaded at `0x10000`. After the jump to `start_of_setup`, it needs to do the following: * Be sure that all values of all segment registers are equal * Setup correct stack if needed * Setup [bss](https://en.wikipedia.org/wiki/.bss) * Jump to C code at [main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c) Let's look at the implementation. Segment registers align -------------------------------------------------------------------------------- First of all it ensures that `ds` and `es` segment registers point to the same address and disables interrupts with `cli` instruction: ```assembly movw %ds, %ax movw %ax, %es cli ``` As I wrote earlier, grub2 loads kernel setup code at address `0x10000` and `cs` at `0x1020` because execution doesn't start from the start of file, but from: ``` _start: .byte 0xeb .byte start_of_setup-1f ``` `jump`, which is at 512 bytes offset from the [4d 5a](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L47). It also needs to align `cs` from `0x10200` to `0x10000` as all other segment registers. After that we set up the stack: ```assembly pushw %ds pushw $6f lretw ``` push `ds` value to stack, and address of [6](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L494) label and execute `lretw` instruction. When we call `lretw`, it loads address of label `6` into the [instruction pointer](https://en.wikipedia.org/wiki/Program_counter) register and `cs` with value of `ds`. After this we will have `ds` and `cs` with the same values. Stack Setup -------------------------------------------------------------------------------- Actually, almost all of the setup code is preparation for the C language environment in real mode. The next [step](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L467) is checking of `ss` register value and make a correct stack if `ss` is wrong: ```assembly movw %ss, %dx cmpw %ax, %dx movw %sp, %dx je 2f ``` This can lead to 3 different scenarios: * `ss` has valid value 0x10000 (as all other segment registers beside `cs`) * `ss` is invalid and `CAN_USE_HEAP` flag is set (see below) * `ss` is invalid and `CAN_USE_HEAP` flag is not set (see below) Let's look at all three of these scenarios: 1. `ss` has a correct address (0x10000). In this case we go to label [2](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L481): ``` 2: andw $~3, %dx jnz 3f movw $0xfffc, %dx 3: movw %ax, %ss movzwl %dx, %esp sti ``` Here we can see aligning of `dx` (contains `sp` given by bootloader) to 4 bytes and checking whether it is zero. If it is zero, we put `0xfffc` (4 byte aligned address before maximum segment size - 64 KB) in `dx`. If it is not zero we continue to use `sp` given by the bootloader (0xf7f4 in my case). After this we put the `ax` value to `ss` which stores the correct segment address of `0x10000` and sets up a correct `sp`. We now have a correct stack: ![stack](http://oi58.tinypic.com/16iwcis.jpg) 2. In the second scenario, (`ss` != `ds`). First of all put the [_end](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L52) (address of end of setup code) value in `dx` and check the `loadflags` header field with the `testb` instruction too see whether we can use heap or not. [loadflags](https://github.com/torvalds/linux/blob/master/arch/x86/boot/header.S#L321) is a bitmask header which is defined as: ```C #define LOADED_HIGH (1<<0) #define QUIET_FLAG (1<<5) #define KEEP_SEGMENTS (1<<6) #define CAN_USE_HEAP (1<<7) ``` And as we can read in the boot protocol: ``` Field name: loadflags This field is a bitmask. Bit 7 (write): CAN_USE_HEAP Set this bit to 1 to indicate that the value entered in the heap_end_ptr is valid. If this field is clear, some setup code functionality will be disabled. ``` If the `CAN_USE_HEAP` bit is set, put `heap_end_ptr` in `dx` which points to `_end` and add `STACK_SIZE` (minimal stack size - 512 bytes) to it. After this if `dx` is not carry (it will not be carry, dx = _end + 512), jump to label `2` as in the previous case and make a correct stack. ![stack](http://oi62.tinypic.com/dr7b5w.jpg) 3. When `CAN_USE_HEAP` is not set, we just use a minimal stack from `_end` to `_end + STACK_SIZE`: ![minimal stack](http://oi60.tinypic.com/28w051y.jpg) BSS Setup -------------------------------------------------------------------------------- The last two steps that need to happen before we can jump to the main C code, are setting up the [BSS](https://en.wikipedia.org/wiki/.bss) area and checking the "magic" signature. First, signature checking: ```assembly cmpl $0x5a5aaa55, setup_sig jne setup_bad ``` This simply compares the [setup_sig](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L39) with the magic number `0x5a5aaa55`. If they are not equal, a fatal error is reported. If the magic number matches, knowing we have a set of correct segment registers and a stack, we only need to set up the BSS section before jumping into the C code. The BSS section is used to store statically allocated, uninitialized data. Linux carefully ensures this area of memory is first blanked, using the following code: ```assembly movw $__bss_start, %di movw $_end+3, %cx xorl %eax, %eax subw %di, %cx shrw $2, %cx rep; stosl ``` First of all the [__bss_start](https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L47) address is moved into `di` and the `_end + 3` address (+3 - aligns to 4 bytes) is moved into `cx`. The `eax` register is cleared (using an `xor` instruction), and the bss section size (`cx`-`di`) is calculated and put into `cx`. Then, `cx` is divided by four (the size of a 'word'), and the `stosl` instruction is repeatedly used, storing the value of `eax` (zero) into the address pointed to by `di`, automatically increasing `di` by four (this occurs until `cx` reaches zero). The net effect of this code is that zeros are written through all words in memory from `__bss_start` to `_end`: ![bss](http://oi59.tinypic.com/29m2eyr.jpg) Jump to main -------------------------------------------------------------------------------- That's all, we have the stack, BSS so we can jump to the `main()` C function: ```assembly calll main ``` The `main()` function is located in [arch/x86/boot/main.c](https://github.com/torvalds/linux/blob/master/arch/x86/boot/main.c). What this does, you can read in the next part. Conclusion -------------------------------------------------------------------------------- This is the end of the first part about Linux kernel internals. If you have questions or suggestions, ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](anotherworldofworld@gmail.com) or just create [issue](https://github.com/0xAX/linux-internals/issues/new). In the next part we will see first C code which executes in Linux kernel setup, implementation of memory routines as `memset`, `memcpy`, `earlyprintk` implementation and early console initialization and many more. **Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to [linux-internals](https://github.com/0xAX/linux-internals).** Links -------------------------------------------------------------------------------- * [Intel 80386 programmer's reference manual 1986](http://css.csail.mit.edu/6.858/2014/readings/i386.pdf) * [Minimal Boot Loader for Intel® Architecture](https://www.cs.cmu.edu/~410/doc/minimal_boot.pdf) * [8086](http://en.wikipedia.org/wiki/Intel_8086) * [80386](http://en.wikipedia.org/wiki/Intel_80386) * [Reset vector](http://en.wikipedia.org/wiki/Reset_vector) * [Real mode](http://en.wikipedia.org/wiki/Real_mode) * [Linux kernel boot protocol](https://www.kernel.org/doc/Documentation/x86/boot.txt) * [CoreBoot developer manual](http://www.coreboot.org/Developer_Manual) * [Ralf Brown's Interrupt List](http://www.ctyme.com/intr/int.htm) * [Power supply](http://en.wikipedia.org/wiki/Power_supply) * [Power good signal](http://en.wikipedia.org/wiki/Power_good_signal)